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![Composition of accumulated fractional melts as a function of melt fraction F. (a) CaO, (b) Al 2 O 3 , (c) MgO, (d) FeO T and (e) SiO 2 , for polybaric fractional melting of peridotite KLB-1 (blue) and silica-deficient pyroxenite KG1(8) (green) and T P ¼ 1315 and 1500 C. Each point represents an F ¼ 0Á01 calculation step. Points show the integrated cumulative melt compositions of all melt fractions up to the one given. Lines show the instantaneous melt compositions. A 1D melt column is assumed; that is, the composition of an F ¼ 0Á03 melt is the average of the F ¼ 0Á01, 0Á02 and 0Á03 incremental melt compositions. Lines show the Paran a– Etendeka CFB province picrite (continuous line) and ferropicrite (dashed line) suggested primary melt compositions for comparison , calculated assuming Fe 3þ /Fe T ¼ 0Á1. Plot (c) also shows the Fo content [100(Mg/(Mg þ Fe 2þ )), mol] of olivine in the solid mantle residue at melting intervals of 0.1, where the colour indicates the corresponding bulk composition.](profile/Eleanor-Jennings/publication/311770082/figure/fig9/AS:446864492371975@1483552205169/Composition-of-accumulated-fractional-melts-as-a-function-of-melt-fraction-F-a-CaO.png)
Composition of accumulated fractional melts as a function of melt fraction F. (a) CaO, (b) Al 2 O 3 , (c) MgO, (d) FeO T and (e) SiO 2 , for polybaric fractional melting of peridotite KLB-1 (blue) and silica-deficient pyroxenite KG1(8) (green) and T P ¼ 1315 and 1500 C. Each point represents an F ¼ 0Á01 calculation step. Points show the integrated cumulative melt compositions of all melt fractions up to the one given. Lines show the instantaneous melt compositions. A 1D melt column is assumed; that is, the composition of an F ¼ 0Á03 melt is the average of the F ¼ 0Á01, 0Á02 and 0Á03 incremental melt compositions. Lines show the Paran a– Etendeka CFB province picrite (continuous line) and ferropicrite (dashed line) suggested primary melt compositions for comparison , calculated assuming Fe 3þ /Fe T ¼ 0Á1. Plot (c) also shows the Fo content [100(Mg/(Mg þ Fe 2þ )), mol] of olivine in the solid mantle residue at melting intervals of 0.1, where the colour indicates the corresponding bulk composition.
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Evidence for chemical and lithological heterogeneity in the Earth’s convecting mantle is widely acknowledged, yet the major element signature imparted on mantle melts by this heterogeneity is still poorly resolved. In this study, a recent thermodynamic melting model is tested on a range of compositions that correspond to potential mantle lithologie...
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... evolution of these incremental and accumulated fractional melts with further decompression, thus increasing cumulative F, is shown in terms of five oxides in Fig. 12. T P is a proxy for average depth of melting, as hotter decompressing mantle will intersect the solidus and begin to melt at higher pressure and temperature. As was also indicated by Fig. 10, Fig. 12 shows that the melt composition is a function of both source compos- ition and depth of melting, making it more difficult to in- terpret ...
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... Fo contents of olivine in the solid mantle residue are also shown in Fig. 12c. This shows Fo to increase with increasing extent of melt extraction. KG1(8) pyrox- enite mantle olivine ranges from 82Á8 to 89Á5 and varies predominantly as a function of melt fraction. Primary pyroxenite-derived melts are therefore not required to be in equilibrium with high-Fo (>90) olivine, provided that the melts maintain Fe-Mg ...
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... at MgO ¼ 15Á1 wt % (also with melt Fe 3þ /RFe ¼ 0Á1), which assumes that there had been no significant prior frac- tionation and that these olivines are in equilibrium with their mantle source. On the solidus of KG1 (8) at T P ¼ 1500 C, olivine is Fo 82Á9 in the present model, which steadily increases during decompression frac- tional melting (Fig. 12). Therefore, Fo 86Á0 or lower does not necessarily imply prior olivine fractionation in pyroxenite-derived melts. Figure 12 shows the suggested primary melt com- position of Etendeka picrites and ferropicrites. FeO T (RFe as FeO), rather than FeO, is shown in Fig. 12, to re- duce the effect of (1) choice of Fe 3þ /RFe in the mantle ...
