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The continental crust is an important reservoir for incompatible elements, including the halogen elements (F, Cl, Br, and I), but their concentrations remain poorly known, thus hindering better understanding of the role of the crust in Earth's halogen cycle. We present halogen data (F, Cl, Br, and I) for twenty-four well-characterized glacial diamictite composites that derive from the upper continental crust (UCC) and were deposited during discrete glacial events at ∼2.9 Ga, ∼2.4–2.2 Ga, 0.75–0.58 Ga, and ∼0.30 Ga. A good correlation between Cl and the highly soluble Na (R2 = 0.70), together with low and scattered Cl concentrations in the diamictites (2–279 ppm), indicates significant Cl loss during chemical weathering of the continents. The other halogens (F, Br, and I), however, are not strongly affected by chemical weathering as revealed by their correlations with less soluble elements like K, P, Nd, and Lu, which may be due to their retention in secondary minerals and/or organic matter. Increasing concentrations of F in the composites through time may reflect the evolving composition of the UCC. Using the median values of the Neoproterozoic and Paleozoic diamictite composites, halogen concentrations of the present-day weathered UCC are estimated to be: 575 ± 87 ppm F, 29 ± 20 ppm Cl, 0.65 ± 0.14 ppm Br, and 0.05 ± 0.01 ppm I (errors quoted at the median absolute deviation). Linear correlations between halogens and other elements provide estimates for minimum halogen concentrations of the present-day crystalline UCC: 394 ± 67 ppm F, 83 ± 24 ppm Cl, 0.41 ± 0.04 ppm Br, and 0.03 ± 0.01 ppm I (with 2σ uncertainties). These estimates are all lower than normalized concentrations of elements of similar incompatibility during igneous differentiation, which may reflect loss of halogens via magmatic degassing/exsolution and/or chemical weathering during the formation of the continental crust. The distinct behavior of Cl compared to Br and I during continental weathering leads to a wide range of Br/Cl (2–265 * 10−3) and I/Cl (122–197,952 * 10−6) ratios, and Br/I ratios that are distinctly higher than those of pelagic sediments and marine pore fluids. Similar halogen signals have been reported from the lithospheric mantle and may reflect recycling of terrigenous sediments. The calculated weathering flux of Cl from the continents is quite small compared to the total Cl content of seawater (<8–34 %), suggesting that Cl in seawater is mainly derived from outgassing of the mantle and/or late volatile accretion. On the other hand, the significantly higher Br and I contents in terrigenous organic-rich sediments compared to those of crystalline bedrocks suggest that significant amounts of Br and I were transported from the oceans to the continents. [ABSTRACT FROM AUTHOR]

Coupled U‐Pb and trace‐element analyses of accessory phases in crustal xenoliths from the Late Devonian Udachnaya kimberlite (Siberian craton, Russia) are used to constrain Moho temperature and crustal heat production at the time of kimberlite eruption. Rutile and apatite in lower‐crustal garnet granulites record U‐Pb dates that extend from 1.8 Ga to 360 Ma (timing of kimberlite eruption). This contrasts with upper‐crustal tonalites and amphibolites that contain solely Paleoproterozoic apatite. Depth profiling of rutile from the lower‐crustal xenoliths show that U‐Pb dates increase gradually from rim to core over μm‐scale distances, with slower‐diffusing elements (e.g., Al) increasing in concentration across similar length‐scales. The U‐Pb and trace element gradients in rutile are incompatible with partial Pb loss during slow cooling, but are consistent with neocrystallization and re‐heating of the lower crust for <1 Myr prior to eruption. Because Paleoproterozoic rutile and apatite dates are preserved, we infer that long‐term ambient lower‐crustal temperatures before this thermal perturbation were cooler than the Pb closure temperature of rutile and probably apatite (<400°C). The lower‐crustal temperature bounds from these data are consistent with pressure‐temperature arrays of Udachnaya peridotite xenoliths that suggest relatively cool geothermal gradients, signifying that the mantle xenoliths accurately capture the thermal state of the lithosphere prior to eruption. Combined, the xenolith data imply low crustal heat production for the Siberian craton (∼0.3 μW/m3). Nevertheless, such values produce surface heat flow values of 20–40 mW/m2, higher than measured around Udachnaya (average 19 mW/m2), suggesting that the surface heat flow measurements are inaccurate. Plain Language Summary: The decay of radioactive elements, like uranium (U), thorium (Th), and potassium (K), produces heat and the dissipation of this heat from Earth's interior drives flow at depth and the movement of tectonic plates at the surface over geologic time. The crust—the planet's rocky outer layer—is hypothesized to contain a significant portion of Earth's heat‐producing elements, yet this budget is uncertain. The total amount of heat leaving the surface can be measured, but how much of this flux reflects heat production within the crust versus heat derived from the underlying mantle is also uncertain. Kimberlite pipes carry fragments of Earth's interior to the surface ("xenoliths") in Siberia. Some of the xenoliths are from near the base of the crust, and temperature‐sensitive chronometers in these rocks record ancient dates (>1 billion years old). Such ancient dates are significant because they can only be preserved if temperatures in the deep crust were cool (<400°C). Models of heat transfer through the Siberian crust using our results and mantle xenolith pressure‐temperature data indicate relatively low heat production in the crust. Key Points: The U‐Pb system in deep crustal xenoliths is sensitive to transient heating of the lower crust and not just slow coolingThe central Siberian craton lower crust resided below the Pb closure temperature of rutile and apatite prior to kimberlite eruption 360 MaExtremely low crustal heat production for the craton (∼0.3 μW/m3) is consistent with peridotite xenolith P‐T data [ABSTRACT FROM AUTHOR]

