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Metal–silicate interaction in quenched shock-induced melt of the Tenham L6-chondrite
Abstract
The metal–silicate microstructures in the shock-induced melt pockets of the Tenham (L6) chondrite have been investigated by analytical transmission electron microscopy. The melt areas, formed under high-pressure, high-temperature dynamic shock conditions, consist of spherical Fe–Ni metal/iron sulfide globules embedded in a silicate glass matrix, showing that the melt was quenched at high cooling rate. The Fe–Ni fraction in the globules is two-phase, composed of a bcc phase (∼5 wt% Ni) and an fcc phase (∼49 wt% Ni), indicating that fractional crystallisation of the metal occurred during the fast cooling. The metal fraction also contains appreciable amounts of non-siderophile elements (mostly Si, Mg and O) suggesting that these elements were trapped in the metal, either as alloying components or as tiny silicate or oxide inclusions. In the iron sulfide fraction, the Na content is high (>3 wt%), suggesting chalcophile behaviour for Na during the shock event. The composition of the silicate glass reflects non-equilibrium melting of several silicate phases (olivine, pyroxene and plagioclase). Moreover, the FeO content is high compared to the FeO contents of the unmelted silicates. Some Fe redistribution took place between metal and silicate liquids during the shock event. The silicate glass also contains tiny iron sulfide precipitates which most probably originated by exsolution during quench, suggesting that the molten silicate retained significant amounts of S, dissolved at high temperature and high pressure. Based on these observations, we suggest that non-equilibrium phenomena may be important in determining the compositions of metal and silicate reservoirs during their differentiation.
... Meteorites and soil samples returned from the Lunar and asteroid (e.g., Itokawa) surface have facilitated the study of native iron species, such as the presence of nanophase iron particles (np-Fe 0 ). Current research suggests that iron nanoparticles form through the reduction of fayalitic olivine (Boland and Duba, 1981;Leroux et al., 2000;Sasaki et al., 2001;Leroux et al., 2003;Noguchi et al., 2011), Postulated reduction reaction include: Fe 2 SiO 4 = 2Fe + SiO 2 + O 2 or Fe 2 SiO 4 = Fe + FeSiO 3 + 1/2O 2 ", with the first reaction liberating both native Fe-metal and a high proportion of SiO 2 from the silicate phase (Palme et al., 1988;Benzerara et al., 2002;Warren et al., 2014). At the same time, the phenomenon of ''dusty olivine" in chondritic meteorites has also been explained by presence of reduced iron particles, which provide a significant record of remanent preaccretionary processes (Lappe et al., 2011;Leroux et al., 2003). ...
... Xray EDS analyses of those np-Fe 0 grains show variable amounts of non-siderophile elements (Mg, Si) and a little Ni (<1%) ( Table 3). Previous studies of nanophase iron particles within the Tenham meteorite reported a similar composition, whereby this was explained by the presence of minute silicate inclusions that were enclosed in those np-Fe 0 grains, as a result of rapid quenching (Leroux et al., 2000). Since it was not possible to perform individual analyses of these nanophase grains, and the Si/Mg atomic ratios of those np-Fe 0 grains are consistent with the surrounding HP-CEn (Table 3), we suggest the light elements detected as part of the analysis are the result of analytical artefacts. ...
... The Earth owes its origin and evolution to having undergone multiple collisions between proto-planets that inhabited the early Solar System, including one or more very large impacts, during and soon after its initial accretion (Wetherill, 1985). Such dynamic impacts can provide a major source of energy needed to melt rock during the formation of both the planet's core, and likely an important role in subsequent planetary differentiation (Leroux et al., 2000). As impacts dominated the early evolution of the terrestrial planets, these processes could be occurred frequently within the early Solar System, and likely have a great significance to understanding the evolution of matter. ...
Although pure metallic iron (i.e. that with an Fe content of greater than 99%) commonly occurs in achondrites, and within the returned soil from asteroids or the Lunar surface, it is rarely found in ordinary chondrites meteorites. Abundant nanophase iron particles (np-Fe⁰) were identified in pyroxene glass, within the shock melt vein of Grove Mountains (GRV) 022115, which is an ordinary (L6) chondrite, with a shock stage determined as S5. The association of np-Fe⁰, highly defective high pressure clinoenstatite (HP-CEn), silica glass, as well as vesicles, embedded in a pyroxene glass selvage within the shock melt vein in this meteorite suggests that these phases formed as the result of decomposition of the host pyroxene grain, a process induced by the shock event that affected GRV 022115. The reaction to account for this mineral breakdown can be written as: FeSiO3 → Fe + SiO2 + 1/2O2 ↑ (MgSiO3 remain in the HP-CEn). The pressure and temperature condition attending this reaction are estimated at 20–23 GPa and over 1800 °C, as indicated by the surrounded high-pressure mineral assemblage: ringwoodite, majorite, and magnesiowüstite. This study provides evidence to the formation of np-Fe⁰ derived from pyroxene, and HP-CEn quenched metastably in such shocked vein could preserve the metastable phase transitions history record.
... Numerous observations and simulations of extraterrestrial sample have demonstrated that a considerable amount of non-siderophile O components from the surrounding O-containing matrix can be incorporated into the metal-sulfide phase and form Fe-S-O systems under high-temperature conditions during impact processes 35,[42][43][44] . Leroux et al. (2000) reported that the metal-sulfide globules embedded in an amorphous silicate-glass matrix contained 13 wt% of FeO under melting conditions, and the phase diagram showed that the melting point of the Fe-S-O system decreases with addition of the O component 35,45 . ...
... Numerous observations and simulations of extraterrestrial sample have demonstrated that a considerable amount of non-siderophile O components from the surrounding O-containing matrix can be incorporated into the metal-sulfide phase and form Fe-S-O systems under high-temperature conditions during impact processes 35,[42][43][44] . Leroux et al. (2000) reported that the metal-sulfide globules embedded in an amorphous silicate-glass matrix contained 13 wt% of FeO under melting conditions, and the phase diagram showed that the melting point of the Fe-S-O system decreases with addition of the O component 35,45 . Based on the oxygen content of the Chang'E-5 spherical iron-sulfide grains, thermodynamic calculations showed that thẽ 20% of dissolved FeO per mole of FeS in the iron-sulfide grain should experience a temperature of above 915°C (Supplementary Fig. 5). ...
Ferric iron as well as magnetite are rarely found in lunar samples, and their distribution and formation mechanisms on the Moon have not been well studied. Here, we discover sub-microscopic magnetite particles in Chang’E-5 lunar soil. Magnetite and pure metallic iron particles are embedded in oxygen-dissolved iron-sulfide grains from the Chang’E-5 samples. This mineral assemblage indicates a FeO eutectoid reaction (4FeO = Fe3O4 + Fe) for formation of magnetite. The iron-sulfide grains’ morphology features and the oxygen’s distribution suggest that a gas–melt phase reaction occurred during large-impact events. This could provide an effective method to form ubiquitous sub-microscopic magnetite in fine lunar soils and be a contributor to the presentation of ferric iron on the surface of the Moon. Additionally, the formation of sub-microscopic magnetite and metallic iron by eutectoid reaction may provide an alternative way for the formation of magnetic anomalies observed on the Moon. Magnetite is rarely present on the Moon. Here the authors report the magnetite formed by eutectic reaction during the impact process in Chang’E-5 lunar soil, and the potential contribution of this magnetite formation to magnetic anomalies on the Moon.
... BSE images show the ubiquitous presence of widely dispersed and finely disseminated spheres of metal, oxides, and iron-sulfide, identified by EDS, which accounts for the largely opaque nature of the vein in transmitted light (Fig. 1). The presumably spherical shape of these minerals likely formed by surface tension of immiscible melts rich in sulfur, metal and silicate (Leroux et al. 2000 (Table 1) and peak positions in the Raman spectrum near 664 cm À1 and 924 cm À1 (Rauch et al. 1996). Excess silicon (3.37-3.71 ...
... Instead, we interpret ringwoodite + stishovite to be a quenched metastable mineral assemblage. Because shock melt, once generated, will cool by conduction of heat to the surrounding colder host rock (Langenhorst and Poirier 2000;Leroux et al. 2000;Xie and Sharp 2007;Shaw and Walton 2013), the margins of shock veins correspond to the most rapid cooling rates with progressively slower quench rates realized within shock vein interior (Shaw and Walton 2013). This assemblage (ringwoodite + stishovite) is also observed around larger clasts (≥50 lm; Fig. 3c), suggesting that not only the walls of the host rock serve as sinks for rapid cooling but also entrained colder clasts. ...
... Clearly, the resulting melt pockets were not in thermochemical equilibrium with their surroundings, and therefore their compositions are not dominated by the FeNi–FeS eutectic, but instead vary considerably within the impact-affected volume. This was also suggested by Leroux et al. (2000). Our observations indicate that it is possible for FeNi–silicate, FeS–silicate and Fe- Ni + FeS–silicate melts to form independently of each other even at distances of <50 lm. ...
... During chemical equilibration, Ni should preferentially partition into the melt from the residue metal (see Scott, 1972; Ni diffuses faster in FeNi metal than trace siderophile elements; Righter et al., 2005; see further discussion on diffusion rates below). Therefore our element maps, which record no enrichment in the melt of Ni over Fe (Fig. 2B–D), indicate chemical disequilibrium (see also Leroux et al., 2000). This is particularly so for the silicate-dominated emulsions, where metal/sulfide melt was almost instantaneously separated from its residue.Fig. ...
Export Date: 9 December 2012, Source: Scopus
... All melted grains from Track 35 examined in this study consist largely of a texturally and compositionally heterogeneous emulsion of immiscible metal/sulfide beads nanometers to tens of nanometers in diameter in a silica-rich vesicular glassy amorphous material, identical to common Stardust material as described by Zolensky et al. (2006, Figs. 1A and B), Leroux et al. (2008a), and Tomeoka et al. (2008 and similar to known silicate-dominated shock melts with quenched metal-sulfide objects in ordinary chondrites (Leroux et al. 2000). ...
... The largest beads in grain C2004,1,44,1,3 have the lowest S abundances , suggesting either that sulfide minerals in the largest beads lost the most S during aerogel capture or that the largest beads had the lowest initial S abundances (and possibly consisted precapture of metal rather than sulfide) before melting. Leroux et al. (2000) report that the smallest (<20 nm diameter) beads in silicate-dominated shock melts with quenched metal-sulfide objects in a shocked ordinary chondrite are dominantly Fe sulfide, whereas larger (>20 nm diameter) beads contain appreciable Fe-Ni metal and are therefore have higher Fe:S ratios than the smaller beads. Comparable observations do not yet exist for other grains from any other Stardust aerogel tracks. ...
Abstract— Five amorphous (extensively melted) grains from Stardust aerogel capture Track 35 were examined by transmission electron microscopy (TEM); two from the bulb, two from near the bulb-stylus transition, and one from near the terminal particle. Melted grains consist largely of a texturally and compositionally heterogeneous emulsion of immiscible metal/sulfide beads nanometers to tens of nanometers in diameter in a silica-rich vesicular glass. Most metal/sulfide beads are spherical, but textures of non-spherical beads indicate that some solidified as large drops during stretching and breaking while in translational and rotational motion, and others solidified from lenses of immiscible liquid at the silicate-melt/vesicle (vapor) interface.