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... Fo 86Á0 or lower does not necessarily imply prior olivine fractionation in pyroxenite-derived melts. Figure 12 shows the suggested primary melt com- position of Etendeka picrites and ferropicrites. FeO T (RFe as FeO), rather than FeO, is shown in Fig. 12, to re- duce the effect of (1) choice of Fe 3þ /RFe in the mantle source and (2) assignment of Fe 3þ /RFe in the natural samples on the comparability of the two. ...
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... model, which steadily increases during decompression frac- tional melting (Fig. 12). Therefore, Fo 86Á0 or lower does not necessarily imply prior olivine fractionation in pyroxenite-derived melts. Figure 12 shows the suggested primary melt com- position of Etendeka picrites and ferropicrites. FeO T (RFe as FeO), rather than FeO, is shown in Fig. 12, to re- duce the effect of (1) choice of Fe 3þ /RFe in the mantle source and (2) assignment of Fe 3þ /RFe in the natural samples on the comparability of the two. The picrites and ferropicrites do not perfectly coincide with any given source composition, T P , or F, reflecting both the large uncertainty associated with determining a ...
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... a-Etendeka picrite and ferropicrite whole-rock compositions and suggested primary melt compos- itions are also shown in Fig. 13. It is clearer here than in Fig. 12 that (1) ferropicrites are similar to low-to moderate-fraction melts of high-pressure (high-T P ) silica-deficient pyroxenite KG1 and (2) picrites are simi- lar to partial melts of KLB-1 peridotite. The dominating effect of olivine loss or gain on the whole-rock compos- itions is clear in Fig. 13a and c. In every plot, the sense of the ...
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... effect of olivine loss or gain on the whole-rock compos- itions is clear in Fig. 13a and c. In every plot, the sense of the change in chemistry between picrites and Fig. 13. Composition of accumulated fractional melts compared with whole-rock data. Points are accumulated (integrated) melt compositions at F ¼ 0Á01 intervals (see caption to Fig. 12 for details). Small arrows indicate the location of the F ¼ 0Á01 melt (i.e. the onset of melting). Small dots represent unfiltered whole-rock samples from the Etendeka province [normalized to 100% in the eight-component NCFMASCrO system; data from Gibson et al. (2000) and Thompson et al. (2001)], enclosed by coloured fields to highlight the general ...
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We constrained the solidus of a model Martian composition with low bulk Mg# (molar MgO/(MgO + FeOT) × 100 ~75) and high total alkali (Na2O + K2O = 1.09 wt.%) concentration at 2 to 5 GPa by experiments. Based on the new solidus brackets, we provide a new parameterization of the solidus temperature as a function of pressure of Martian mantle: Ts(°C)...
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... Ball et al. 2021], compositional variability [e.g. Brown and Lesher 2014;Gleeson et al. 2021], and primary melt compositions [e.g. Jennings et al. 2016]. These melting models calculate mantle melting behaviour either by minimising thermodynamic potentials at each calculation step [e.g. ...
Modelling the melting of Earth’s mantle is crucial for understanding the distribution of volcanic activity on Earth and for testing models of mantle convection and mantle lithological heterogeneity. pyMelt is a new open-source Python library for calculating the melting behaviour of multi-lithology mantle and can be used to predict a number of geophysical and petrological observations, including melt productivity, spreading centre crustal thickness, lava trace element concentrations, and olivine crystallisation temperatures. The library is designed to be easily extensible, allowing melting models to be added, different methods for calculating lava chemistry to be applied, and new melting dynamics and properties to be incorporated.
... The crystal settling of the earlier minerals leads to accumulation and formation of massive layer of ilmenite intercalated within gabbroic rocks (Fig. 9). In addition, Fe-Ti oxides enrichment in Abu Ghalaga layered intrusions may be related to Ti enrichment in the slab-derived fluids leading to heterogeneity of ANS mantle (Jennings et al. 2016). The formation of Fe-Ti oxides requires high oxygen fugacity (ƒO2) and water contents, which is applicable with subduction signature (Howarth and Prevec 2013). ...