The concentrations of Cu and Ag, both insoluble chalcophile elements, can be used to place tight constraints on the proportion of basalts in the upper continental crust (UCC) through time via analyses of fine-grained terrigenous sedimentary rocks. Copper and Ag concentrations in magmas are largely controlled by sulfide dissolution during melting and sulfide fractionation during differentiation. We show that Cu is high in basalts but low in komatiites and felsic rocks, making Cu useful for constraining the proportion of basalts in the UCC. Furthermore, Cu/Ag ratios are high in basalts and komatiites but decrease with differentiation. The fine-grained matrix of ancient glacial diamictites shows dramatic declines in both Cu concentrations and Cu/Ag ratios at 3.0–2.4 Ga, after which diamictite Cu concentrations and Cu/Ag ratios remain roughly constant. Mass-balance calculations using average Cu concentrations of Archean komatiites, basalts and felsic rocks require that a high proportion basalt (65–75%) was present in the UCC at ∼3.0 Ga, and this crust transitioned to a felsic-rock-dominated crust at ∼2.4 Ga ago and has remained basically unchanged since then. This conclusion is supported by the consistency between the Cu/Ag ratios in the diamictites and the weighted average Cu/Ag ratios based on the lithological proportions obtained using Cu mass-balance calculations. Our observations document the emergence of felsic continental crust in the late Archean, which, in turn, indicates a fundamental change in Earth's dynamic regime at that time. [ABSTRACT FROM AUTHOR]

The use of molybdenum as a quantitative paleo-atmosphere redox sensor is predicated on the assumption that Mo is hosted in sulfides in the upper continental crust (UCC). This assumption is tested here by determining the mineralogical hosts of Mo in typical Archean, Proterozoic, and Phanerozoic upper crustal igneous rocks, spanning a compositional range from basalt to granite. Common igneous sulfides such as pyrite and chalcopyrite contain very little Mo (commonly below detection limits of around 10 ng/g) and are not a significant crustal Mo host. By contrast, volcanic glass and Ti-bearing phases such as titanite, ilmenite, magnetite, and rutile contain significantly higher Mo concentrations (e.g., up to 40 µg/g in titanite), and can account for the whole-rock Mo budget in most rocks. However, mass balance between whole-rock and mineral data is not achieved in 4 out of 10 granites analyzed with in-situ methods, where Mo may be hosted in undetected trace molybdenite. Significant Mo depletion ( i.e. , UCC-normalized Mo/Ce < 1) occurs in nearly every granitic rock analyzed here, but not in oceanic basalts or their differentiates (Greaney et al., 2017; Jenner and O’Neill, 2012). On average, granites are missing ∼60% of their expected Mo contents. There are two possible reasons for this: (1) Mo partitions into an aqueous magmatic vapor/fluid phase that is expelled from cooling plutons, and/or (2) Mo is partitioned into titaniferous phases during partial melting and fractional crystallization of an evolving magma. The first scenario is likely given the high solubility of oxidized Mo. However, correlations between Mo/Ce and Nb/La in several plutonic suites suggest fractionating phases such as rutile or Fe-Ti oxides may sequester Mo in lower crustal rocks or in subducting slabs in arc settings. [ABSTRACT FROM AUTHOR]

