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(a) SE image of lithomarge L13 grain (b) magnified image of the black box displaying the interface b e t w e e n a n o p a q u e a n d transparent zone. (c) Topsoil 8 grain showing differential dissolution that reveals internal zoning. (d) SE image of lithomarge L9 grain displaying some fracture due to the heterogeneous metamictization and roughness likely related to corrosion. (e) SE image of details of the lithomarge L8 grain displaying a contrast between four flat faces and one rough. (f) magnified view of the surface of lithomarge L5 grain.
Source publication
Zircon crystals from lateritic soils at Nsimi, Cameroon, were investigated using electron microprobe analysis (EMPA), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Raman spectroscopy to determine the extent of radiation damage from alpha-decay events. The soils belong to a small watershed developed on granitic rock...
Contexts in source publication
Context 1
... The surfaces of the grains display a wide range of morphologies from almost flat ( Fig. 2d) to very rough (Figs. 3d-3f and 2b) at the micrometer to submicrometer scale. The surface roughness can affect the whole surface of the grain (Fig. 2b) or can be restricted to specific areas, likely related to the growth or sector zoning of zircon (Fig. 3c). Many grains are fractured. This is a well-known feature of heterogeneous zircon grains, which is related to differential stress caused by the swelling of the more damaged domains ( Chakoumakos et al. 1987). In some cases, the fracturing and detachment of the external shell reveals a very rough internal surface, indicative of the ...
Context 2
... correspond to the percolated zircon domains presently observed in the lateritic profile. Internal voids and surface roughness observed on these heavily damaged areas (Figs. 2 and 3) thus attest to the pervasive dissolution of damaged zircon exposed to tropical weathering conditions in the lateritic profile. Dissolution of damaged zircon is expected to preferentially occur at low temperature; whereas, it is counterbalanced by efficient recrystallization under hydro- thermal conditions (Geisler et al. 2003b). ...
Citations
... Twelve analyses with discordance > 10% were excluded from further analysis. This discordance may be related to Pb loss during longterm chemical weathering (Delattre et al., 2007). Most zircon grains are prismatic, although some grains are rounded or subrounded, reflecting long-distance transportation (Fig. 5c). ...
The timing of the collision between the Songnen–Zhangguangcai Range Block (SNZRB) and the Jiamusi Block (JMSB) is poorly constrained; however, detrital zircons may provide crucial information on the history of this collision. This study focuses on the Yangmugang and Tumenling formations of the Yimianpo Group in the southeastern SNZRB, which were previously thought to have been deposited during the closure of the Mudanjiang Ocean. Detrital zircon U-Pb dating was carried out to constrain the age, provenance, and tectonic setting of the Yangmugang and Tumenling formations, and ultimately the timing of the final closure of the Mudanjiang Ocean. The detrital zircon grains in the two formations yield similar age spectra, with two major age peaks at 275 Ma and 280 Ma, 512 Ma and 525 Ma, respectively. Given the age of the youngest group and that of a granite intrusion, the Yangmugang and Tumenling formations were probably deposited during the middle Permian. The detrital zircon ages also suggest that the Paleozoic igneous rocks distributed across the eastern SNZRB were the major source of the zircons in the formations. Combined with evidence for synchronous arc magmatism, this shows that the Yangmugang and Tumenling formations were likely deposited at a convergent margin and that the collision between the SNZRB and JMSB occurred after the late Permian.
... It seems unlikely that the xenotime core is a pseudomorph of a zircon grain which served as a substrate for epitaxial growth of the outer xenotime regions, and subsequently underwent complete dissolution and replacement because the concentration of Zr remained unchanged from the ashfall deposit (~ 130 ppm, as measured by ICP-MS). Further, zircon solubility is extremely low, and even in reported cases of (metamict) zircon dissolution by alkaline fluids at ambient temperature, re-precipitation occurs right on the grain rim or within a few microns (Delattre et al., 2007;Franz et al., 2015;Hay et al., 2009). It is also unlikely that the xenotime nanocrystals replaced a xenotime phenocryst that sourced the Y and P during congruent dissolution, as the solubility of xenotime phenocrysts is very low (Budzyń & Sláma, 2019;Cetiner et al., 2005), and the ICP-MS data indicated that Y was transported to the pods, along with the other REEs. ...