Neoproterozoic Abu Ghalaga ilmenite-bearing mafic rocks were subjected to geological, petrographical and geochemical studies integrated with mineral chemistry of ilmenite, pyrite and magnetite in order to deduce their petrogenesis. Abu Ghalaga mafic intrusions hosting the largest ilmenite reserve in the Egyptian part of the Arabian-Nubian Shield. Field observations revealed that the intrusions impound ilmenite ore, which occurs as interlayer of massive bands or disseminated within gabbroic rocks. Petrographically, Abu Ghalaga mafic intrusions include different kinds of gabbro; olivine pyroxene gabbro, pyroxene gabbro, pyroxene hornblende gabbro, altered gabbro and Fe-Ti-rich gabbro. They have calc-alkaline to tholeiitic magmas, which are depleted in high field strength elements (e.g., Ta, Nb and Th) relative to low field strength elements (e.g., Ba, Sr and Rb), and exhibit light rare earth elements enrichment relative to heavy rare earth elements with positive Eu anomaly (Eu/Eu*= 0.8-2.4). Variable mineralogical and wide variation of bulk rock geochemistry are ascribed to fractional crystallization and hydrothermal overprinting. Abu Ghalaga gabbroic rocks are derived from fractional crystallization of depleted mantle magmas that were modified by ascending fluids from subducted slab in volcanic arc setting. Fe-Ti oxides required high oxygen fugacity (ƒO2) and water contents by crystal settling of Ti-rich mantle which is applicable with subduction setting. It can be inferred that Abu Ghalaga mafic rocks are neither related to ophiolite nor to Alaskan type but to one of the layered intrusions hosting Fe-Ti oxides.
... Based on the melting behavior of fertile peridotite and pyroxenite, magmas generated from some types of pyroxenite should generally have lower CaO contents at a given MgO content as compared with peridotite-derived melts (Herzberg and Asimow, 2008). However, fractionation of CaO relative to MgO is sensitive to the bulk mineral/ melt distribution coefficient for CaO (D CaO ) in the magma, and pyroxenites can have D CaO > 1 or D CaO < 1, irrespective of the initial concentration of CaO in the pyroxenite (e.g., Herzberg, 2011;Jennings et al., 2016;Jennings and Holland, 2015). The whole-rock composition of the mantle source is also important, and melting of CaO-poor harzburgite can also produce low-CaO melts because of the absence of clinopyroxene in the source (Hole and Natland, 2020). ...
Olivine in ocean island basalts can provide unique insights into lithological heterogeneity (i.e., peridotite versus pyroxenite) in the mantle. Here we present high-precision major and trace element data for olivine crystals in basalts from Hainan Island, China, and use these data to investigate their mantle source. Many of the olivines are in disequilibrium with their whole-rock compositions (i.e., Mg# values), and the basalts contain both normally and reversely zoned olivine crystals, which are indicative of magma mixing. The decoupled Fo–NiO profiles and concave-down trends defined by data in a Fo–NiO diagram suggest that diffusive re-equilibration controlled the olivine chemistry. The olivine compositions were significantly affected by crustal magmatic processes, such as fractional crystallization, magma mixing, and diffusive re-equilibration. However, the lack of clear correlations between Mn/Fe, Mn/Zn, Zn/Fe, and Ga/Sc ratios and olivine Fo contents suggests these trace element ratios were not significantly affected by crustal processes, and thus preserve information about the mantle source. The Mn/Zn, 100 × Mn/Fe and 10,000 × Zn/Fe ratios of the examined olivines indicated that the Hainan basalts had a mixed pyroxenite-peridotite mantle source. This is consistent with the whole-rock geochemical data and previously reported isotope geochemistry data for the Hainan Island basalts. However, crustal magmatic processes also affected the compositions of the primitive olivines that crystallized in the mantle-derived magma. Therefore, careful assessment of post-melting processes using olivine textures and minor element chemistry needs to be undertaken before discriminating between pyroxenitic and peridotitic source components based on olivine chemistry.