Melted grains appear to become richer in Fe relative to Mg, and depleted in S relative to Fe and Ni with increasing penetration distance along the aerogel capture track. Fe/S ratios are near unity in grains from the bulb of Track 35, consistent with the dominance of Fe-monosulfide minerals inferred by previous research on Stardust materials. Near-stoichiometric Fe/S in melted grains from the bulb suggests that Fe-sulfides in the bulb were dispersed and melted during formation of the bulb but did not lose S. Along-track increases in Fe/S in melted grains from the bulb through the bulb-stylus transition and continuing into the stylus indicate that S initially present as iron monosulfide may have been progressively partially volatilized and lost from the melted grains with greater penetration of the grains deeper into the aerogel during capture-melting of comet dust. Extensively melted grains from the bulbs of aerogel capture tracks may preserve better primary compositional information with less capture-related modification than grains from farther along the same capture tracks.
... Micrometer-size metallic grains are observed in olivines heated in a reducing atmosphere (Korotev et al., 1997;Lemelle et al., 2000;Leroux et al., 2003;Sasaki et al., 2001), surrounded by a pure silica or silica-rich amorphous phase, which is an expected product of the reduction reaction. Similar textures are observed in socalled "dusty" olivines in chondrites (Leroux et al., 2000). Alternatively, a pyroxene layer can form as a reaction product around metal (Nitsan, 1974). ...
... Small-scale reduction of olivine grains by heating following micro-impacts has been proposed as a mechanism of space weathering (Sasaki et al., 2001). Shock-induced reduction over larger distances was described in a melt pocket of the Tenham chondrite, with a metal size distribution between 10 nm and 1 μm in a silica amorphous matrix (Leroux et al., 2000). More advanced reduction can be explained by the larger shock duration in the Tenham meteorite . ...
Magnetic anomalies observed by the Mars Global Surveyor mission are attributed to crustal remanence. SNC (Shergotty–Nakhla–Chassigny) meteorites are likely samples of the Martian crust and are amenable to mineralogical and magnetic measurements essential to the understanding of the origin of magnetic anomalies. The recently discovered chassignite NWA 2737 and lherzolitic shergottite NWA 1950 display unusual magnetic characteristics that argue for a different magnetic carrier than the oxides and sulfides previously invoked in SNC meteorites. NWA 2737, the second member of the chassignite group, is a dunite with unusually dark-brown olivines and large magnetic susceptibility while Chassigny contains green olivines and is nearly a pure paramagnet. Dark olivines are also found in NWA 1950, a lherzolitic shergottite, which has singular magnetic properties when compared with other shergottites. The dark olivine color is due to the presence of Fe and FeNi metal nanoparticles, identified both by TEM and by magnetic measurements. Their size distribution encompasses the superparamagnetic to single domain transition at 30 K (10 nm range) and explains the magnetic properties of the bulk rocks. The formation of these nanoparticles is attributed to heating during the shock events that affected NWA 2737 and NWA 1950. The production of metal particles by shock-induced reduction of olivine has been invoked on surfaces deprived of atmosphere but never observed on Earth or Mars. Therefore, metal formed by shock in the heavily cratered Noachian crust is a possible carrier for crustal magnetic remanence. Widespread surface formation of metal nanoparticles could provide the precursor for the oxidized particles (goethite, hematite) observed in the Martian soils.
... These veins are actually sheets, ranging in thickness from $1 lm to several mm, which appear vein-like in thin section. Petrographic evidence supports the view that the veins comprise material that was locally melted (Fredriksson et al., 1963) and then quenched by conduction to the surrounding cooler host rock (Langenhorst and Poirier, 2000;Leroux et al., 2000;Sharp et al., 2003). The melt veins commonly contain two distinct parageneses. ...
... In both compositions, majorite garnet is the liquidus phase between about 15 and 23-25 GPa, but in KLB-1, magnesiowü stite also has a broad range of stability (17-23 GPa) near the liquidus (Fig. 4), which is significantly lower than the 23-27 GPa stability range of magnesiowü stite in Allende. In MÕbale, which has an FeO content (14.1 wt%) very similar to that of Tenham 14.4 wt% FeO in melted areas (Leroux et al., 2000), magnesiowü stite occurs between 21 and 25 GPa. Therefore, the majorite + magnesiowü stite assemblage in Tenham probably corresponds to a crystallization pressure range that is intermediate between that of KLB-1 (17-23 GPa) and Allende (23-27 GPa) and very similar to that of MÕBale (21-25 GPa). ...
The microtexture and mineralogy of a 580-μm-wide melt vein in the Tenham L6 chondrite were investigated using field-emission scanning electron microscopy and transmission electron microscopy to better understand the shock conditions. The melt vein consists of a matrix of silicate plus metal-sulfide grains that crystallized from immiscible melts, and sub-rounded fragments of the host chondrite that have been entrained in the melt and transformed to polycrystalline high-pressure silicates. The melt-vein matrix contains two distinct textures and mineral assemblages corresponding to the vein edge and interior. The 30-μm-wide vein edge consists of vitrified silicate perovskite + ringwoodite + akimotoite + majorite with minor metal-sulfide. The 520-μm-wide vein interior consists of majorite + magnesiowüstite with irregular metal-sulfide blebs. Although these mineral assemblages are distinctly different, the pressure stabilities of both assemblages are consistent with crystallization from similar pressure conditions: the melt-vein edge crystallized at about 23–25 GPa and the vein interior crystallized at about 21–25 GPa. This relatively narrow pressure range suggests that the melt vein either crystallized at a constant equilibrium shock pressure of ∼25 GPa or during a relatively slow pressure release. Using a finite element heat transfer program to model the thermal history of this melt vein during shock, we estimate that the time required to quench this 580-μm-wide vein was ∼40 ms. Because the entire vein contains high-pressure minerals that crystallized from the melt, the shock-pressure duration was at least 40 ms. Using a synthetic Hugoniot for Tenham and assuming that the sample experienced a peak-shock pressure of 25 GPa near the impact site, we estimate that the Tenham parent body experienced an impact with collision velocity ∼2 km/s. Based on a one-dimensional planar impact model, we estimate that the projectile size was >150 m in thickness.
... Metal and sulfide phase in the shock veins were commonly molten and occur as rapidly solidified metal-troilite eutectic with dendritic or cellular texture, in which metallic dendrites were enclosed in a troilite groundmass (Scott, 1982;Rubin, 1985;Chen et al., 1995). The investigation of textures, compositions and microstructures in the metal-troilite eutectic might shed light on the postshock thermal histories of reheated chondritic meteorites (Begemann and Wlotzka, 1969;Taylor and Heymann, 1971;Smith and Goldstein, 1977;Scott, 1982;Rubin, 1985;Chen et al., 1995;Leroux et al., 2000). ...
... This fact shows that only small amounts of metal-troilite eutectic contain the portion of metal-troilite-magnetite assemblage. Leroux et al. (2000) reported that the metal-sulfide globules embedded in an amorphous silicate glass matrix of shock veins of Tenham contain 13 wt.% of FeO. They believe that the shock-melting event was accompanied by oxidation of metallic iron-nickel and that appreciable amounts of FeO incorporated into the metal phase at high pressure and temperature. ...
The first natural occurrence of metal-troilite-magnetite assemblage composed of Fe-Ni metal and magnetite dendrites and a groundmass of troilite was identified in the shock veins of the Sixiangkou L6 chondrite, which contain abundant high-pressure minerals. This assemblage suggests a liquid miscibility among metal, FeS, and iron oxide, and subsequently quenching under pressure. Components of magnetite could be, in origin, related to chromite that was embedded and dissolved in an Fe-Ni-S liquid. Cr2O3 dissociated from chromite was mainly incorporated into garnet and magnesiowüstite in the fine-grained matrix of shock veins, in which chromium behaves as a lithophile element at the P-T conditions experienced by the shock veins. The occurrence of metal-troilite-magnetite assemblage suggests that the shock veins were still under pressure at temperatures from 900 to 950°C during solidification of Fe-Ni-S or Fe-Ni-S-O liquid, hence indicating a long duration of high pressure in the shock veins.
... The spherical shape of metal and sulfide inclusions that is governed by surface tension of a liquid phase is typical for a rapidly quenched impact melt of these two immiscible molten components. Such morphologies occur in shocked chondrites (e.g., Bennett and McSween 1996; Leroux et al. 2000) and in micro-craters from solar cells returned from low Earth orbit (Kearsley et al. 2007). The fine-scale and ubiquitous dispersion of droplets within the silicate melt is due to the low viscosity of the metal-sulfide melt that was injected by the hypervelocity impact into the aerogel capture cell. ...
... Crystallization in both cases ends at the eutectic temperature of ~1000 °C. With regard to the metal inclusions present in the silica matrix we note that the co-existence of kamacite and taenite in quenched impact melts was observed in the Tenham L6 chondrite (see Leroux et al. 2000). The core-mantle structure of the solidified droplets might be due to differential interfacial energies for the metal/ silicate and sulfide/silicate interfaces. ...
We report the results of high-resolution, analytical and scanning transmission electron microscopy (STEM), including intensive element mapping, of severely thermally modified dust from comet 81P/Wild 2 caught in the silica aerogel capture cells of the Stardust mission. Thermal interactions during capture caused widespread melting of cometary silicates, Fe-Ni-S phases, and the aerogel. The characteristic assemblage of thermally modified material consists of a vesicular, silica-rich glass matrix with abundant Fe-Ni-S droplets, the latter of which exhibit a distinct core-mantle structure with a metallic Fe,Ni core and a iron-sulfide rim. Within the glassy matrix, the elemental distribution is highly heterogeneous. Localized amorphous "dust-rich" patches contain Mg, Al, and Ca in higher abundances and suggest incomplete mixing of silicate progenitors with molten aerogel. In some cases, the element distribution within these patches seems to depict the outlines of ghost mineral assemblages, allowing the reconstruction of the original mineralogy. A few crystalline silicates survived with alteration limited to the grain rims. The Fe- and CI-normalized bulk composition derived from several sections show CI-chondrite relative abundances for Mg, Al, S, Ca, Cr, Mn, Fe, and Ni. The data indicate a 5 to 15% admixture of fine-grained chondritic comet dust with the silica glass matrix. These strongly thermally modified samples could have originated from a finegrained primitive material, loosely bound Wild 2 dust aggregates, which were heated and melted more efficiently than the relatively coarse-grained material of the crystalline particles found elsewhere in many of the same Stardust aerogel tracks (Zolensky et al. 2006).
... Our Cs atalja sample is influenced by shock metamorphism and alteration by iron-rich fluids. The Fe enrichment in veins and melt pyroxenes might be driven by Fe redistribution between metal and silicate liquids during the shock events in agreement with Leroux et al. (2000). The shock-related diffusion appeared only along the shock vein walls and at the clasts inside the melt as expected. ...