Eocene-aged bentonite deposits in Gonzalez (Texas, USA) carrfy scattered manganese oxide-rich pods where rare earth element (REE) concentrations as high as 7800 ppm have been detected — 10 times greater than in the already enriched bentonite. This study investigated the nature of the association between REEs and Mn oxides as well as REE modes of occurrence in these pods, in order to understand the processes that mobilize and accumulate REEs in the low-temperature depositional environment of the Texas coastal plain. Embedded, polished blocks of Mn pods were analyzed by Scanning Electron Microscopy (SEM); sections were then extracted by the FIB liftout technique from regions bearing REEs for further analysis by Transmission Electron Microscopy (TEM). The SEM and TEM results revealed the presence of abundant yttrium phosphate nanoparticles identified as xenotime, forming large globular aggregates that reach a few microns in diameter; these xenotime aggregates also host trace concentrations of Zr, Dy, Er, Yb, and Ce. Further, rhabdophane nanoparticles were detected that host predominantly light REEs and Y. The relative proportions of REEs in rhabdophane vary between particles and show decoupling of Ce from other light REEs, suggesting oxidation from Ce3+ to Ce4+. The REE enrichment of these Mn pods is due to extensive weathering along fractures cutting through the bentonite deposit that drove their remobilization and transport downward. At the base of the bentonite, where fractures terminate against the boundary with a cemented sandstone, changes in flow regime combined with gradual downward changes in fluid composition, prompted co-precipitation of Mn oxides and REE phosphates (xenotime and rhabdophane).
... However, zircon has an Achilles heel; namely its affinity for U and Th results in radiation damage to the crystal structure (metamictisation) over time, which then allows for uptake of non-formula elements, as well as susceptibility to fluid-aided alteration even under low temperature (i.e., T < 300 • C) conditions (Delattre et al., 2007;Geisler et al., 2003a;Geisler et al., 2003b;Hay and Dempster, 2009a;Hay and Dempster, 2009b;Nasdala et al., 2001b;Pidgeon et al., 2019). These processes are particularly relevant for ancient zircons, such as the Archean zircons of the BRM (Nazari-Dehkordi et al., 2017), that have billions of years to undergo metamictisation. ...
... The high and variable levels of non-formula elements, and variably low SiO 2 and ZrO 2 contents of the BRM zircons are entirely inconsistent with a primary igneous origin (Hoskin and Schaltegger, 2003). Instead, these characteristics are distinctive of metamict or radiation damaged zircons (Delattre et al., 2007;Geisler et al., 2003b;Hay and Dempster, 2009b), whereby non-formula elements are taken up into radiationdamaged or amorphous domains of the zircon once the inherent radiation dose exceeds the percolation threshhold of 2 × 10 18 α-decays/g Fig. 11). Extensive radiation damage to the BRM zircons is confirmed by: (i) structural and textural features of the BRM zircons, including the presence of void spaces, fractures and highly porous 'sponge-like' domains throughout the zircon grains from the μmdown to the nm-scale (Fig. 3, Fig. 9a-g, Fig. 10e-f) and the semicrystalline to amorphous nature of the grain structure at the nm-scale ( Fig. 10 b, d); and (ii) complex and variable Raman spectra that include a vibrational band for OH − and other non-formula components, as well as ν 3 (SiO 4 ) vibrational bands that are broad and attenuated, or absent altogether (Fig. 8). ...
... 1.83 and 1.72 Ga (labelled 2 on Fig. 11); and (iii) element uptake during modern exposure and weathering (labelled 3 on Fig. 11). Although modern weathering has been proposed as a mechanism for element uptake in metamict zircon (Delattre et al., 2007;Hay and Dempster, 2009b), most of the samples examined here are drillcore samples derived from ~100 m or ~ 441 m below the surface, which is well below the weathering zone. Therefore, the bulk of non-formula element uptake cannot be due to modern surface weathering processes. ...
... A precondition of these applications is that the elements in question represent the original concentrations in the primary zircons and that the zircons have remained closed systems to loss or rearrangement of the constituent minor elements. However, the loss of radiogenic Pb and the gain of common Pb is a common feature of many zircon suites and there have been numerous reports of elevated contents of incompatible trace elements (such as Ca, Al, Fe, P and Mn) in radiation damaged zircon (Grünenfelder, 1963;Krogh and Davis, 1975;Geisler and Schleicher, 2000;Köppel and Sommeraur, 1974;Sommerauer, 1974;Geisler et al., 2003;Delattre et al., 2007;Horie et al., 2006;Hay and Dempster, 2009;Jiang et al., 2020) that indicate post crystallization, late stage, open system, addition of these elements to the zircons. Also, Kubeˇs et al. (2021) reported that besides enrichment in non-formula elements Ca, Fe, Al, P in metamict zircon there is evidence for strong depletion of Zr and Si and proposed that "extensive HFSE remobilization from altered zircon was controlled by interaction with oxidized aqueous solutions circulating through abundant microfractures that formed by volume expansion due to zircon metamictization". ...