... MIX1G is a silica-deficient pyroxenite which plots close to the average global pyroxenite composition and can be considered as a mixture between KLB1 and MORB (Lambart et al., 2016), i.e., recycled crust mixed with ambient mantle. The ferric Fe content of MIX1G is taken as between that of KLB1 peridotite and MORB, following the approach used for KG1 by Jennings et al. (2016), with Fe 3+ /Fe T = 0.1. Further detail on the choice of pyroxenite lithologies is given throughout the thesis. ...
... Many tools are available to study the contributions of temperature and lithological heterogeneity in these two types of basalts, for example: rare earth element inversion modelling (INVMEL: O'Nions, 1991, 1995), major element calculations (PRIMELT: Asimow, 2008, 2015; the thermodynamic model of Jennings and Holland, 2015;Jennings et al., 2016), trace element and radiogenic isotope composition forward modelling (REEBOX PRO: Brown and Lesher, 2016), and models combining trace elements, crystallisation temperatures and magma productivity (Shorttle et al., 2014;Matthews et al., 2016Matthews et al., , 2021). However, achieving success in linking the elemental and radiogenic isotope variability in MORB and OIB to temperature and/or lithological heterogeneity is complicated by uncertainty in the nature of enriched lithologies, metasomatism by small volumes of melt (which are usually highly enriched in incompatible elements, so can overwhelm evidence of the original source lithology), magma recharge and mixing, diffusional re-equilibration and fractional crystallisation (e.g., Niu and O'Hara, 2003;Workman et al., 2004;Niu and O'Hara, 2008;Lambart et al., 2013;Matzen et al., 2017;Gleeson and Gibson, 2019). ...
... The MIX1G 1400* inputs are actually for an isentrope of T p = 1436 • C. The P-T given are the closest P-T pair in the model output files to the average P-T of melting along each isentrope, as calculated in section 4.10. For V and Cr, where element partitioning is not calculated by THERMOCALC, the melt element concentrations are calculated as described in section 4.2.2, using the following bulk compositions for each lithology: KLB1 Cr 2 O 3 = 0.32 wt %, G2 Cr 2 O 3 = 0.08 wt %, MIX1G Cr 2 O 3 = 0.11 wt % (Jennings et al., 2016); KLB1 V = 85 ppm (typical primitive mantle value from Lee et al., 2003;Prytulak et al., 2013), MIX1G and G2 V = 350 ppm (typical MORB value; Prytulak et al., 2013 this detection limit only improves to ∼ 20 % of pyroxenite in the source. The small variability in melt isotope ratios with increasing pyroxenite fraction compared to resolvable variability is consistent with DMM, EM1 and HIMU basalts recording no resolvable δ 44 Ca variability (Valdes et al., 2014), and measured mantle pyroxenites and associated peridotites recording indistinguishable δ 44 Ca (Dai et al., 2020). ...
The geochemistry of global mantle melts suggests that both mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) sample lithological heterogeneities originating in both the upper and lower mantle, with recycled crustal material accounting for a significant part of this variability. Recently, heavy stable isotopes have been suggested as a new tool to complement existing tracers of mantle heterogeneity and lithology (e.g., major and trace elements, radiogenic isotopes), because mineral- and redox-specific equilibrium stable isotope fractionation effects can link the stable isotope ratios of melts to their source mineralogy and melting degree. In this thesis, I present a unique `bottom-up' approach to understanding how mantle lithology, such as recycled crust (pyroxenite), could be reflected in the stable isotope composition of the erupted melts, and the insights that heavy stable isotope data from basalts could provide into mantle source and process. Throughout this thesis, I investigate five stable isotope systems (Mg-Ca-Fe-V-Cr) that have shown promise in models or natural samples as tracers of mantle lithology. I develop a quantitative model, combining thermodynamically self-consistent mantle melting and equilibrium isotope fractionation models, to explore the behaviour of the stable isotope ratios of these elements during melting of three mantle lithologies (peridotite, and silica-excess and silica-deficient pyroxenites). I also present new Fe isotope data for Samoan shield and Azores