The analysis of the Csatalja H4 chondrite (which was found in August 2012) suggests shock‐related textures and spatial inhomogeneities, indicating a complex geological history. In the most heavily fractured and sheared units, small opaque grains and older fractures have locally enhanced the shock effect, producing melt. While the impact textures were evident in most units of the meteorite, mechanical shearing is apparent in only two units, suggesting that these units might have been present at somewhat different locations inside the parent body. Shearing also occurred at the border of the so‐called xenolith unit, confirming its mechanical mixing with the other units. Besides fragmentation and melting, chemical changes due to impact have also been identified, producing compositional homogenization of olivines in 30% of the investigated area of the sample's thin section (23 mm2), and moderate accumulation of Fe, Ca, and Na in the strongly shocked zones, initiating crystallization of feldspar in veins with a specific spatial distribution (feldspar glass with metal–sulfide globules). Analyzing the high P–T minerals, the peak shock pressure and temperature values differed substantially in the various units, ranging between 2 and 17 GPa, 100 and >1200 °C. The xenolith unit crystallized more slowly after the impact event and does not show shock impact alterations, suggesting that it was formed in a deeper region of the parent body. This was later shifted to its current surroundings and was lithified (fixed) to the rest of the sample. This “randomly selected” Csatalja sample provides information on the range of the formation temperatures, pressures, and processes that contributed to the heterogeneity of meteorites at the mm spatial scale, in general. The identified heterogeneity is a result not purely of the shock effects but also of the different pre‐shock structural characteristics. The shock also mixed fragments mechanically that have been formed at different environments, with at least several dozens or even 100 m depth in the parent body.
... S2), with a composition of Fe 97 (3) Si 4(1) , inside hiroseite. During shock melting, Fe-rich nanoparticles, derived from FeS and FeNi in the host rock, could have been entrapped into silicates crystallizing from shock melt (33). However, the absence of S and Ni in the metallic-iron nanoparticles ( fig. ...
Bridgmanite, MgSiO 3 with perovskite structure, is considered the most abundant mineral on Earth. On the lower mantle, it contains Fe and Al that strongly influence its behavior. Experimentalists have debated whether iron may exist in a mixed valence state, coexistence of Fe ²⁺ and Fe ³⁺ in bridgmanite, through charge disproportionation. Here, we report the discovery of Fe-rich aluminous bridgmanite coexisting with metallic iron in a shock vein of the Suizhou meteorite. This is the first direct evidence in nature of the Fe disproportionation reaction, which so far has only been observed in some high-pressure experiments. Furthermore, our discovery supports the idea that the disproportionation reaction would have played a key role in redox processes and the evolution of Earth.
... Metal and sulfide phases in the chondritic melt were completely molten and occur as rapidly solidified metal-troilite eutectic with dendritic or cellular texture, in which metallic dendrites were enclosed in a troilite groundmass (Scott 1982;Rubin 1985;Chen et al. 1995aChen et al. , 1995b. The investigation of textures, compositions, and microstructures in the metal-troilite eutectic can be used to construct the postshock thermal histories of chondritic meteorites (Begemann and Wlotzka 1969;Taylor and Heymann 1971;Smith and Goldstein 1977;Scott 1982;Rubin 1985;Chen and Xie 1995;Chen et al. 1995aChen et al. , 1995bLeroux et al. 2000;Kong and Xie 2003). The cooling rates deduced from the structures of metal dendrites in the shock-induced melts of chondrites range from 0.1 to 5000°C s À1 (Scott 1982). ...
An assemblage with FeNi metal, troilite, Fe-Mn-Na phosphate, and Al-free chromite was identified in the metal-troilite eutectic nodules in the shock-produced chondritic melt of the Yanzhuang H6 meteorite. Electron microprobe and Raman spectroscopic analyses show that a few phosphate globules have the composition of Na-bearing graftonite (Fe,Mn,Na)3(PO4)2, whereas most others correspond to Mn-bearing galileiite Na(Fe,Mn)4(PO4)3 and a possible new phosphate phase of Na2(Fe,Mn)17(PO4)12 composition. The Yanzhuang meteorite was shocked to a peak pressure of 50 GPa and a peak temperature of approximately 2000 °C. All minerals were melted after pressure release to form a chondritic melt due to very high postshock heat that brought the chondrite material above its liquidus. The volatile elements P and Na released from whitlockite and plagioclase along with elements Cr and Mn released from chromite are concentrated into the shock-produced Fe-Ni-S-O melt at high temperatures. During cooling, microcrystalline olivine and pyroxene first crystallized from the chondritic melt, metal-troilite eutectic intergrowths, and silicate melt glass finally solidified at about 950–1000 °C. On the other hand, P, Mn, and Na in the Fe-Ni-S-O melt combined with Fe and crystallized as Fe-Mn-Na phosphates within troilite, while Cr combined with Fe and crystallized as Al-free chromite also within troilite.
... (2) Petrographical evidence demonstrates that shock melts comprise material that is locally melted (Fredriksson et al. 1963) and then quenched by conduction to the surrounding cooler host rock (Langenhorst & Poirier 2000;Leroux et al. 2000;Sharp et al. 2003 (Walton & Shaw 2009). In this study, an upper limit of several minutes is considered for cooling of the 1 mm-diameter shock vein in NWA 4797 (see the following paragraph). ...
Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of NW Africa (NWA) 4797. Shock heating of NWA 4797 completely melted and vesiculated precursor igneous plagioclase, which cooled to an assemblage of plagioclase crystals with interstitial glasses of variable composition (Ca/K ratios). Using a focused ultraviolet laser beam, is has been possible to distinguish between argon isotopic signatures from groundmass minerals (igneous olivine + pyroxene), plagioclase and a shock vein. This study focuses on the potential for this meteorite to shed light on shock ages of shergottites.
Apparent ⁴⁰ Ar/ ³⁹ Ar ages of groundmass minerals show that there are large amounts of excess argon in this phase, yielding a wide range of calculated ages from 690 ± 30 Ma to several apparent ages older than 4.5 Ga. A traverse of laser-probe extractions across the 1 mm-diameter shock vein in NWA 4797 yielded apparent ⁴⁰ Ar/ ³⁹ Ar ages younger than the groundmass. A signature of the Martian atmosphere, identified by ⁴⁰ Ar/ ³⁶ Ar ratios of 1600–1900, was not found in the NWA 4797 shock vein. This is distinct from other shergottites where the products of shock melting contain a nearly pure sample of Martian atmosphere. We attribute this to a distinct formation mechanism, and hence gas-trapping mechanism, of the NWA 4797 shock vein.
We undertook 44 analyses of plagioclase areas identified by SEM analysis. Ages ranged from 45 ± 27 to 3771 ± 109 Ma and yield an average age of 375 ± 77 Ma, considerably younger than ages obtained in this study from either the groundmass or the shock vein. A plot of age v. ³⁷ Ar/ ³⁹ Ar for plagioclase showed a continuum of ages from the oldest to youngest ages measured. Older ages are correlated with higher Ca/K ratios of plagioclase, indicating contamination from groundmass minerals rich in excess argon. The youngest ages correlate to plagioclase extractions with the lowest Ca/K ratios, interpreted to have crystallized from a nearly pure plagioclase melt with contributions from a K-rich mesostasis. We see no evidence for multiple shock events in NWA 4797. Rather, we favour the interpretation that the cosmic-ray exposure (CRE) age of 3.0±0.5 Ma, obtained on NWA 4797 in this study using cosmogenic ³⁸ Ar, approximates the timing of shock melting in this meteorite.
Supplementary material
Laser probe argon isotopic data for NWA 4797 obtained in this study are available at http://www.geolsoc.org.uk/SUP18602 .
... Shock veins and shock-melt pockets comprise material that was locally melted (Fredriksson et al. 1963) and then cooled by conduction of heat to the surrounding host rock (Langenhorst and Poirier 2000;Leroux et al. 2000;Sharp et al. 2003;Xie et al. 2006). Calculations by Beck et al. (2007) indicate cooling rates for a 1 mm diameter shock melt of 5000°C s À1 over the cooling interval 2500-500°C, giving a cooling time for this interval of 0.2 s. ...
The distribution of shock melts in four shergottites, having both vein
and pocket geometry, has been defined and the conductive cooling time
over the range 2500 °C to 900 °C calculated. Isolated 1
mm2 pockets cool in 1.17 s and cooling times increase with
pocket area. An isolated vein 1 × 7 mm in Northwest Africa (NWA)
4797 cools to 900 °C in 4.5 s. Interference between thermal haloes
of closely spaced shock melts decreases the thermal gradient, extending
cooling times by a factor of 1.4 to 100. This is long enough to allow
differential diffusion of Ar and Xe from the melt. Small pockets (1
mm2) lose 2.2% Ar and 5.2% Xe during cooling, resulting in a
small change in the Ar/Xe ratio of the dissolved gas over that
originally trapped. With longer cooling times there is significant
fractionation of Xe from Ar and the Ar/Xe ratio increases rapidly. The
largest pockets show less variation of Ar/Xe and likely preserve the
original trapped gas composition. Considering all of the model
calculations, even the smallest isolated pockets have cooling times
greater than the duration of the pressure pulse, i.e., >0.01 s. The
crystallization products of these shock melts will be unrelated to the
peak shock pressure experienced by the meteorite.
... BSE images reveal a mixture of tiny silicate crystals 189 (60 nm − 325 nm; avg 180 nm) and glassy material finely intergrown with circular blebs of 190 bright iron‐sulfides (Fig. 3a). The presumably spherical shape of the iron‐sulfides is 191 interpreted to have formed by surface tension of the immiscible sulfur‐rich melt in the 192 surrounding silicate melt (Leroux et al., 2000), indicating partial melting. Opaque areas of 193 the matrix have a higher concentration of iron sulfide spheres, whereas amber‐ or dark‐ 194 brown colored matrix are mostly silicate crystals or silicate crystals + glass. ...
Lithology A of Martian meteorite Elephant Moraine (EET) A79001 contains fragments entrained within a 100 μm-thick shear-induced shock vein. These fragments, the shock vein matrix and walls of olivine along the vein, as well as shock deformation and transformation in rock-forming minerals in the bulk rock, were investigated using scanning electron microscopy, the electron microprobe and Raman spectroscopy. The presence of ringwoodite, the spinel-structured high-pressure (Mg,Fe)2SiO4 polymorph, has been confirmed in EETA79001 for the first time. Ringwoodite occurs within and around the shock vein, exhibiting granular and lamellar textures. In both textures ringwoodite consists of ∼500 nm size distinct grains. Ringwoodite lamellae are 115 nm to 1.3 μm wide. Planar fractures in olivine provided sites for heterogeneous nucleation of ringwoodite. Analyses performed on the largest grains (⩾1 μm) show that ringwoodite is consistently higher in iron (Fa27.4–32.4) relative to surrounding olivine (Fa25.1–267.7), implying that there was Fe–Mg exchange during their transformation, and therefore their growth was diffusion-controlled. In the shock environment, diffusion takes place dynamically, i.e., with concurrent deformation and grain size reduction. This results in enhanced diffusion rates (⩾10⁻⁸ m²/s) over nm – μm distances.
... It is possible that metastable compositions of similar phases were produced in flash heating conditions upon impact with aerogel. Leroux et al. (2000) also showed the FeO content in quenched shock glass from a melt in the Tenham L6 chondrite was high compared to the FeO contents of the unmelted silicates, suggesting that some Fe redistribution had taken place between metal and silicate liquids during the shock event. They also showed that the shock-induced melts retained significant amounts of S. Such mixtures could have caused melting via the capture heating effects in aerogel described above. ...