Tungsten (W) stable isotope data for twenty-four composites made from the fine-grained matrix of glacial diamictites, deposited between the Mesoarchean and the Paleozoic, as well as two Archean tonalite-trondhjemite-granodiorites (TTG) are used to refine the W isotope compositions (expressed as δ186W deviations from the NIST 3163 W standard) of the upper continental crust (UCC). To do this we evaluate W isotope fractionation in the diamictites as the result of both magmatic and low-temperature processes. The δ186W values of the diamictites are heterogeneous (0.017 ± 0.011 ‰, 2SE to 0.182 ± 0.007 ‰, 2SE), encompassing the range of previously published values for igneous rocks. We find that δ186W correlates positively with the diamictite's chemical index of alteration (CIA), a measure of the degree of chemical weathering in the provenance, suggesting that the continental regolith (i.e., surface material composed of weathered residues) is isotopically heavy for W. Since W in rivers and ocean water is also isotopically heavy, this suggests that equilibration with Fe-Mn-oxides controls the W isotope composition of surface waters and that such oxides are not important constituents of the regolith, whose W is likely accommodated in clays. Using diamictites with low CIA (<60, n = 9), we calculate an average δ186W of 0.046 ± 0.036 ‰ (2SD) for the UCC, from 2.3 to 0.3 Ga. This average is combined with the TTG data of this study, as well as data from the literature for upper crustal rocks to obtain a representative δ186W value of the UCC of 0.046 ± 0.046 ‰ (2SD). The W stable isotope composition of the UCC is lighter than that of mantle-derived melts (MORB and OIB, with an average δ186W = 0.082 ± 0.026 ‰, 2SD), and much lighter than the weighted average δ186W of intra-oceanic arcs (δ186W = 0.104 ± 0.052 ‰, 2SD), though the spread in the arc rocks is large. The significant difference in δ186W between average intra-oceanic island arc lavas and UCC is intriguing given that continental crust is thought to form primarily via arc magmatism. This difference suggests that not all modern intra-oceanic arcs are representative of those that formed the continental crust. Rather, only arcs that are enriched in incompatible trace elements, which also tend to have lighter W isotope compositions, may have been involved in making new continental crust. Alternatively, there may have been a secular change in the W isotope composition of arc magmas. [ABSTRACT FROM AUTHOR]

Titanium isotopes recorded in glacial diamictites with depositional ages between 2.9 and 0.3 Ga show that the upper continental crust became significantly more felsic relative to the present-day crust during the amalgamation of the Paleoproterozoic Nuna and the Neoproterozoic Gondwana supercontinents. This can be attributed to the continental collisions involved in the assembly of Nuna and Gondwana. The resulting high topographic relief of Nuna and Gondwana orogens must have resulted in an enhanced erosional supply from the continents to oceans. The step changes in the development of organismal complexity from prokaryotes to eukaryotes, and eventually metazoans, appear to be temporally correlated to instances where collisional mountain-building sustained an elevated nutrient supply from the continents to oceans. The nutrient surge associated with the rise of the Gondwana mountains likely provided the necessary impetus for the Neoproterozoic ecological expansion of eukaryotes and the eventual radiation of metazoans. A similar link between the enhanced nutrient supply from Nuna mountains and the radiation of early eukaryotes is plausible, although its mechanistic underpinnings remain unclear. The termination of Nuna orogeny and its transition to Rodinia without significant breakup and subsequent collisional orogenesis corresponds to the long lull in Earth's redox and biological evolution in its middle age. • Pronounced variations in the felsic character of Proterozoic upper crust. • These variations can be temporally linked to the assembly of Nuna and Gondwana. • High continental relief brackets the mid-Proterozoic lull in eukaryote evolution. • Gondwana orogeny likely facilitated the Neoproterozoic redox and biological changes. • Enhanced erosion linked to Nuna orogeny may have aided the radiation of eukaryotes. [ABSTRACT FROM AUTHOR]