volcanoes, and for a suite of samples from 90 million years of evolution of the Galápagos mantle plume system. These OIB allow me to study the role of recycled mantle components in generating Fe isotope variability in melts, to compare to my mantle melting and isotope fractionation model. I find that single-stage melting of a MORB-like eclogitic pyroxenite cannot generate the high δ⁵⁷Fe seen in some OIB, notably Pitcairn, the Azores and rejuvenated Samoan lavas. Instead, the generation of high δ⁵⁷Fe melts in OIB requires: (1) processes that make subducted eclogite isotopically heavier than its pristine precursor MORB (e.g., hydrothermal alteration, metamorphism, sediment input); (2) lithospheric processing, such as remobilisation of previously frozen small-degree melts, or a contribution from lithospheric material metasomatised by silicate melts; and/or (3) melting conditions that limit the dilution of melts with high δ⁵⁷Fe by ambient lower δ⁵⁷Fe materials. Therefore, it cannot be assumed that a pyroxenite lithology derived from recycled crustal material is the sole producer of high δ⁵⁷Fe melts in OIB, as has sometimes been assumed in the literature. Instead, the observation of high δ⁵⁷Fe OIB melts cannot be ascribed to a unique source or process. This ambiguity reflects the multitude of processes operating from the generation of recycled lithologies through to their mantle melting: from MORB generation, its low temperature alteration, through mantle heterogeneity development and lithospheric processing, to eruption at ocean islands. I also find that, given current analytical precision, the five stable isotope systems examined here are not predicted to be sensitive to mantle potential temperature variations through equilibrium isotope fractionation processes, for the melting of peridotite. By contrast, source lithological heterogeneity is predicted to be detectable in some cases in the stable isotope ratios of erupted basalts, although generally only at proportions of > 10% MORB-like pyroxenite in the mantle source, given current analytical precision. However, even when considering analytical uncertainty on natural sample measurements, the range in stable isotope compositions seen across the global MORB and OIB datasets suggests that kinetic isotope fractionation, or processes modifying the isotopic composition of recycled crustal material such that it is distinct from MORB, may be required to explain all the natural data. Finally, I combine the insight into and modelling of Fe stable isotope behaviour presented throughout the thesis to highlight the potential of heavy stable isotopes to constrain mantle dynamics, in the Galápagos plume system. I show that although the proportion of pyroxenite-derived melt has increased through time as the plume has cooled by 400°C over its lifetime, these results are consistent with a cooling plume containing a small and approximately constant proportion of pyroxenite. This result is consistent with geodynamic models of entrainment of dense material, such as from a lower mantle low velocity superstructure underlying the plume. The small proportion of pyroxenite throughout plume evolution also suggests that geochemical signatures of primordial mantle may be diluted approximately uniformly by recycled components throughout plume evolution and therefore could be identified in early plume localities. From my combined mantle melting and isotope fractionation model, and comparison to natural datasets, I conclude that the five stable isotope systems considered in this thesis have potential to be powerful tracers of the source lithology of erupted basalts, complementary to other geochemical tools. However, continued improvements in analytical precision in conjunction with experimental and theoretical predictions of isotopic fractionation between mantle minerals and melts are required before these heavy stable isotopes can be unambiguously used to understand source heterogeneity in erupted basalts.
... There is no big differences in the estimate of the depletion buoyancy, i.e. 2-5 kg/ m 3 , when computations are made along geotherms of thicker lithospheres or using density without correction from thermal expansion and compressibility (Figs. 7 and S8). KLB-1 (Jennings and Holland, 2015;Takahashi et al., 1993;Jennings et al., 2016). MS (McDonough and Sun, 1995). ...