X-ray Absorption Fine Structure techniques have been used on Comet Wild2/81P tracks from the Stardust mission. Fe-XANES and EXAFS have been performed on aerogel sections from Tracks 41 and 162 as well as the mid and terminal positions of Track 134. This is the first use of EXAFS in the study of early Solar System materials. With EXAFS, we have measured Fe–O and Fe–S bond lengths and thus, together with complementary XANES measurements, identified Fe-rich phases. In particular, we show that ferric-rich phases in 2 Tracks (41, 162) have Fe–O bond 1st shell bond lengths of 1.99–2.01 Å and Fe K absorption edge and pre edge centroid positions consistent with being hematite-dominated grains. These iron oxides can be clearly distinguished from a magnetite grain, present in Track 134. We also demonstrate the identification of the Mg-rich end member olivine using EXAFS with XANES in Track 162. The terminal grain of Track 134 is pyrrhotite, its first atomic shell has an Fe–S structure, with 4 nearest neighbouring S atoms at a distance of 2.29 ± 0.05 Å.Our XANES results show the presence of Fe3+-bearing grains along the Stardust tracks and suggest either flash-cooling of an Fe–S–SiO–O2 gas during capture or the presence of a Fe–Ni–S–O melt along the cometary tracks during impact capture in the aerogel, rather than the capture process being solely associated with reduction of cometary phases. Accurate determination of Comet Wild2 redox conditions requires the identification of phases, in particular terminal grains, which have not experienced this melting. For instance, the larger hematite-rich grains (>10 μm) are more likely to be cometary in origin. EXAFS provides a valuable new analytical technique to study fine-grained early Solar System materials.
... (2) Petrographical evidence demonstrates that shock melts comprise material that is locally melted (Fredriksson et al. 1963) and then quenched by conduction to the surrounding cooler host rock (Langenhorst & Poirier 2000;Leroux et al. 2000;Sharp et al. 2003 (Walton & Shaw 2009). In this study, an upper limit of several minutes is considered for cooling of the 1 mm-diameter shock vein in NWA 4797 (see the following paragraph). ...
Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of Northwest Africa (NWA) 4797. Shock heating of NWA 4797 completely melted and vesiculated precursor igneous plagioclase, which cooled to an assemblage of plagioclase crystals with interstitial glasses of variable composition (Ca/K ratios). Using a focused ultraviolet laser beam, is has been possible to distinguish between argon isotopic signatures from groundmass minerals (igneous olivine + pyroxene), plagioclase and a shock vein. This study focuses on the potential for this meteorite to shed light on shock ages of shergottites.
Apparent 40Ar/39Ar ages of groundmass minerals show that there are large amounts of excess argon in this phase, yielding a wide range of calculated ages from 690 ± 30 Ma to several apparent ages >4.5 Ga. A traverse of laser probe extractions across the 1 mm diameter shock vein in NWA 4797 yielded apparent 40Ar/39Ar ages lower than the groundmass. A signature of the Martian atmosphere, identified by 40Ar/36Ar ratios = 1600−1900, was not found the NWA 4797 shock vein. This is distinct from other shergottites where the products of shock melting contain a nearly pure sample of Martian atmosphere. We attribute this to a distinct formation mechanism, and hence gas-trapping mechanism, of the NWA 4797 shock vein.
We undertook 44 analyses of plagioclase areas identified by SEM analysis. Ages ranged from 45 ± 27 Ma to 3771 ± 109 Ma and yield an average age of 375 ± 77 Ma, considerably lower than ages obtained in this study from either the groundmass or the shock vein. A plot of age versus 37Ar/39Ar for plagioclase showed a continuum of ages from the oldest to youngest ages measured. Older ages are correlated with higher Ca/K ratios of plagioclase, indicating contamination from groundmass minerals rich in excess argon. The youngest ages correlate to plagioclase extractions with the lowest Ca/K ratios, interpreted to have crystallized from a nearly pure plagioclase melt with contributions from a K-rich mesostasis. We see no evidence for multiple shock events in NWA 4797. Rather, we favor the interpretation that the CRE age of 3.0 ± 0.5 Ma, obtained on NWA 4797 in this study using cosmogenic 38Ar, approximates the timing of shock melting in this meteorite.
... Various models have been proposed to explain their formation. Most experiments are carried out under low oxygen fugacities and high temperatures to induce the formation of metallic particles (e.g., Boland and Duba, 1986;Connolly et al., 1994;Leroux et al., 2000Leroux et al., , 2003. For example, a high density of several hundreds of nanometer sized iron particles in the centre of ca. 100 μm large olivine crystals of the so-called "dusty regions" of unequilibrated chondrules can be experimentally reproduced by heating fayalitic olivines for a few minutes to 1610°C at a C/CO buffer of log fO 2 (atm) = −15.2 ...
Olivines from the meteorite Northwest Africa 2737 (NWA 2737) show unique characteristics such as strong brownish staining, the occurrence of perpendicular sets of bright lamellae and the exsolution of metallic nanoparticles. These features were investigated by transmission electron microscopy and spectroscopic techniques to understand their formation and consequent implications for the shock history of NWA 2737. Areas showing optically an intense brown staining are characterised by the occurrence of finely dispersed metallic nanoparticles, an extreme high density (1016m−2) of polygonised dislocations with Burgers vector [001] and a high abundance of trivalent iron (16%) as determined by electron energy loss spectroscopy. In contrast, the bright lamellae contain individual dislocation lines with Burgers vector [001] of a slightly lower density (1014m−2), almost no metallic nanoparticles and significantly lower trivalent iron content. Model mass balance calculations suggest that the high content of trivalent iron can be incorporated as a laihunite component into the cores of polygonised dislocations. This is charge balanced by the formation of metallic nanoparticles. Trivalent iron is likely responsible for the intense brown colour, whereas nanoparticles darken these areas considerably and cause an intense red slope in the infrared range. The proposed diffusion of trivalent iron into the cores of polygonised dislocations and the associated exsolution of metallic iron nanoparticles are likely fundamental processes for intense olivine staining during meteoritic impacts in general. Detailed analyses of variously stained regions suggest that the bright lamellae have experienced slightly higher shock-induced strain causing a recrystallisation of olivine, which occurred simultaneously with the polygonisation and exsolution processes resulting in the olivine staining of adjacent areas. The existence of individual dislocations can be interpreted as the result of a two-stage shock history for NWA 2737. After a first impact, the material of NWA 2737 was likely embedded in a hot ejecta blanket. This would ensure sufficient time at elevated temperatures for the polygonisation of dislocations and the recrystallisation of bright lamellae. A second impact resulted in the emission of individual dislocations into recrystallised olivine and the ejection of that meteorite from Mars.
... Petrographic precedents of equilibrated L chondrites which have experienced near complete shock melting or total shock melting are known and complete a consistent sequence while additionally accounting for the observations in this work. The L6 Chico (Norman and Mittlefehldt, 2002), the L chondrite melt rock PAT 91501 (Mittlefehldt and Lindstrom, 2001), L6 Shaw (Taylor et al., 1979) and the L6 Tenham melt breccia (Leroux et al., 2000) are examples of samples where, to different degrees, metal and silicate have been partitioned to an extent much greater than that observed in the samples included in this study because of their reaching higher whole rock temperatures. ...
To assess the role of impact related reheating on compositional trends in the major elements of the L chondrite parent(s), I examined a suite of 37 L falls analyzed by Jarosewich (1990). After eliminating 8 samples prepared from quantities which differently sampled the chondrites considered, I found that suites of mildly-shocked (S3) and strongly-shocked (S4-S6) equilibrated L chondrite falls can be distinguished at statistically significant levels when compared based on their major element content. Graphical comparisons demonstrated that the differences arise because siderophiles are depleted with corresponding mass-balanced lithophile enrichment in strongly-shocked samples. Since the samples considered here were derived from volumes of between 1.5 cm3 and 4.7 cm3, we can conclude that statistically significant post-metamorphic metal(sulfide)/silicate segregation occurred on a scale of at least this size.
... X-ray di¡raction revealed the diagnostic di¡raction lines of the phase previously synthesized by Gautron et al. [18] and Irifune et al. [13]. [26]. The molten areas are preferentially located at the boundary between pyroxenes and maskelynite. ...
A (Ca,Na)-hexaluminosilicate, whose Ca end member was previously synthesized in numerous high-pressure experiments, has been identified by Raman spectroscopy in heavily shocked Martian meteorites. This mineral has a structural formula close to (CaxNa1−x)Al3+xSi3−xO11 and is similar to the calcium aluminum silicate phase previously synthesized in high-pressure experiments performed on anorthite and rocks of basaltic composition. This new mineral occurs in shock melt pockets in two distinct settings and is intimately intergrown with SiO2-stishovite. The first setting, encountered in Zagami, consists of idiomorphic equant crystals overgrown by acicular stishovite that crystallized from a melt of labradorite composition. The second setting contains the (Na,Ca)-hexaluminosilicate phase intergrown with stishovite and hollandite and was formed during partial melting at high pressures. The mineralogical association (Na,Ca)-hexaluminosilicate+stishovite was observed in shock melt pockets, which have distinct bulk compositions in seven Martian shergottites. This new mineral represents, after majorite, the second natural occurrence of a silicate mineral with silicon in both four and six coordination. The assemblage stishovite+(Na,Ca)-hexaluminosilicate sets constraints on the pressure and temperature conditions that prevailed during shock in some of the studied meteorites. The (Na,Ca)-hexaluminosilicate mineral is a potential carrier of Al and Na during subduction of oceanic crust in the lower mantle of the Earth.
... Plots of Si/ (Mg+Fe) vs. count rate, obtained with gun-lens 5 and spot size 7 (left) and gun-lens 5 and spot size 9 (right), show a nearly constant Si/(Mg+Fe) of 1.0 (left) and 1.1 (right). textures and compositions were reported in another Tenham sample (Leroux et al. 2000) and in Nakhon Pathon (Z. Xie et al. 2006a), and were interpreted as the result of rapid quench. ...
We report a new mineral that occurs in shock-induced melt veins of the Tenham L6 chondrite. The new mineral, identified by
transmission electron microscopy (TEM), occurs as acicular nanocrystals in a glassy matrix at the edge of shock-induced melt
veins that crystallized during rapid quench at high pressure. The elongate crystals have aspect ratios up to 25. Widths range
from ~5 to ~40 nm and lengths are up to 500 nm. Energy-dispersive X-ray spectroscopy (EDS) analyses provide the relative cation
abundances that are consistent with a pyroxene-like stoichiometry: Na0.06Ca0.02Mg0.71Fe0.20Al0.11 Si0.94O3. Selected area electron diffraction (SAED) patterns from single-crystal and polycrystalline aggregates indicate an olivine
structure with refined cell parameters: a = 4.78, b = 10.11, and c = 5.94 Å and a calculated density of 3.32 g/cm3. Synchrotron X-ray microdiffraction data are consistent with an olivine structure and provide similar cell parameters: a = 4.778, b = 10.267, c = 5.937 Å. The pyroxene composition represents a large deviation from olivine stoichiometry, (Na0.08Ca0.03Mg0.95 Fe0.26Al0.15Si0.25□0.28)2Si1O4, with 0.28 formula units of vacancies (□), 0.11 of Na+ plus Ca2+, and 0.25 of Si4+, in octahedral sites. Our observations indicate that a metastable and nonstoichiometric olivine structure can crystallize
from a silicate melt during rapid quench. Trace amounts of such defects may be present in stable olivines in the deep upper
mantle.