... Summing up the data suggests that in samples TUC19-1D and -1E, the characteristics of goethite evolved with time. As presented in Figure However, it has already been stated that zircon, especially when highly radiation damaged, i.e. metamict, was not stable under lateritic conditions (Carroll, 1953;Balan et al., 2001;Delattre et al., 2007). As the parental rocks of the schists are very old (Paleoproterozoic), the zircons are probably very metamict and therefore less resistant against weathering. ...
Laterites are deep stratified weathering profiles which form under tropical and subtropical climatic conditions commonly found at the Earth’s continental surface. Although they are very widespread in (sub)tropical areas, many aspects of their genesis and evolution are still not well constrained. Previous studies have shown that they can be very old (Millions of years) and are therefore records of past climate, but little is known about their temporal evolution, notably in relation with climatic and geodynamic changes, due to the small quantity of age data and the difficulty to date this material. The Guiana shield is of particular interest to study laterites as this area remained tectonically stable and in tropical latitudes since the Cretaceous, favoring the formation of very deep lateritic profiles. In this work, two areas were studied: the northeastern rim of the Guiana shield in French Guiana and the eastern rim of the shield in the Brazilian state of Amapá. In both areas, deep (>70 m) lateritic profiles developed on top of Paleoproterozoic basement rocks. The lateritic profiles were studied using mineralogical, geochemical and geochronological tools, notably (U-Th)/He dating and micro X-ray diffraction of supergene hematite and goethite and electron paramagnetic resonance spectroscopy (EPR) of kaolinite. In order to better understand the signification of hematite and goethite (U-Th)/He ages, a theoretical study on the He retention in these supergene minerals has been performed. The results allow to propose correction factors for the (U-Th)/He ages and to better apprehend the signification of these ages.Results from the lateritic-bauxitic duricrust of Kaw mountain in French Guiana indicate that weathering started at least during the Oligocene and was favored during several episodes in the Middle Miocene and the Late Neogene. Coupling of geochronological, geochemical and mineralogical data allows to distinguish between a period of ferruginous lateritic conditions and another one with bauxitic conditions indicating an important change in the weathering conditions. Comparison with other lateritic covers in northeastern French Guiana reveals that these periods also effected other sites in the area and that climate was the main driving force for the Late Neogene bauxitization. In contrast, results from northern Brazil record weathering since the Late Cretaceous and indicate very discrete intervals of weathering at ca. 30 and 12 Ma. The Late Neogene bauxitization event recorded in French Guiana is absent in the studied samples and the therefore very well constrained (U-Th)/He ages, coupled to high resolution micro-XRD analyses, allow to establish a detailed precipitation chronology of the supergene iron (oxyhydr)oxide phases. The results of the EPR analyses of purified kaolinites from two studied profiles indicate that kaolinite in the saprolites of the profiles formed probably several Myr to tens of Myrs ago. At Kaw, in French Guiana, EPR analyses of structural Fe impurities reveal that the kaolinites of the saprolite formed under near identical conditions over 70 m depth. In Amapá, the presence of rather young kaolinite at the top of the studied profile indicates relatively recent rejuvenation of kaolinite at the top but older kaolinite was preserved inside the duricrust as well as in the lower, saprolitic, part of the profile. Finally, this thesis shows that a detailed combination of geochronological results coupled to mineralogical and geochemical analyses improves our understanding of tropical weathering processes and laterite formation by placing mineralogical and geochemical processes into a temporal framework.
... Another caveat is that our zircon roundness data are not paired with U-Pb age for each grain. It has been suggested that radiation damage of zircon crystal structure causes internal disorder, volume expansion, and a decrease in density, making grains more prone to chemical attack, such as in tropical weathering condition and fragmentation during sediment transport and deposition (e.g., Holland and Gottfried, 1955;Lee and Tromp, 1995;Delattre et al., 2007). Therefore, zircons with high radiation damage can be easily fragmented, and are potentially better rounded than the ones without radiation damage. ...