The choice of crustal and mantle densities in numerical geodynamic models is usually based on convention. The isostatic component of the topography is not calibrated to fit observations resulting in not very well constrained elevations. The density distribution on Earth is not easy to constrain because it involves multiple variables (temperature, pressure, composition, and deformation). We aim in this study to provide a reference case for geodynamic modelling where crustal and mantle densities are calibrated to fit the relative continent/mid-ocean ridge elevation in agreement with observations. We first review observed Earth topography of stable continents and of active mid-ocean ridges and define the characteristic average elevation of these domains. We use self-consistent thermodynamic calculations of dry mantle rocks that include partial melting to calibrate densities of the continental lithospheric mantle and beneath the mid-ocean ridge. The thermodynamic solutions are coupled with a 2-D incompressible plane strain finite element method for viscous-plastic creeping flows to solve for the dynamic evolution during extension from continental rifting to mid-ocean spreading. The combined results from 2-D thermo-mechanical models and 1-D isostatic calculations show that the relative elevation difference between mid-ocean ridges and continents depends on crustal density, mantle composition, and the degree of depletion of the lithospheric mantle. Based on these results we calibrate the reference density that only depends on temperature, which can be used in classic thermo-mechanical models based on the Boussinesq approximation. Finally the model calibration provides a solution that fits (1) the elevation of active mid-ocean ridges far from hotspots (-2750±250 m), (2) the elevation of stable continents far from hotspots (+400±400 m), (3) the average depletion buoyancy of the continental lithospheric mantle (between -20 and -50±15 kg/m3 depending on lithospheric thickness) and (4) the average continental crust density (2835±35 kg/m3 for a 35 km thick crust).
... Chondrite and OIB values are taken from Sun and McDonough (1989). (b) FeO/ MgO versus CaO/MgO plot and comparison of ferrobasalts and ferropicrite cumulates with Etendeka ferropicrite and picrites (Jennings, Holland, Shorttle, Maclennan, & Gibson, 2016), which indicate pyroxenite source fall in OIB field of mantle plume source (Condie, 2005), which also corresponds well with the REE patterns ( Figure 8). ...
An occurrence of Palaeoarchaean ferropicrite cumulates and ferrobasalts is being reported from the Jojohatu area of the Iron Ore Group (IOG) greenstone belt, which is situated in the northern part of the Singhbhum Craton (eastern India). The ferropicrite cumulates contain serpentine and clinopyroxene as major constituents and spinel, magnetite, and ilmenite as accessories. Their bulk rock chemistry exhibits an extremely high amount of FeOT (19.0–20.8 wt%) and MgO (22.0–23.9 wt%) and quite low Al2O3 (4.5–5.2 wt%) and Na2O + K2O (<1.0 wt%), thus making them as one of the most Fe-rich ferropicrite cumulates in the world. They are also characterized by high contents of compatible trace elements (Ni: 986–1,470 ppm; Cr: 990–1,256 ppm) and moderate TiO2 (1.5–1.7 wt%), Nb (15–18 ppm), and Zr (82–90 ppm). These cumulates are possibly formed due to accumulation of olivine from the ferrobasaltic melts. In comparison, ferrobasalts consisting of plagioclase, pyroxene, and opaques are alkaline in nature. Some of the plagioclase and pyroxene grains have shown a signature of metasomatic alteration, which resulted into formation of secondary minerals such as epidote, chlorite, and hornblende. They exhibit relatively low contents of FeOT (14.6–18.4 wt%), MgO (8.4–10.8 wt%), Ni (32–122 ppm), and Cr (41–131 ppm) than the ferropicrite cumulates, while TiO2 (2.9–4.1 wt%), Nb (42–61 ppm), and Zr (216–318 ppm) contents are higher. Incompatible elemental patterns and their ratios, as well as Zr/Y versus Nb/Y plot, indicate their ocean island basalts (OIB) affinity and genesis from a hot mantle plume, having mantle potential temperature of about 1,600°C and pressure of 3.5–5 GPa. Considerably low CaO (3.00–7.16 wt%) and FeO/MgO and CaO/MgO systematics indicate that the primary source of the basaltic melt could be pyroxenite. These volcanic occurrences possibly represent remnants of the OIB formed during the Palaeoarchean in the eastern part of the Indian shield.
... 10.1029/2020GC009157 4 of 32 mantle source of melts creates additional complexity; at any given pressure and temperature, the chemistry of melts in equilibrium with pyroxenite is different from melts in equilibrium with mantle lherzolite (e.g., Hirschmann & Stolper, 1996;Jennings et al., 2016;Lambart et al., 2013). The chemistry of a mixed magma, containing substantial contributions from both lherzolite and pyroxenite, is difficult to use to directly estimate melting temperature and pressure. ...