The composition of the fine-grained matrix of glacial diamictites from the Mesoarchean, Paleoproterozoic, Neoproterozoic, and Paleozoic, collected from four modern continents, reflects the secular evolution of the average composition of the upper continental crust (UCC). The effects of localized provenance are present in some cases, but distinctive geochemical signatures exist in diamictites of the same age from different localities, suggesting that these are global signatures. Archean UCC, dominated by greenstone basalts and to a lesser extent komatiites, was more mafic, based on major elements and transition metal trace elements. Temporal changes in oxygen isotope ratios, rare earth elements, and high field strength elements indicate that the UCC became more differentiated and that tonalite–trondhjemite–granodiorite suites became less important with time, findings consistent with previous studies. We also document the concentrations of siderophile and chalcophile elements (Ga, Ge, Cd, In, Sn, Sb, W, Tl, Bi) and lithophile Be in the UCC through time, and use the data for the younger diamictites to construct a new estimate of average UCC along with associated uncertainties. [ABSTRACT FROM AUTHOR]

Lithium isotopes are increasingly used to trace both present-day and past weathering processes at the surface of the Earth, and could potentially be used to evaluate the average degree of past weathering recorded by the upper continental crust (UCC). Yet the previous estimate of average δ 7 Li of the UCC has a rather large uncertainty, hindering the use of Li isotopes for this purpose. New δ 7 Li for desert and periglacial loess deposits (windblown dust) from several parts of the world (Europe, Argentina, China and Tajikistan) demonstrate that the former are more homogeneous than the latter, and may therefore serve as excellent proxies of the average composition of large tracts of the UCC. The Li isotopic compositions and concentrations of desert loess samples are controlled by eolian sorting that can be quantified by a binary mixing between a weathered, fine-grained end-member, dominated by phyllosilicates and having low δ 7 Li, and an unweathered, coarse-grained end-member, that is a mixture of quartz and plagioclase having higher δ 7 Li. We use correlations between insoluble elements (REE, Nd/Hf and Fe 2 O 3 /SiO 2 ), Li concentrations (henceforth referred as [Li]), and δ 7 Li to estimate a new, more precise, average Li isotopic composition and concentration for the UCC: [ Li ] = 30.5 ± 3.6 ( 2 σ ) ppm , and δ Li 7 = + 0.6 ± 0.6 ( 2 σ ) . The δ 7 Li for desert loess deposits is anti-correlated with the chemical index of alteration (CIA). Using this relationship, along with our average δ 7 Li, we infer that (1) the present-day CIA of the average UCC is 61 − 2 + 4 ( 2 σ ) , higher than the common reference value of 53, and (2) the average proportion of chemically weathered components is as high as 37 − 10 + 17 ( 2 σ ) % at the surface of the Earth. [ABSTRACT FROM AUTHOR]

Statistical analyses of Sm-Eu-Gd concentrations in more than 3000 samples from the upper, middle, and lower continental crust reveal that the enrichment of Eu in the lower continental crust cannot compensate for the Eu deficit in the upper and middle continental crust, leaving the bulk continental crust with a significant negative Eu anomaly. Because the building blocks of the continental crust (mantle-derived basalts or tonalitic slab melts) do not possess a negative Eu anomaly, removal of Eu-enriched lower continental crust is required. Using Sm- Eu-Gd systematics and a mass conservation model, at least 2.9-0.9+1.1 (95% confidence) crustal masses (~6 x 1022 kg) appear to have been lost to the mantle over Earth's history via lower crustal recycling. Such a lower crustal component in the mantle may reappear in some ocean island basalts that have positive Eu anomalies and unradiogenic Pb isotopes. [ABSTRACT FROM AUTHOR]

Abstract: Mineralogical, chemical, and Li and Nd isotopic compositions of two drill cores (8–9m deep) through bauxites developed on the Miocene Columbia River Basalts document the changes associated with basalt weathering, provide insights into the processes involved, and allow us to examine the overall influence of chemical weathering on juvenile (basaltic) crust. Gibbsite, hematite, ±kaolinite, halloysite, goethite, and maghemite are the weathering products in the bauxites. Quartz is observed near the tops of the cores and its abundance decreases progressively with depth; no quartz is observed below five meters depth in either core. Most major and trace elements, including “mobile” and some “immobile” elements are severely depleted in the bauxites. Niobium is less mobile relative to the rare earth elements, thus chemical weathering attenuates the negative Nb anomaly in the continental crust. Li and Nd are strongly depleted relative to fresh basalt, and both increase systematically towards the surface in the quartz-bearing samples while δ7Li and ε Nd values decrease systematically towards the surface in these same samples. Both Li and Nd were likely lost from the bauxites through leaching. The systematic enrichment of Li, Nd, and quartz, as well as the less radiogenic Nd isotopic composition at the tops of both profiles reflects 20–60wt.% addition of an eolian component to the soils. The eolian dust is unlikely to have experienced significant post-depositional weathering due to the relatively high Li contents near the tops of the profiles, and, therefore, the low δ7Li and ε Nd values suggest that the dust came from an old, weathered region of the continent. Our results demonstrate that lithium isotopes are sensitive tracers of chemical weathering, particularly in extreme weathering settings, and support the hypothesis that chemical weathering influences the mass and composition of the continental crust. [Copyright &y& Elsevier]