Paleozoic sediment provenance of the Permian Basin remains controversial. This study presents new detrital zircon morphology, UPb ages, and Hf isotope data of eight Paleozoic sandstone samples, and integrates published zircon UPb data in the basin to illustrate the spatial and temporal changes of sediment provenance. The Cambrian sample has predominantly Mesoproterozoic (1500–1300 Ma) grains because of fluvial transport of basement detritus from the local Texas Arch. The Ordovician shoreface sandstone contains mostly rounded grains of Paleoproterozoic (1800–1600 Ma) age, mostly likely by longshore drift reworking detritus from the Transcontinental Arch. The Pennsylvanian sediment provenance shows heterogeneity. While samples from the Northwestern Shelf have both the Mesoproterozoic and Paleoproterozoic groups and low grain roundness, reflecting sediment delivery from basement rocks in the Ancestral Rocky Mountains, a sample in the Midland Basin contains a broad range of ages, including a major Neoproterozoic (800–500 Ma) group that were primarily from peri-Gondwana terranes. The difference suggests that there was limited sediment mixing in the Permian Basin. The similarity of zircon age distributions between the Midland Basin and Fort Worth Basin confirms synorogenic progradation of detritus from the Fort Worth Basin to the Eastern Shelf during the Pennsylvanian. During the early Permian Cisuralian epoch, the compiled UPb data show that the southern and eastern basin contains a significant fraction of Gondwanan detritus, and the amount reduced by the latest Cisuralian as the orogenesis waned. The middle Permian Guadalupian samples on the Northwestern Shelf contain well-rounded grains that are of mainly Grenvillian and Paleozoic ages, suggesting sediment recycling from Laurentia by wind and likely by river as well. The change of sediment provenance from the early to the middle Permian suggests that after the continental collision, Laurentian sediments dominated detritus in the Permian Basin.
... The glassy nature of highly metamict zircons also renders them more vulnerable to weathering (Ewing et al., 2011). However, it still takes intense tropical weathering (laterization) to (partially) destroy metamict zircons (Delattre et al., 2007). ...
Analysis of cosmogenic nuclides produced in surface rocks and sediments is a
valuable tool for assessing the rates of processes and the timing of events that
shaped the Earth surface. The various nuclides that are used have specific
advantages and limitations that depend on the time range over which they are
useful, the type of material they are produced in and not least the
feasibility of the analytical effort. Anticipating novel applications in
Earth surface sciences, we develop in situ-produced terrestrial cosmogenic
krypton (Krit) as a new tool, the motivation being the availability of
six stable and one radioactive isotope (81Kr, half-life 229 kyr) and of
an extremely weathering-resistant target mineral (zircon). We provide proof
of principle that terrestrial Krit can be quantified and used to
unravel Earth surface processes.
... The U -Pb and U-series isotope systems are particularly valuable in detecting the effects of fluid flow in rocks. In crystalline rocks, a large fraction of U resides along grain boundaries and in minerals that may Table 3. become degraded and unstable because of radiation damage (Lumpkin, 2001;Delattre et al., 2007). The relative ease with which U is lost from the rocks is the result of (i) a large part of the U being localized in potentially labile sites and (ii) the high solubility of U that may exist in the +6 valence state (Stuckless, 1986;Drake et al., 2009). ...
High-U hydrothermal apatite with complex UPb systematics is closely spatially associated with mineralization at the Coles Hill deposit, the largest unmined uranium deposit known in the United States. The deposit is hosted in metasomatized rocks of the 450- to 430-Ma-old Martinsville Intrusive Complex in south-central Virginia. Direct dating of metamict uranium-ore minerals, mostly coffinite, is not possible due to open-system radon loss. Instead, UPb isotopes in cogenetic apatite were investigated as a means of evaluating the age of mineralization. Here we report in situ electron probe microanalyses (EPMA) of coffinite, isotope-dilution thermal-ionization mass spectrometry (ID-TIMS) UPb data for mineralized whole rock samples, and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) UPb isotope data for apatite in both unmineralized and U-mineralized host rocks.