... Olivine populations in Fo-T crys space can be bounded by two liquid lines of descent (LLD) (Figure 5c) each corresponding to a primary magma of distinct composition (Matthews et al., 2016). Pyroxenite-derived melts generally have a lower Mg# and a higher FeO content than lherzolite-derived melts (e.g., Jennings et al., 2016;Kogiso et al., 2004;Lambart et al., 2009); therefore, they will saturate in olivine of lower Fo at the same temperature, compared to lherzolite-derived melts (Roeder & Emslie, 1970). Since the lherzolite-derived melts are the most likely to have been in equilibrium with Fo ≥91 olivine, the most suitable starting point for extrapolating back to primary crys T is an olivine crystallized on the lherzolite-derived melt LLD. ...
Crystallization temperatures of primitive olivine crystals have been widely used as both a proxy for, or an intermediate step in calculating, mantle temperatures. The olivine‐spinel aluminum‐exchange thermometer has been applied to samples from mid‐ocean ridges and large igneous provinces, yielding considerable variability in olivine crystallization temperatures. We supplement the existing data with new crystallization temperature estimates for Hawaii, between 1282 ± 21 and 1375 ± 19°C. Magmatic temperatures may be linked to mantle temperatures if the thermal changes during melting can be quantified. The magnitude of this temperature change depends on melt fraction, itself controlled by mantle temperature, mantle composition and lithosphere thickness. Both mantle composition and lithosphere thickness vary spatially and temporally, with systematic differences between mid‐ocean ridges, ocean islands and large igneous provinces. For crystallization temperatures to provide robust evidence of mantle temperature variability, the controls of lithosphere thickness and mantle lithology on crystallization temperature must be isolated. We develop a multi‐lithology melting model for predicting crystallization temperatures of magmas in both intra‐plate volcanic provinces and mid‐ocean ridges. We find that the high crystallization temperatures seen at mantle plume localities do require high mantle temperatures. In the absence of further constraints on mantle lithology or melt productivity, we cannot robustly infer variable plume temperatures between ocean‐islands and large igneous provinces from crystallization temperatures alone; for example, the extremely high crystallization temperatures obtained for the Tortugal Phanerozoic komatiite could derive from mantle of comparable temperature to modern‐day Hawaii. This work demonstrates the limit of petrological thermometers when other geodynamic parameters are poorly known.
... the average peridotite ofJennings et al. (2016), with sufficient added H 2 O that H 2 O is always in excess, and O and S contents taken from a typical serpentinized peridotite from New Caledonia(Evans, 2012); (2) a more oxidized serpentinized peridotite, similar to that of (1), but with Fe 3+ increased as high as possible while still preserving typical reactions such as brucite-out and orthopyroxene-in. The Fe 3+ :total Fe ratio for this bulk composition ranges from 0.63-0.65, ...
Previous studies have concluded that dehydration of serpentinites in subduction zones produces oxidizing fluids that are the cause of oxidized arc magmas. Here, observations of natural samples and settings are combined with thermodynamic models to explore some of the factors that complicate interpretation of the observations that form the basis of this conclusion. These factors include: the variability of serpentinite protoliths; the roles of carbon and sulfur in serpentinite evolution; variability in serpentinization in different tectonic settings; changes in the bulk compositions of ultramafic rocks during serpentinization; fundamental differences between serpentinization and deserpentinization; and the absence of precise geothermobarometers for ultramafic rocks.
The capacity of serpentinite-derived fluids to oxidize sub-arc magma is also examined. These fluids can transport redox budget as carbon-, sulfur-, and iron-bearing species. Iron- and carbon-bearing species might be present in sufficient concentrations to transport redox budget deep within subduction zones, but are not viable transporters of redox budget at the temperatures of antigorite breakdown, which produces the largest proportion of fluid released by serpentinite dehydration. Sulfur-bearing species can carry significant redox budget, and calculations using the Deep Earth Water (DEW) model show that these species might be stable during antigorite breakdown. However, oxygen fugacities of ∼ΔFMQ +3 (where FMQ refers to the fayalite–magnetite–quartz buffer, and ΔFMQ is Log fO2 – Log fO2,FMQ), which is close to, or above, the hematite–magnetite buffer at the conditions of interest, are required to stabilize oxidized sulfur-bearing species. Pseudosection calculations indicate that these conditions might be attained at the conditions of antigorite breakdown if the starting serpentinites are sufficiently oxidized, but further work is required to assess the variability of serpentinite protoliths, metamorphic pressures and temperatures, and to confirm the relative positions of the mineral buffers with relation to changes in fluid speciation.