Chemical weathering, as well as physical erosion, changes the composition and shapes the surface of the continental crust. However, the amount of continental material that has been lost over Earth's history due to chemical weathering is poorly constrained. Using a mass balance model for lithium inputs and outputs from the continental crust, we find that the mass of continental crust that has been lost due to chemical weathering is at least 15% of the original mass of the juvenile continental crust, and may be as high as 60%, with a best estimate of approximately 45%. Our results suggest that chemical weathering and subsequent subduction of soluble elements have major impacts on both the mass and the compositional evolution of the continental crust. [ABSTRACT FROM AUTHOR]

Abstract: Lithium concentrations and isotopic compositions of 39 A-type granites and mafic enclaves from 11 plutons in Northeast China and the North China craton are used to constrain their genesis and to characterize further the average composition of the continental crust. Lithium concentrations (2.8 to 80 ppm) in 29 A-type granites are slightly higher than those of average I-type granites and lower than those of average S-type granites, reflecting source differences. Lithium isotopic compositions (δ 7Li=−1.8 to +6.9) of A-type granites fall within the range of worldwide I-type and S-type granites. By contrast, 10 mafic enclaves have significantly higher Li concentrations (32 to 179 ppm) and less variation in Li isotopic compositions (δ 7Li=−3.2 to +3.1) than their corresponding granites, due to the high modal abundance of amphiboles and biotites. Overall, the weighted mean δ 7Li for A-type granites, as well as I-type and S-type granites is lighter than that of the mantle. Lithium concentrations in granites vary as a function of fractional crystallization and are mainly controlled by modal mineralogy. Lithium mostly behaves as an incompatible element during granite differentiation, although Li is compatible in hornblende and biotite during crystallization of mafic enclaves. By contrast, Li isotopic compositions of A-type granites and mafic enclaves are not controlled by fractional crystallization, comparable to that for I- and S-type granites, indicating negligible Li isotope fractionation during granite differentiation. Instead, they reflect source heterogeneity produced by mixing of isotopically heterogeneous lower crust and/or variable amounts of re-equilibration between enclaves and host granites. The Li isotopic compositions of A-, I- and S-type granites can be used to derive an independent estimate of the average Li isotopic composition of the continental crust (i.e., δ 7Li=+1.7, weighted mean of all granites), comparable to our previous estimate (δ 7Li=+1.2) that relies on large-scale sampling of upper, middle and lower continental crust. Compared to the mantle, the continental crust has lower δ 7Li, indicating the influence of weathering and recycling in controlling the Li isotopic composition of the continents. [Copyright &y& Elsevier]