Northwest Africa (NWA) 032 is an unbrecciated porphyritic basalt found in the Moroccan desert in 1999. Constituent igneous minerals—olivine, pyroxene, and plagioclase—exhibit shock deformation and transformation effects. NWA 032 is among the youngest radiometrically dated sample from the Moon, with concordant Sm‐Nd and Rb‐Sr ages of 2.947 ± 0.016 Ga and 2.931 ± 0.092, respectively, representing the timing of igneous crystallization. We present the first comprehensive study of shock metamorphism in NWA 032, with a focus on the structural state of fine‐grained plagioclase feldspar, shock deformation in olivine and pyroxene, and the microtexture and mineralogy of shock melts. Micro‐Raman spectroscopy, optical properties, and electron imaging confirm that plagioclase in this meteorite has been shock amorphized, which, for calcic plagioclase (An80‐90), requires shock pressures on the order of ~25–27 GPa. Shock pressures in this range are accompanied by a postshock temperature increase <200 °C. Shock deformation in olivine and pyroxene phenocrysts comprises undulose extinction to weak mosaicism, irregular fractures, polysynthetic mechanical twinning in pyroxene, and development of planar fractures in olivine. The shock effects in mafic minerals constrain the upper limit of shock in NWA 032 to have been <30 GPa. Shock melt in NWA 032 has quenched to glass of basaltic composition, representing localized in situ melting of igneous minerals by shearing along lithological boundaries to form shock veins and shock impedance contrasts to form isolated pockets of shock melt. These melts quench‐crystallized olivine and pyroxene during the pressure release (<14 GPa). Using recent experimental data on shock amorphization of feldspars, coupled with constraints on the formation of metastable minerals associated with shock melt, we have revised the shock pressure experienced by paired meteorites NWA 10597, NWA 4734, and LaPaz Icefield 02205/02224/0226/02436/03632/04841. These largely unbrecciated, basaltic meteorites experienced an equilibration shock pressure on the order of ~22–25 GPa, constrained by partial amorphization of precursor igneous bytownite. Our results are consistent with crater pairing and ejection in a single impact cratering event.
An assemblage with metal, troilite, Fe–Mn–Na phosphates, and Al-free chromite was identified in the metal-troilite eutectic nodules in the shock-produced chondritic melt of the Yanzhuang H6 meteorite. Electron microprobe and Raman spectroscopic analyses show that a few phosphate globules have composition of Na-bearing graftonite (Fe,Mn,Na)3(PO4)2, whereas most other correspond to Mn-bearing galileiite Na(Fe,Mn)4(PO4)3 and a possible new phosphate phase of Na2(Fe,Mn)17(PO4)12 composition. The Yanzhuang meteorite was shocked to a peak pressure of some tens GPa and a peak temperature of ~2000 ℃. All minerals were melted after pressure release to form a chondritic melt due to very high postshock heat that brought the chondrite material above its liquidus. The volatile elements P and Na released from whitlockite and plagioclase along with elements Cr and Mn released from chromite are concentrated into the shock-produced Fe–Ni–S–O melt at high temperatures. During cooling, microcrystalline olivine and pyroxene first crystallized from the chondritic melt, and the formed metal-troilite eutectic intergrowths and silicate glass finally solidified at about 950–1000 ℃. On the other hand, P, Mn, and Na in the Fe–Ni–S–O melt combined with Fe and crystallized as Fe–Mn–Na phosphates within troilite, while Cr combined with Fe and crystallized as Al-free chromite also within troilite.
The partitioning of K and Na between liquid Fe-S-O alloys and silicate
melt has been determined over the pressure and temperature range 2.5-24
GPa and 1500-1900°C. In experiments with S-free Fe alloys, the
alkali elements show completely lithophile behaviour. When S is added,
however, K and Na begin to enter the Fe-liquid phase and their
distribution coefficients DI (=[I] metal/[I]
silicate) correlate strongly with O content (and FeO
activity) of the Fe-S-O liquid and with the composition of the silicate
melt. For potassium, DK is ˜1.0 for Fe-sulphide liquid
containing 30% S and 8% O. Increasing temperature leads to increasing O
solubility in the Fe-sulphide liquid and correspondingly higher values
of DK. Increasing pressure on the other hand slightly reduces
DK values. Given a planetary core containing 10 wt% S and 4-8
wt% O, then several hundred ppm K would be present in the Fe-sulphide
liquid if it segregated at low pressures, e.g. in a small planetary body
such as Mars. If, as has recently been suggested, the Earth's core
separated at the base of a deep magma ocean, then its highest possible K
content is about 250 ppm. The latter would generate approximately 20% of
the total heat production of the core. K can only be present in the
core, however, if, at some time during its formation, a discrete O-rich
FeS liquid separated from the silicate mantle. Finally, the sulphide
compositions produced in our experiments imply that a combination of S
and O could contribute significantly to the light element content of the
Earth's core.
To constrain the effects of capture modification processes, the size
distribution of nanoscale refractory Fe-Ni-S inclusions ("droplets") was
measured in five allocations extracted from throughout the depth of
Stardust Track 35. The Fe/S ratio has been previously shown to increase
significantly with penetration depth in this track, suggesting
increasing capture-related modification along the track. Astronomical
image analysis tools were employed to measure the sizes of more than
8000 droplets from TEM images, and completeness simulations were used to
correct the distribution for detection bias as a function of radius. The
size distribution characteristics are found to be similar within
independent regions of individual allocations, demonstrating uniformity
within grains. The size distribution of the Fe-Ni-S droplets in each
allocation is dominated by a mode near 11 nm, but is coarse-skewed and
leptokurtic with a mean of ˜17 nm and a standard deviation of
˜9 nm. The size distribution characteristics do not vary
systematically with penetration depth, despite the strong trend in bulk
Fe/S ratio. This suggests that the capture modification process is not
primarily responsible for producing the morphology of these nanoscale
droplets. The Stardust Track 35 droplet size distribution indicates
slightly smaller sizes, but otherwise resembles those in carbonaceous
chondrite Acfer 094, and chondritic porous interplanetary dust particles
that escaped nebular annealing of sulfides. The size distribution of
metal-sulfide beads in Stardust's quenched melted-grain emulsions
appears to be inherited from the size distribution of unmelted sulfide
mineral grains in comet-dust particles of chondritic character.
The historical Madrid meteorite chondrite fell in 1896 showing thin melt veins with a 65% of brecciated forsterite fragments surrounded by a fine grained matrix formed by troilite, chromite and Fe-Ni blebs. It exhibits a delicate iron infill, neo-formation of troilite in pockets and shock veins and neo-formation of Na-feldspar formed at high temperature and fast quenching. The semi-quantitative mineral determinations were performed with IMAGEJ freeware and chemical mappings resulting in the following approximated compositions: olivine (∼55%); augite (∼10%); enstatite (∼10%); plagioclase (∼10%); chromite (∼2%); troilite (∼4%), kamacite-taenite α-γ-(Fe, Ni) (∼7%) and merrillite (∼7%). The specimen was also studied by computer tomography, micro-Raman spectroscopy and spectral cathodoluminescence. X-ray diffraction patterns were also recorded in non-destructive way on a polished surface because of the small size of the specimen. This combination of non-destructive techniques provides an improved knowledge on the Madrid-1896 meteorite compared to the previous study performed on the same specimen carried out twenty years ago by electron probe microanalysis and optical microscopy in destructive way. Limits of these techniques are the specimen's size in the analytical chambers and the threshold resolution of the microscopes analyzing shock veins micro-crystals.
We investigated a shock vein of the Yamato 791384 L6 chondrite to clarify the nature and sequence of the dynamic processes that resulted from the shock events. The chondritic host-rock of Y-791384 mainly consists of olivine (Fa24–25), low-Ca pyroxene (Fs18–22), albitic feldspar (An9–10Ab84–86Or5–7), troilite and metallic Fe–Ni. The shock vein contains majorite (or majorite-pyropess) and magnesiowüstite (+ minor jadeite) as high-pressure polymorphs. Two different dynamic events were recorded in the shock vein. The majorite grain contained vitrified (Mg,Fe)SiO3-perovskite inclusions. The (Mg,Fe)SiO3-perovskite was crystallized from a chondritic melt, and is a remnant of a first dynamic event. The majorite and magnesiowüstite were also crystallized directly from a chondritic melt but induced by a second dynamic event. The pressure condition for the first and second dynamic events would be >~24GPa and
Heavily shocked meteorites of shock stages S5 and S6 contain shock-induced melt veins (SMVs). SMVs might have reset the remanence of an asteroidal metamorphism at the time of giant collisions against a chondrite parent body. Here we present micropaleomagnetic and petrologic studies of SMVs in L6S5 Tenham chondrite with ∼500 μm thick black veins enclosing high-pressure minerals such as ringwoodite. Paleomagnetic data show that the high-temperature (HT, 200°C–650°C) and high-coercivity (HC, 20–100 mT) stable components of SMVs formed a cluster even from different portions of SMV, whereas the stable HT component of surrounding host rock showed a scattered orientation under stereonet projection. The host rock HC components form a girdle between the mixing of SMVs and unknown overprints, tracing the magnetic susceptibility foliation. Magnetic force microscopy and backscattered electron images confirmed kamacite and taenite assemblages in iron sulfides as remanence-carrying minerals in SMVs. Hysteresis data of SMVs revealed the presence of single-domain (SD) FeNi metals with Mrs/Ms = ∼0.1 and Hrc/Hc = ∼2, although these parameters are only applicable to magnetite. Because the metastable ringwoodite in SMVs transforms back to olivine at 188°C for 1000 Myr (metamorphism) or at 900°C for 1 h (postshock heating), the preservation of ringwoodite suggests that SMVs have not experienced either thermal condition. The magnetic time-temperature relation for SD FeNi metals suggested that 200°C unblocking temperature corresponds to the storage time of 100 years for kamacite and 4500 Myr for taenite at room temperature. The difference of HT components discards the possibility of postshock heating. Therefore, the SMV's remanence is a characteristic shock-induced thermal remanence that has newly been acquired during hypervelocity collision under a cryptic magnetic field.
Pseudotachylytes generally possess stable remanent magnetizations but the processes by which pseudotachylytes are magnetized remain poorly understood. Magnetic hysteresis and scanning electron microscope studies reveal that experimental frictional melting of granites produces dispersed submicron inclusions of weakly interacting pseudo-single-domain (PSD) magnetite, in artificial pseudotachylyte. The magnetite inclusions are absent in the undeformed granite protolith and result from oxidation of Fe in melt-susceptible mafic minerals during the melt-quenched event. The pseudotachylytes acquired a stable thermal remanence in fine-grained PSD magnetites during the rapid cooling of the melt, implying that fine-grained magnetite has the potential for paleointensity determinations of contemporaneous magnetic fields with co-seismic faulting in granitoids.
The Yanzhuang meteorite was severely shocked and reheated in an extraterrestrial impact event; it consists of four shock facies characteristic of disequilibrium shock effects. This study reveals that the volatile element Na in the shock-induced melt phase is not unequivocally depleted. Na preservation in the melt phase could correspond to: (1) high shock pressures (>=30 GPa) and high post-shock temperatures that acted in most parts of the meteorite, resulting in reduced rock porosity and lower pressure and temperature gradients between the melt phase and the surrounding shock facies. (2) quenching of melt phase (6 - 400 deg C/s).