... However, some primary melts show extremely high FeO tot (>13 wt%) with high MgO (>12 wt%) (ferropicrite) [1]. Considering the influence of chemical and mineral compositions of the mantle source on melt compositions, it has been argued that ferropicrites may be near-primary partial melts of pyroxenite formed in the convecting mantle [2][3][4], whereas others favor an origin by partial melting of an iron-rich peridotitic mantle source [5,6]. Melting experiments have shown that melting conditions such as pressure and temperature can strongly influence the melt compositions, such that ferropicrites may be generated by partial melting of an olivine-dominated mantle source at~5 GPa [5]. ...
... This implies that ferropicrite could be produced by the partial melting of oxidized mantle peridotite, and that a Fe-enriched mantle source [1] is not necessary. Some pyroxenite-derived melts formed at normal mantle fO 2 may have higher FeO tot than melts of peridotite, and have been advocated to explain ferropicrite petrogenesis [4]. However, the Al 2 O 3 contents (about 14 wt%) (Tables S3 and S4 online) of these melts are much higher than in ferropicrites (<10 wt%) [5]. ...
... This requires information about the mineral modes along the KG1 solidus and the extents of melting required to exhaust each phase at any given pressure. We quantified these values using ThermoCalc (Powell et al., 1998) and the Jennings and Holland (2015) thermodynamic database using the KG1(8) bulk composition modelled by Jennings et al. (2016). To ensure consistency with the MELT-Px melting parameterization (which depends on the melt fraction at which clinopyroxene is exhausted from the residue, F cpx-out ), we renormalized the phase-out boundaries predicted by ThermoCalc to be consistent with F cpx-out predicted by the MELTS-Px model. ...
The compositions and volumes of basalt generated by partial melting of the Earth's mantle provide fundamental constraints on the thermo-chemical conditions of the upper mantle. However, using melting products to interpret uniquely these conditions is challenging given the complexity of the melting and melt aggregation processes. Forward models simulating melting of lithologically heterogeneous mantle sources can account for this complexity, but require assumptions about key model input parameters, and the quality of the model fits to the observations are rarely, if at all, considered. Alternatively, inverse melting models can provide estimates of the quality of model fits to the observations, but as of yet, do not account for the presence of lithologic heterogeneity in the mantle source. To overcome these limitations, we present an inverse method coupling a Markov chain Monte Carlo (MCMC) sampling method with the REEBOX PRO forward mantle melting model. We use this tool to constrain mantle potential temperature, melt volumes, and the trace element and isotopic compositions of mantle source lithologies beneath the Reykjanes Peninsula of Iceland. We consider a range of plausible pyroxenite compositions (KG1, G2, and MIX1G) that span much of the range of natural pyroxenite compositions, and constrain the mantle potential temperature between 1455 and 1480 • C and pyroxenite abundance between 6.5 and 8.5%. These results are independent of the choice of pyroxenite composition and indicate that elevated potential temperatures and modest pyroxenite abundances are robust features of the Reykjanes Peninsula mantle source. The permitted ranges of pyroxenite trace element compositions vary as a function of pyroxenite fertility and mineralogy, but differ from the compositions of subduction-modified recycled oceanic crust typically used in previous models, indicating a more complex petrogenetic origin for the pyroxenite source than previously considered. As all of the pyroxenites employed here yield equally good fits to the geochemical and geophysical observations along the Reykjanes Peninsula, forward models should not be used to constrain the major element character of pyroxenite present in mantle source regions based solely on the trace element/isotopic compositions (and volumes) of basalts. Given the range of lithologies included in REEBOX PRO and the flexibility of MCMC inversion, this method may be applied to constrain thermal and compositional source characteristics in a wide variety of basalt source regions.