Abstract: Samples from Archean high-grade metamorphic terranes in China and granulite-facies xenoliths from Australia (Chudleigh and McBride suites) and China (Hannuoba suite) have been analyzed to assess the Li concentrations and isotopic compositions of the middle and lower continental crust, respectively. Thirty composite samples from metamorphic terranes, including tonalite–trondjhemite–granodiorite (TTG) gneisses, amphibolites and felsic to mafic granulites, show a large variation in Li concentrations (5–33 ppm) but a relatively narrow range in δ 7Li values, from +1.7 to +7.5 with a mean of +4.0±1.4 (1σ). These results suggest that the middle continental crust is relatively homogenous in Li isotopic composition and indistinguishable from the upper mantle. This may be a primary feature or may reflect homogenization of Li isotopes during exhumation of the metamorphic terranes. In contrast, Li isotopic compositions of granulite xenoliths from the lower crust vary significantly, with δ 7Li ranging from −17.9 to +15.7. δ 7Li of minerals also shows a very large spread from −17.6 to +16.7 for plagioclases and −14.6 to +12.7 for pyroxenes. Large Li isotopic variations exist between plagioclase and pyroxene, with pyroxenes (13 out of 14) isotopically equal to or lighter than coexisting plagioclases. Lithium concentrations of granulite xenoliths also vary widely (0.5 to 21 ppm) and are, on average, lower than those of terranes (5±4 vs. 13±6 ppm respectively, 1σ), consistent with a higher proportion of mafic lithologies and a higher metamorphic grade for the xenoliths. Pyroxene separates from granulite xenoliths have equal or significantly greater Li than coexisting plagioclase. These large Li isotopic variations between minerals and in whole-rock granulite xenoliths mostly reflect diffusion-driven kinetic isotopic fractionation during the interactions of xenoliths with host magma. Only those xenoliths that reach inter-mineral isotopic equilibria are likely to preserve the initial Li isotopic signatures of the lower crust. Eight such equilibrated samples have δ 7Li from −14 to +14.3, with a concentration weighted average of +2.5, which is our best estimate of the average δ 7Li of the lower continental crust. The substantial isotopic heterogeneity of the lower crust may reflect the combined effects of isotopic fractionation during granulite-facies metamorphism, diffusion-driven isotopic fractionation during igneous intrusion and variable protolith compositions. Consistent with previous B elemental and O isotopic studies, the Li isotopic heterogeneity in the lower crust indicates that pervasive fluid migration and equilibration have not occurred. Using all data for granulite xenoliths, the Li concentration of the lower crust is estimated to be ∼8 ppm. Together with previous estimates of Li concentration in the upper and middle crust, the average Li concentration of the bulk continental crust is estimated to be 18 ppm, which is similar to previous estimates. The average Li isotopic composition of the continental crust is estimated to be +1.2, which is isotopically lighter than upper mantle and may reflect the loss of isotopically heavy Li from the continents during weathering and metamorphic dehydration. [Copyright &y& Elsevier]

Molybdenum isotopes in three deep and well-characterized weathering profiles – a saprolite formed on meta-diabase from South Carolina, USA, and two ferruginous bauxites formed on Columbia River Basalts in Oregon and Washington, USA – elucidate Mo isotope behavior during continental weathering. The saprolite records an overall loss of Mo relative to the fresh bedrock, as indicated by negative τMo Ti , which defines the loss of Mo relative to the relatively immobile element Ti. The saprolites are also isotopically light: δ98Mo values range from −0.89‰ (relative to NIST 3134) to −0.05‰, mean δ98Mo = −0.40‰, compared to +0.55‰ NIST3134 for the underlying unweathered bedrock. By contrast, the ferruginous bauxites generally record addition of Mo relative to the fresh bedrock (zero to positive τMo Ti) and generally have higher δ98Mo values than the parental basalts: δ98Mo of the bauxites range from −0.14‰ to +0.38‰ compared to −0.33‰ and +0.02‰ for the unweathered parental basalt. Low δ98Mo values in the saprolites likely reflect preferential retention of isotopically light Mo adsorbed onto accessory Fe-oxy-hydroxides and clays during weathering, whereas the high δ98Mo values in the bauxites reflect the addition of isotopically heavy Mo from groundwater. When the three profiles are combined, there is a positive correlation between τMo Ti and δ98Mo, suggesting that when Mo is lost during continental weathering, the resulting regolith is isotopically light, whereas groundwater addition can shift the regolith to heavier values. Because saprolites are a more common weathering product than bauxites, we conclude that, in general, continental weathering fractionates Mo isotopes such that the weathered upper crust retains isotopically light Mo. In contrast, the groundwater that leaches Mo from the weathered crust is isotopically heavy. Thus, chemical weathering of continents generates the isotopically heavy riverine signature observed globally, and partially contributes to the isotopically heavy seawater signature. Finally, these data, in conjunction with previously published data for glacial diamictites, can be used to assess changes in the crustal Mo isotope signature over the last 2.9 Ga. [ABSTRACT FROM AUTHOR]