Dendrites in the metal-troilite spherules in both shock-induced melt
veins and a melt pocket of the Yanzhuang chondrite show zoning in their
microstructures. This feature is indicative of nonequilibrium
solidification of the metal phases. Dendrites in the melt pocket have a
typical crust-core structure consisting of martensitic interiors (7.5 -
8.1 wt% Ni) and Ni-rich rims (12.5 - 23.3 wt% Ni). In comparison, the
dendrites in melt veins have three microstructural areas: (1) core (6.4
- 7.3 wt% Ni); (2) martensite between the core and rim (7.4 - 8.5 wt%
Ni); (3) Ni-rich rim (12.8 - 21.4 wt% Ni). It is suggested that the
difference in cooling rates followed shock-induced high temperature
melting might be an important factor in producing the different
dendritic microstructures in melt veins and melt pocket. Cooling rates
deduced from measurements of secondary dendritic arm spacings are 100 -
400 C/s in the melt veins and 6 - 30 C/s in the melt pocket,
respectively, and lie in the temperature interval 950 to 1400 C.
A revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed. Six stages of shock (S1 to S6) are defined, based on shock effects in olivine and plagioclase as recognized by thin section microscopy. The characteristic shock effects of each shock stage are: S1 (unshocked)—sharp optical extinction of olivine; S2 (very weakly shocked)—undulatory extinction of olivine; S3 (weakly shocked)—planar fractures in olivine; S4 (moderately shocked)—mosaicism in olivine; S5 (strongly shocked)—isotropization of plagioclase (maskelynite) and planar deformation features in olivine; and S6 (very strongly shocked)—recrystallization of olivine, sometimes combined with phase transformations (ringwoodite and/or phases produced by dissociation reactions). S6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked. In stages S3 to S6, localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure, due to differences in shock impedance. These melting effects are (a) opaque melt veins (shock veins); (b) melt pockets with interconnecting melt veins; (c) melt dikes; and (d) troilite/metal deposits in fractures. Based on a critical evaluation of data from shock recovery experiments, a shock pressure calibration for the six shock stages is proposed, which defines the S1 /S2, S2/ S3, S3/S4, S4/S5, and S5/S6 transitions at < 5, 5–10, 15–20, 30–35, and 45–55 GPa, respectively. Whole-rock melting and formation of impact melt rocks or melt breccias occurs at about 75–90 GPa. The symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite, e.g., H5(S3). We propose this new shock classification and pressure calibration system to replace previous systems, which are out-of-date with respect to the pressure calibration, nominally restricted to L chondrites, and based on incomplete and, in part, illdefined sets of shock effects.We have classified seventy-six ordinary chondrites using the new classification system and conclude the following: 1.1) Shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in H, L, and LL groups. Differences in the frequency distribution of shock stages are relatively minor; e.g., L chondrites appear to have the largest fraction with stages S5 and S6. This suggests that the collisional histories of the H, L, and LL parent bodies were similar.2.2 ) Petrologic type 3 chondrites are deficient in stages S4 to S6 and, with increasing petrologic type, the frequency of stages S4 to S6 increases. We suggest that the more porous and volatile-rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type. Volatiles trapped in pores cause shock-induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites.3.3 ) Stage S3 is the most abundant in nearly all petrologic types.4.4) At shock pressures in excess of about 35 GPa (S5 and S6), 4He and 40Ar are almost completely lost; pressures below 10 GPa (S1 and S2) do not cause noble gas losses.
We report new metal-silicate partition coefficients for Ni, Co and P at 7.0 GPa (1650–1750°C), and Ni, Co, Mo, W and P at 0.8, 1.0 and 1.5 GPa (1300–1400°C). Guided by thermodynamics, all available metal-silicate partition coefficients, D(i), where i is Ni, Co, P, Mo and W, are regressed against , , lnf(O2), ln(1 − Xs) (XS is mole fraction of S in metallic liquid) and nbo/t (non-bridging oxygen/tetrahedral cation ratio, a silicate melt compositional-structural parameter) to derive equations of the following form: . Expressions for solid metal-liquid silicate and liquid metal-liquid silicate partition coefficients are derived for S-free and S-bearing systems.We investigate whether Earth's upper-mantle siderophile element abundances can be reconciled with simple metal-silicate equilibrium. Sulfur-free metallic compositions do not allow a good fit. However, Ni, Co, Mo, W and P abundances in the upper mantle are consistent with simple metal-silicate equilibrium at mantle pressures and temperatures (27 GPa, 2200 K, ΔIW(iron-wüstite) = −0.15, ; XS = 0.15). Although these conditions are near the anhydrous peridotite solidus, they are well above the hydrous solidus and probably closer to the liquidus. A hydrous magma ocean and early mantle are consistent with predicted planetary accretion models. These results suggest that siderophile element abundances in Earth's upper mantle were established by liquid metal-liquid silicate equilibrium near the upper-mantle-lower-mantle boundary.
The non-equilibrium state of an undercooled melt allows the investigation of various phenomena involved in the formation of metastable solid phase. The present work reviews the present state of the art in this field. In detail, the thermodynamics of the undercooled melt, nucleation and growth processes are discussed, with special emphasis on the conditions of the formation of metastable states, such as metastable crystalline metals, quasi-crystals, metallic glasses, supersaturated solutions and grain-refined materials. The techniques needed for the in-situ investigation of non-equilibrium solidification phenomena in undercooled melts are discussed, and their potential for such investigations is evaluated. Aspects of microgravity research in this field are also included.
The parameterless correction method to perform absorption and fluorescence corrections in X-ray microanalysis of tranparent (S.) TE.M. specimens is presented and supported. This correction procedure requires no external parameters such as foil thickness or density, and no coefficients such as mass absorption coefficients or fluorescence yield coefficents and is therefore suitable for fast routine thin film microanalysis. Furthermore, thickness dependent artefacts such as surface and contamination layers and crystallographic orientation effects (the Borrmann effect) can be detected by the method. Une méthode permettant de corriger les effets d'absorption et de fluorescence en microanalyse X d'échantillons transparents pour la microscopie en transmission (S.) TE.M. est présentée. Cette procédure n'exigeant la connaissance d'aucun paramètre comme l'épaisseur ou la densité de l'échantillon ni d'aucun coefficient comme les coefficients d'absorption de masse ou encore des coefficients de rendement de fluorescence, est extrêmement utile et pratique pour effectuer des analyses de routine d'échantillons minces et a été appelée : "parameterless correction method". De surcroit tout effet dépendant de l'épaisseur de l'échantillon, comme la présence de couches de surface ou l'effet d'orientation crystallographique (l'effet Borrmann), peuvent-être détectés par cette méthode.
The minerals (Mg,Fe)SiO3-ilmenite and -perovskite were identified in the shock-induced veins in the Tenham chondritic meteorite. Both phases are inferred to have transformed from pyroxene at high pressures and temperatures by shock metamorphism. Columnar-shaped ilmenite grains, one of two types of morphologies, have a topotaxial relationship with neighboring pyroxene grains, indicating shear transformation. Granular-shaped perovskite grains showed a diffraction pattern consistent with orthorhombic perovskite, but these grains were not stable under the electron beam irradiation and became amorphous. The higher iron concentration in both phases compared with those experimentally reported may suggest their metastable transition from enstatite because of shock compression.
Hydrothermal experiments on natural samples of trachyandesite and dacite bulk composition show that anhydrite (CaSO4) may occur as a stable phenocryst phase at oxygen fugacities greater than or equal to 1.0 to 1.5 log fO2 units above the Ni-NiO equilibrium. The dissolved sulfur concentration in anhydrite saturated melts from MnO-Mn3O4 buffered experiments decreases with decreasing temperature, from approximately 2300 p.p.m.S at 1025°C to 250 p.p.m.S at 850°C (all at 2 kb Pfluid = Ptotal). In FeS-saturated melts equilibrated at the Ni-NiO buffer and 2 kb pressure, the concentration of dissolved sulfur also decreases with decreasing temperature, varying from approximately 400 p.p.m. S at 1025°C to less than 100 p.p.m. S at 850°C. At NNO or lowerfO2s, decreasing melt FeO content due to crystal fractionation may explain the observed decrease in sulfur solubility with decreasing temperature.Sulfur solubility values equivalent to the approximately 0.6 wt. per cent S present in fresh bulk pumice samples from the 1982 eruptions of El Chichon volcano are not readily achieved under any reasonable combinations of pressure, temperature, and oxidation state. Dissolved sulfur contents approaching 0.6 wt. per cent might occur if the source regions of melts parental to the El Chichon trachyandesite were at an fO2 several log units above the Ni-NiO equilibrium. Because such elevated oxidation states are far greater than the generally accepted values for mantle-derived partial melts we believe the high sulfur content of the El Chichon pumices is not a primary feature; it reflects reaction with sulfur enriched material at some unknown depth beneath the volcano. Published sulfur isotopic and petrologic data suggest that hydrothermally altered rocks similar to the pyrite- and anydritebearing lithic fragments found in the 1982 pumices could have provided a source of sulfur for crystallization of magmatic anhydrite. The anhydrite was an important source of sulfur for evolution of a sulfur-rich vapor phase during eruption of the magma.Although many calc-alkaline dacites and rhyolites appear to attain oxidation states high enough to stabilize anhydrite, the characteristically low bulk sulfur contents of these rocks will limit anhydrite abundances to less than approximately 0.1 wt. per cent, assuming sufficient sulfur is present to achieve saturation. Such small amounts of a water soluble mineral could be easily removed by subaerial weathering processes, dissolved during vapor exsolution from a magma, or simply overlooked during routine petrographic examination.
The abundances of siderophile elements in the Earth's silicate mantle are too high for the mantle to have been in equilibrium with iron in the core if equilibrium occurred at low pressures and temperatures. It has been proposed that this problem may be solved if equilibrium occurred at high pressures and temperatures. Experimental determination of the distribution of siderophile elements between liquid metal and liquid silicate at 100 kilobar and 2000 degrees C demonstrates that it is unlikely that siderophile element abundances were established by simple metal-silicate equilibrium, which indicates that the segregation of the core from the mantle was a complex process.
A large number of ordinary chondrites contain micron-sized particles of metal and/or troilite dispersed in their silicate grains. Such metallic phases are responsible for the so-called darkening of the silicate grains and might be either precipitates, which formed during reduction of the silicates, or inclusions injected as a melt during a shock event. We have investigated these tiny foreign phases by analytical transmission electron microscopy in three unweathered, metamorphosed ordinary chondrites (Saint Siverin, LL6, Tsarev, L6 and Kernouvi, H6). We also looked for remnant shock indices. Our TEM observations suggest the following sequence of events in the three meteorites. First, a number of relatively strong shock events occurred on the parent body/bodies producing an Fe-FeS melt which was injected into silicate grains along a dense network of open fractures. Most of these shock defects were subsequently erased by high-temperature (700-900 C) thermal metamorphism. Some remnants of the shock events are the observed trails of tiny metal and/or sulfide inclusions which formed as a result of fracture healing. Chemical homogenization of the silicates and limited oxidation of the metallic blebs also occurred during this high temperature annealing event, resulting in Ni-rich inclusions. This effect was especially pronounced in the L and LL-chondrites studied. During subsequent cooling of the body/bodies, inclusions of chromite and phosphate precipitated, nucleating preferentially on lattice defects (dislocations, sub-grain boundaries) and on the metal and sulfide inclusions. A later shock event of moderate intensity, probably corresponding to the separation of the meteorite from its parent body, produced new shock features in the silicate grains of the Saint Siverin meteorite including mechanical twins in diopside and straight free screw dislocations in olivine.