Massive deficits in radiogenic Pb preclude reliable UPb “chemical ages” calculated from EPMA data obtained from coffinite. In contrast, LA-ICPMS data for secondary apatite in unmineralized rocks indicate low-U concentrations (10⁰–10² ppm), “normal” (consistent with models of terrestrial Pb isotopic evolution) initial Pb isotope compositions, and UPb age estimates of ~330 Ma, which is consistent with dates previously proposed for the regional Paleozoic shear zone that hosts the deposit. Ore-stage apatite associated with coffinite has high-U concentrations (typically 10²–10³ ppm but up to 2.4 wt% U) and large excesses of ²⁰⁶Pb (²⁰⁷Pb/²⁰⁶Pb < 0.01) unsupported by in situ U decay. Data show that initial Pb had variable isotopic compositions including both “normal” Pb derived from host rocks and ²⁰⁶Pb-enriched Pb introduced by secondary metasomatic fluids. Evaluation of the complex evolution and mixing of Pb sources has broader implications for UPb dating of hydrothermal apatite.
Excess ²⁰⁶Pb in apatite is derived from decay products of ²²²Rn lost from coffinite and mobilized by Na-, P-, and U-enriched metasomatic fluids during the main mineralizing event at ~230 Ma. Ore-stage alteration did not uniformly reset the UPb systematics in host rocks precluding a well-constrained whole-rock isochron age. However, whole-rock isotope data imply U mobility at ~200–220 Ma and support a Triassic age for the final stages of mineralization. Results also indicate that apatite with up to several weight percent uranium is able to retain U and its decay products for hundreds of millions of years; an important consideration when assessing this mineral as a potential matrix for long-term storage of radioactive waste.
... The glassy nature of highly metamict 699 zircons also renders them more vulnerable to weathering (Ewing et al., 2011). However, it still takes 700 intense tropical weathering (laterization) to (partially) destroy metamict zircons (Delattre et al., 2007). The results presented here provide an idea of the utility of this new tool for dating changes of process 717 rates on the Earth surface (MPT in temperate regions), or testing their notional long-term invariance 718 (African Surface). ...
Analysis of cosmogenic nuclides produced in surface rocks and sediments is a valuable tool for assessing rates of processes and the timing of events that shaped the Earth surface. The various nuclides that are used have specific advantages and limitations that depend on the time-range over which they are useful, the type of material they are produced in, and not least the feasibility of the analytical effort. Anticipating novel applications in Earth surface sciences, we develop in-situ produced terrestrial cosmogenic krypton (Krit) as a new tool; the motivation being the availability of six stable and one radioactive isotope (81Kr, half-life 229 kyr) and of an extremely weathering-resistant target mineral (zircon). We provide proof of principle that terrestrial Krit can be quantified and used to unravel Earth surface processes.
... While diffusion of most cations (including U and Pb) in crystalline zircon is extraordinary slow (Cherniak, 2003), disordered and partially metamict zircon has long been known to be susceptible to aqueous alteration via recrystallization on laboratory timescales at hydrothermal temperatures (Pidgeon et al., 1966;Geisler et al., 2001;Geisler et al., 2003aGeisler et al., , b, 2004 and over longer timescales even at ambient temperatures (Stern et al., 1966;Black, 1987;Tromans, 2006;Delattre et al., 2007). If such aqueous processes are responsible for zero-age Pb loss in zircon, the absence of such Pb loss in monazite and sphene remains notable. ...
... This process is evidently sensitive to the crystallinity and surface area of the zircon in question and forms the basis for the whole-grain direct evaporation technique of Kober (1986). Since the products of the lowtemperature aqueous recrystallization of metamict zircon appear to remain rather poorly crystalline (featuring microlites, nanopores, and residual amorphous zones; Geisler et al., 2003bGeisler et al., , 2004Delattre et al., 2007;Hay et al., 2009), partially metamict zircon that has undergone exposure-associated aqueous Pb loss and recrystallization should remain susceptible to oxide decomposition during low-pressure, hightemperature annealing. If this interpretation is correct, the high temperature at which this conversion occurs (limiting isotopic fractionation), followed by quantitative dissolution of highly acid-soluble (Rioux et al., 2010) ZrO 2 crystallites during chemical abrasion, may explain why isotopic fractionation and reverse discordance are rare in the chemical abrasion of annealed zircon. ...
The Hadean Jack Hills zircons represent the oldest known terrestrial material, providing a unique and truly direct record of Hadean Earth history. This zircon population has been extensively studied via high-spatial-resolution high-throughput in situ isotopic and elemental analysis techniques, such as secondary ionization mass spectrometry (SIMS), but not by comparatively destructive, high-temporal-precision (