Smooth grains in the Peace River shocked matrix previously described as maskelynite are not diaplectic glass but a crystalline phase with a stoichiometric composition. They formed upon decompression by inversion of a parental high-pressure polymorph that crystallized from a dense K-rich melt. They are surrounded by radiating cracks that have extensively shattered the neighboring minerals due to volume increase induced by decompression. Similar grains in Tenham turned out to be glass-quenched from a dense alkali-rich melt compositionally unrelated to plagioclase. Expansion of the alkali-bearing aluminosilicate in Peace River and the quenched dense glass in Tenham triggered the second high-pressure event. Neither Peace River nor Tenham contain any maskelynite.
A new absorption correction applicable to EDX spectra of ionic compounds (oxides, ceramics, minerals, …) is presented. The method enables the accurate quantification of oxygen and nitrogen, and does not require parameters such as specimen thickness, X-ray take-off angle and specimen density. Physically this new correction procedure is based on the sole principle of electroneutrality of the specimen: this means that the sum of all anions and cations times their respective valence states must cancel out. An additional benefit of this method is the possibility to calculate the average thickness of the analysed area if, after the quantitative analysis, numbers for the specimen density and X-ray take-off angle are available.
Shock-induced melt veins in the Acfer 040 L5-6 (S6) chondrite contain a previously unknown set of high pressure phases consisting
of amorphous grains similar in composition to majorite, MgSiO3-ilmenite, and ringwoodite. The amorphous grains have compositions that are similar to those of synthetic MgSiO3-perovskites from chemically complex systems and are inferred to be MgSiO3-perovskite that crystallized from the melt at high pressure and temperature and subsequently amorphized after pressure release.
The ilmenite represents a natural occurrence of a potentially important mineral in Earth's mantle. The MgSiO3-perovskite–MgSiO3-ilmenite–ringwoodite assemblage is not predicted by phase equilibria studies, but appears to result from crystallization
of a melt at pressures above 26 gigapascals.
Shock-induced melt veins in the Acfer 040 L5-6 (S6) chondrite contain a previously unknown set of high pressure phases consisting
of amorphous grains similar in composition to majorite, MgSiO3-ilmenite, and ringwoodite. The amorphous grains have compositions that are similar to those of synthetic MgSiO3-perovskites from chemically complex systems and are inferred to be MgSiO3-perovskite that crystallized from the melt at high pressure and temperature and subsequently amorphized after pressure release.
The ilmenite represents a natural occurrence of a potentially important mineral in Earth's mantle. The MgSiO3-perovskite–MgSiO3-ilmenite–ringwoodite assemblage is not predicted by phase equilibria studies, but appears to result from crystallization
of a melt at pressures above 26 gigapascals.
Shock veins in the Sixiangkou (L6) chondrite contain two high-pressure assemblages: (i) majorite-pyrope solid solution plus
magnesiowüstite that crystallized at high pressures and temperatures from a shock-induced silicate melt of bulk Sixiangkou
composition and (ii) ringwoodite plus low-calcium majorite that were produced by solid-state transformation of olivine and
low-calcium pyroxene. The morphology and chemistry of the majorite-pyrope garnet and the size of the magnesiowüstite crystals
indicate a longer duration at high pressure and temperature than predicted by impact scenarios. This pressure-temperature
regime is constrained by the olivine-ringwoodite and orthopyroxene-majorite phase transformations, fusion of the meteorite
constituents, and crystallization of majorite-pyrope solid solution plus magnesiowüstite from that melt under high pressure.
Abstract— Shock defects in the most common silicate minerals of chondrites (olivine, pyroxenes and feldspars) have been investigated in detail, but there have been almost no studies of the shock defects in other components, like metal and sulfide. This probably stems from the fact that these latter phases are opaque in the optical microscope. The same reason explains why veins and melt pockets, which are constituted of microcrystalline or glassy phases (i.e., isotropic) are also poorly documented. We have investigated such phases by analytical transmission electron microscopy (ATEM) in two shocked chondrites, Tenham (L6) and Gaines County (H5).
We have characterized shock defects in troilite very similar to those occurring in silicates (i.e., a mosaic texture and sets of straight and very narrow, ⋍10 nm, lamellae of amorphized FeS).
There are many small regions in shocked chondrites that are composed of very fine grained (⋍1 μm) mixtures of metal and sulfide or of various silicates. They must result from local melting followed by a rapid cooling that prevented grain growth. We have determined the chemical compositions and the volume proportions of the tiny grains in these veins and melt pockets, which has allowed their temperature and pressure (T, P) history to be partially deciphered.
Finally, we have observed a dense network of very narrow fractures (down to 10 nm) in the olivine and enstatite grains. These fractures are systematically filled with an amorphous (or cryptocrystalline) material that stems from the melt pockets and was injected when the fractures were opened by the rarefaction wave. This material was then quenched at the contact with the colder crystalline rims.
A compilation of the chemical analysis of 241 stony and 36 iron
meteorites is presented; 196 analyses were published previously, 81 are
new. This compilation includes analyses of new falls, new finds,
previously analyzed meteorites, previously analyzed meteorites with
suspect values, analyses of separates and inclusions, and analyses of 53
stony and 29 iron meteorites from Antarctica, including one of the
'lunar' type. Mean compositions of chondrite falls, finds, and Antarctic
chondrites are compared. References are listed for earlier published
analyses and an appendix provides an outline of the sampling procedures,
sample preparation, and the analytical methods.
Abstract— A large number of ordinary chondrites contains micron-sized particles of metal and/or troilite dispersed in their silicate grains. Such metallic phases are responsible for the so-called darkening of the silicate grains and might be either precipitates, which formed during reduction of the silicates, or inclusions injected as a melt during a shock event. We have investigated these tiny foreign phases by analytical transmission electron microscopy in three unweathered, metamorphosed ordinary chondrites (Saint Séverin, LL6, Tsarev, L6 and Kernouvé, H6). We also looked for remnant shock indices.
Our TEM observations suggest the following sequence of events in the three meteorites. First, a number of relatively strong shock events occurred on the parent body/bodies producing an Fe-FeS melt that was injected into silicate grains along a dense network of open fractures. Most of these shock defects were subsequently erased by high-temperature (700–900 °C) thermal metamorphism. Some remnants of the shock events are the observed trails of tiny metal and/or sulfide inclusions that formed as a result of fracture healing. Chemical homogenization of the silicates and limited oxidation of the metallic blebs also occurred during this high-temperature annealing event, resulting in Ni-rich inclusions. This effect was especially pronounced in the L and LL-chondrites studied. During subsequent cooling of the body/bodies, inclusions of chromite and phosphate precipitated, nucleating preferentially on lattice defects (dislocations, subgrain boundaries) and on the metal and sulfide inclusions. A later shock event of moderate intensity, probably corresponding to the separation of the meteorite from its parent body, produced new shock features in the silicate grains of the Saint Séverin meteorite, including mechanical twins in diopside and straight free screw dislocations in olivine.
Partly shocked-melted Yanzhuang H-chondrite, now classified as
petrological type H6 but before the shock event of type H4, was
subjected to the shock-heating 2.6 Ma ago at the time it was spalled off
its parent body and came into being as a meteoroid of ca. 30 cm radius.
At that time the unmelted portion of the meteoroid suffered an almost
complete loss of its radiogenic He-4 and Ar-40, while the contents of
the most volatile nonnoble gas elements Zn and Se were not measurably
affected. The melted portion of Yanzhuang is also essentially void of
radiogenic He-4(10 +/- 4 x 10 exp -8 cu cm STP/g) but it has retained
some 80 percent of its radiogenic Ar-40, presumably because in the melt,
the increase of the diffusion length more than compensated the increase
of the diffusion constant.
THE apparent excess of siderophile (iron-Ioving) elements in the Earth's mantle has been a long-standing enigma in the geochemistry of mantle–core differentiation1,2. Although current models have proved successful in explaining some aspects of this problem3–7, important questions remain. In particular, the mantle's near-chondritic ratio of nickel to cobalt (close to that expected for the material from which the Earth formed) is hard to explain, given the markedly different ambient-pressure partitioning behaviour of these elements between iron-alloy and silicate melts3–8. Here we report experimental results which show that both elements become less siderophile with pressure, but the effect is much more pronounced for Ni, so that the partition coefficients of the two elements become essentially equivalent at an extrapolated pressure of ~28 GPa. The absolute and relative abundances of Ni and Co in the mantle are therefore consistent with alloy–silicate chemical equilibrium at high pressure, indicating that core formation may have taken place in a magma ocean with a depth of 750–1,100 km. We also find that, unlike Ni and Co, sulphur becomes more siderophile with pressure. Sulphur's increased affinity for iron with depth could make it the dominant light element in the Earth's core.
WE have observed numerous rounded purple isotropic grains up to 100 microns in diameter in thin sections of two stones from the Tenham meteorite shower1 (British Museum B.M.1935,792 and Australian Museum DR 8298). The grains occur chiefly within black veins cutting across the stones, but the same material also replaces olivine within 10–20 microns of certain thicker veins and at the margins of some large chondritic fragments within the veins. Our investigations show that the purple mineral is the high pressure spinel polymorph of olivine, and for this first authentic natural occurrence we propose the name ringwoodite, in honour of the experimental studies2 by Professor A. E. Ringwood, Australian National University. The name covers the entire range of (Mg,Fe)2SiO4 spinels, and has been approved by the Commission on New Minerals and Mineral Names, International Mineral-ogical Association.
A quasi-subregular solution model is used to describe the thermodynamic properties of the liquid phase; values of the solution
parameters are obtained from extensive and consistent thermochemical data reported in the literature. For the fcc and bcc
phases, the same model is used to account for the nonmagnetic part of the Gibbs energy and the magnetic contribution is taken
from the previous paper. Again, the values for the quasi-subregular solution parameters for the fcc phase are obtained from
extensive and consistent thermochemical data reported in the literature at high temperatures. The values of the solution parameters
for the bcc phase are obtained from the thermodynamic values of the liquid and fcc phases and the known phase boundary data.
The calculated phase equilibria are in good agreement with the available data. Based on the thermodynamic data, the metastablel + γ andl + δ phase boundaries as well as theT
0
(γ + l) andT
0(δ +l) curves are calculated.
An electromagnetic levitation facility is used to containerlessly process Fe-Ni droplets; undercoolings ΔT of up to 300 K were achieved. Thermal measurements during solidification showed two types of recalescence behaviour for alloys containing between 7.5 and 17 at% Ni: A single recalescence step for ΔT< ΔT* (primary growth of the stable ccp phase) and two recalescence steps for Δ> ΔT* (primary growth of the metastable bcc phase, and the subsequent transformation of the bcc phase and solidification of the stable ccp phase). The critical undercooling ΔT* strongly increases with the Ni-content. The growth velocities at which the primary dendrites propagate through the droplet have been measured for a number of Fe-Ni alloy compositions. The velocities reflect the phase selection, i.e. primary bcc phases grow markedly more slowly than primary ccp phases. Thermodynamic modelling (CALPHAD) and an analysis of the velocity data within current theories of dendrite growth is undertaken to describe nucleation and growth behaviour. The results suggest that the metastable phase is nucleated in preference at high undercoolings because of its lower solid-liquid interface energy and that the kinetics at the bcc-liquid interface is considerably more sluggish than the kinetics at the ccp-liquid interface.
The solubilities of several oxide components of the mantle in molten iron have been measured at 16 GPa and 1700–2000°C. Their relative solubilities are as follows: FeO > Cr2O3 > MnO, V2O3, TiO2 > SiO2 > MgO > Al2O3. Oxide contents reach significant levels, varying from ∼ 10 wt% FeO to ∼ 1 wt% SiO2 at the respective metal-oxide eutectics, but are very small for MgO and Al2O3. The solubilities of these oxides are expected to increase substantially at temperatures above 2000°C and at pressures above 16 GPa and are relevant to the core-formation process in the Earth. Individual oxide species dissolve quasi-congruently under conditions which maintain oxygen fugacities near the iron-wüstite buffer. However, the dissolution of mantle mineral phases is highly incongruent. If a mixture of metallic iron and mantle silicates were subjected to increasing pressures and temperatures (above 16 GPa and 2000°C) the most soluble species, FeO, would first be extracted into the metallic liquid. When the FeO activity had been lowered sufficiently, significant amounts of SiO2 would then enter this melt. At extremely high temperatures and pressures, appreciable amounts of MgO could even dissolve. These results provide a background for interpreting recent diamond-anvil experiments in which molten iron was observed to react with mantle silicates at temperatures of 2700–3700°C and pressures of 20–70 GPa. They also elucidate the nature of chemical reactions between the core and mantle which may occur in the “D” layer of the lower mantle.
Four pallasites (Brenham, Brahin, Esquel, and Omolon) were investigated by analytical transmission electron microscopy. All show very similar compositions and defect microstructures. The olivine grains contain a low density of c dislocations organized in tilt subgrain boundaries, indicating that the meteorites were annealed for a long time at high temperature. They also contain straight fractures parallel to ‘{1k0’} planes (with k = 0, 2, and 3) filled with a mixture and sulfide, and alignments of tiny inclusions of metal + troilite. Both types of defects are shock indicators, the former ones are common in shocked olivine and the later ones must result from earlier shock events and healed during a post-shock annealing episode. The Widmanstätten patterns in the metal are similar to those observed in iron meteorites. By using the calibration charts published by the Goldstein group the cooling rates of the pallasites were deduced from the Ni concentration profiles across taenite bands. These cooling rates are quite low (≤ 5 K/MY). Kamacite represents approximately 90% of the metal and is constituted of large grains with low densities of free dislocations (≤ 1013 m−2) exhibiting a marked screw character. One also detects some twin lamellae and a number of phosphide precipitates with compositions M3P and M2P as well (M = Fe and Ni). The twins probably result from a shock with moderate intensity. The M2P phosphides have never been characterized before. They probably precipitated at low temperature when their Gibbs energy became lower than the one of schreibersite (Fe, Ni)3P. The shock indices in both olivine and kamacite correspond to a moderate shock intensity. As their parent body (or bodies) suffered at least one strong shock which fragmented it, the source region of pallasites must lie at great depth within their parent bodies.
Sulfur is a potential light element in the liquid outer core of the Earth. Its presence in segregating metal may have had an influence in distribution of metal-loving (siderophile) elements during early accretion and core formation events in the Earth. The observed “excess” abundance of siderophile elements in the terrestrial mantle, relative to an abundance expected from simple core-mantle equilibrium at low temperature and pressure, may indicate a reduction in the iron-loving tendency of siderophile elements in the presence of sulfur in the metallic phase. The present experimental partitioning study between iron-carbon-sulfur-siderophile element bearing liquid metal and liquid silicate shows that for some siderophile elements this sulfur effect may be significant enough to even change their character to lithophile. Large and intricate variations in metal-silicate partition coefficients () have been observed for many elements, e.g., Ni, Co, Ge, W, P, Au, and Re as a function of sulfur content. Moderately siderophile elements Ge, P, and W show the most significant response (sulfur-avoidance) by an enhanced segregation into the associated sulfur-deficient phases. Highly siderophile elements Ir, Pt, and Re show a different style of sulfur-avoidance (alloy-preference) by segregating as sulfur-poor, siderophile element-rich alloys. Both groups are chalcophobic. for Ni, Co, and Au moderately decreases with increasing sulfur-content in the liquid metal. for chalcophile element, Cr, in contrast, increases with sulfur. Irrespective of the sulfur-content, in the presence of a carbon-saturated liquid metal, P is always lithophile. The general nonmetal-avoidance tendency of siderophile elements (and acceptance of chalcophile elements) in the liquid metal, postulated by Jones and Malvin (1990) in the FeNiS(sulfur)M (siderophile) system is found to be present in the metal-silicate system as well. A sulfur-bearning liquid metal segregation can potentially reduce the metal-loving nature of many elements to explain the excess paradox. Sulfur-bearing core segregation, however, might require an efficient draining of exsolved immiscible sulfide liquids from the molten silicate, or an increasing siderophility of sulfur at high pressure to reduce the mantle sulfur content to the observed (<300 ppm) value. Moreover, the chondritic relative abundance pattern of many moderately or highly siderophile elements in the upper mantle is not explained by the presence of sulfur in the segregating metals. Core formation is more complex and intricate than equilibrium segregation.
Highly and intermediately shocked carbonate-rich fragments of the allochtonous polymict breccia from the Haughton impact crater (Canada) were studied by Scanning Electron Microscopy (SEM), Analytical Transmission Electron Microscopy (ATEM) and analyses of carbon and oxygen stable isotopes (δ¹³C and δ¹⁸O). In areas subjected to severe shock conditions, carbonates represent only about 10 vol% of the shocked samples and they are located in holes and fractures within a matrix of SiO2-rich glass. Shock features are absent in these crystals. High-temperature reactions have occurred between molten silicates and carbonates, producing CaMg-rich glasses, or crystalline phases such as augite and larnite (Ca2SiO4). The carbonates are dominated by calcite and they generally have significantly positive δ¹³C, ranging up to +9‰, with a weighted average value of +1.75‰. Their δ¹⁸O values range between +15‰ and +20‰ and they are about 5‰ lower than in unshocked reference sediments, a trend consistent with that resulting from silicate-carbonate reactions. The microstructures of the carbonates suggest that they did not undergo shock conditions but, instead, were produced by back-reactions between impact-released CO2 and highly reactive residual oxides. Such a process would introduce isotope fractionations, which might explain the positive δ¹³C values observed.
During the early episode Of the solar system, the L6 chondrite Tenham has been affected by intense thermal metamorphism. Microanalytical data reveal homogeneous compositions Of Olivine (FO75Fa25), enstatite (En79Fs19WO2), and diopside (En47Fs8WO45). Using these data, empirical pyroxene thermometers yield temperature estimates for this thermal metamorphism, ranging from 810 to 870°C. Due to the presence of thin shock veins, which contain the high-pressure phases majorite and ringwoodite, the L6 chondrite Tenham is an instructive example for strong shock metamorphism. In contrast to previous transmission electron microscopy (TEM) studies, which concentrated on these shock veins, we also systematically characterized the shock signature of the silicates occurring in the bulk Of Tenham.
Microprobe analyses of 33 melt pocket glasses in five L6d and L6e chondrites show them to be chemically varied but typically enriched in the constituents of plagioclase relative to the host meteorites. This enrichment appears to increase with the degree of melting (0–6.5 vol.%), but other chemical variations among the glasses (sodium depletion, reduction of ferrous iron) appear to be unrelated to shock intensity and melt abundance.Chemical trends for melt pocket glasses differ sharply from those reported for chondrules in ordinary chondrites. Thus partial shock melting of chondritic material is an inadequate explanation for the chemical properties of chondrules.
Inclusions of troilite and metallic Fe,Ni 0.2–4 mm in size with a dendritic or cellular texture were observed in 12 ordinary chondrites. Cooling rates in the interval 1400−950°C calculated from the spacing of secondary dendrite arms or cell widths and published experimental data range from 10−7 to 104°C/sec. In 8 of these chondrites, which are breccias containing some normal slow-cooled metal grains, the inclusions solidified before they were incorporated into the breccias. Their cooling rates of 1–300 °C/sec indicate cooling by radiation, or by conduction in contact with cold silicate or hot silicate volumes only 6–40 mm in size. This is quantitative evidence that these inclusions and their associated clasts were melted on the surface of a parent body (by impact), and were not formed at depth from an internally derived melt. In Ramsdorf, Rose City and Shaw, which show extensive reheating to ⩾ 1000°C, Fe-FeS textures in melted areas are coarser and indicate cooling rates of 10−1 to 10−4°C/sec during solidification. This metal may have solidified inside hot silicate volumes that were 10–300 cm in size. As Shaw and Rose City are breccias of unmelted and melted material, their melted metal did not necessarily cool through 1000°C within a few m of the surface. Shock-melted, fine-grained, irregular intergrowths of metal and troilite formed in situ in many irons and some chondrites by rapid solidification at cooling rates of ⩾ 105°C/sec. Their kamacite and taenite compositions may result from annealing at ~250°C of metallic glass or exceedingly fine-grained quench products.
The mineralogical nature of the purple shocked olivine phase known as ringwoodite in the Tenham chondritic meteorite is investigated. Electron microprobe analysis indicates a composition identical to that of the relatively unshocked olivine parts of the meteorite (Mg,Fe)2SiO4. Transmission electron microscopy and X-ray diffraction reveal that the purple ringwoodite consists of a polycrystalline spinel aggregate partially inverted to the beta 'modified' spinel phase by the rapid quenching of gamma spinel. Two different mechanisms for the transformation of olivine to the original gamma spinel, namely an independent nucleation and growth mechanism and a cooperative shear mechanism, are indicated by spinel morphologies, while the inversion of the gamma to the beta spinel on quenching is shown to involve only a change in cation distribution.
In order to identify the possible hierarchy of microstructural pathways and transitions, a systematic evaluation of the microstructural evolution in undercooled Fe-Ni alloys was performed on uniformly processed samples covering seven orders of magnitude in volume. At appropriate undercooling levels, alternate solidification pathways become thermodynamically possible and metastable product structures can result from the operation of competitive solidification kinetics. For thermal history evaluation, a heat flow analysis was applied and tested with large Fe-Ni alloy particles (1 to 3 mm) to assess undercooling potential. Alloy powders (10 to 150 microns), with large liquid undercoolings, were studied under the same composition and processing conditions to evaluate the solidification kinetics and microstructural evolution, including fcc/bcc phase selection and the thermal stability of a retained metastable bcc phase. The identification of microstructural transitions with controlled variations in sample size and composition during containerless solidification processing was used to develop a microstructure map which delineates regimes of structural evolutions and provides a unified analysis of experimental observations in the Fe-Ni system.
The meteorite Tenham was observed by transmission electron microscopy and ringwoodite and majorite, the high-pressure polymorphs of olivine and pyroxene, were identified. Ringwoodite contains antiphase boundaries and straight dislocations that are probably dissociated. Mantle flow of spinel might proceed by pure climb, and whole-mantle convection may be possible if the grain size is small enough.
Uquid metal-liquid silicate partition coefficients for several elements at 100 kilobars and temperatures up to about 3000 kelvin in carbon capsules experimentally converge on unity with increasing temperature. The sense of change of the partition coefficients with temperature resembles the extrapolation of Murthy and may partially contribute to, but by no means provide a complete resolution of, the "excess" siderophile problem in the Earth's mantle. Sulfur and perhaps carbon successfully compete with oxygen for sites in the metallic liquid at these temperatures and pressures. This observation casts doubt upon the hypothesis that oxygen is the light element in the Earth's core.
Geochemistry of mantle-core differentiation at high pressure
- Li