To understand sulfur and carbon sequestration and release within the continental crust, scapolite minerals from a variety of granulite facies rocks were analyzed for their elemental composition and S isotope signatures. These high-grade scapolites host significant amounts of SO 3 and CO 2 , up to approximately 5 wt% and 3 wt%, respectively, with δ34S VCDT from −3 to +10 ‰, and formed in relatively oxidizing environments characterized by low a H 2 O in which scapolite may form as a primary igneous mineral or via metamorphic reactions involving sulfides and silicates. The range of scapolite sulfur isotope compositions mirrors those observed in mantle xenoliths, suggesting transport of S from the mantle into the lower crust via fluids and melts. Although scapolite's contribution to the global S and C cycles may be modest, it is significant in the context of sulfur fluxing from the mantle to the lower crust, particularly in its oxidized form. We estimate that at least 10 % of lower crustal sulfur is sequestered within scapolite. The exhumation of scapolite-bearing lower crustal rocks can therefore liberate substantial quantities of sulfur species and CO 2 , which may serve to both supply and compositionally buffer retrograde metamorphic fluids. These fluids may exhibit a range of S isotope compositions from mantle-like (δ34S VCDT ≈ 0 ‰) to relatively 34S-enriched signatures. Consequently, retrograde fluids may have S isotope signatures indistinguishable from those of mantle fluids, even in the absence of direct mantle S input during fluid formation. Exhumation of scapolite-bearing lower crust may facilitate element mobilization through S and Cl complexing, particularly with respect to base metals, within exhumed lower crustal sections, thus providing sources of metals and fluids in mid- to high-grade metamorphic rocks. Globally, scapolite-bearing lower crust may help balance the global sulfur cycle through catch-and-release from scapolite. [ABSTRACT FROM AUTHOR]

• Glacial diamictites record changes in chemical weathering of the crust. • Diamictites formed before the GOE have similar δ 98 Mo values as igneous rocks. • Diamictites that formed after the GOE retain isotopically light Mo. • The onset of the GOE drove Mo isotope fractionation during crustal weathering. • The weathered continental crust is an isotopically light Mo reservoir. Molybdenum isotopes in twenty-four composites of glacial diamictites spanning depositional ages of 2900 to 300 Ma show a systematic shift to lighter compositions and a decrease in Mo concentration over time. The diamictites fall into three age groups relative to the Great Oxidation Event (GOE): pre-GOE (2.43 – 2.90 Ga), syn-GOE (2.20 – 2.39 Ga), and post-GOE (0.33 – 0.75 Ga). Pre-GOE composites have an average δ 98 Mo NIST3134 of +0.03‰ (± 0.18‰), syn-GOE composites average − 0.29 ‰ (± 0.60‰), and post-GOE composites average − 0.45 ‰ (± 0.51‰). These groups are statistically different at p=0.05. We use the pre-GOE data to estimate the average Archean upper continental crust (UCC) δ 98 Mo signature as +0.03 ± 0.18‰ (2 σ), which falls within the range of previous estimates of modern igneous rocks. As the diamictites represent a mixture of igneous and weathered crust, the shift to lighter Mo values over time likely reflects Mo isotope fractionation during oxidative weathering and increased retention of light Mo isotopes in weathered regolith and soils. We hypothesize that this fractionation is due to the mobilization of oxidized Mo following the GOE, and subsequent adsorption of light Mo onto Fe-Mn oxides and/or organic matter in weathered regolith. We conclude that Mo isotopes in continental weathering products record the rise of atmospheric oxygen and onset of oxidative weathering. As the regolith formed under oxidative conditions is isotopically lighter than average continental igneous rocks, mass balance dictates that Mo isotope fractionation during oxidative weathering should result in isotopically heavy groundwater and river water, which is observed in modern systems. [ABSTRACT FROM AUTHOR]

Assessing the K isotopic composition of the upper continental crust is important for understanding the processes by which the crust is generated and modified, and constraining the K isotopic budget of the silicate Earth. High-precision K isotopic data are reported for 70 well-characterized individual and composite samples from the upper continental crust, including diorite, granodiorite, granite, loess, shale, graywacke, pelite, and tillite, to constrain its K isotopic composition. δ41K varies significantly in eight I-type and two S-type granites from Australia (−0.57 to −0.40‰), nine A-type granites (−0.53 to −0.38‰), and three granitoid composites including diorite, granodiorite, and granite (−0.50 to −0.37‰) from China, mainly reflecting source heterogeneity. The 22 post-Archean Australian shales (PAAS) (δ41K = −0.68 to −0.12‰) and the 12 sedimentary composites including graywacke, pelite, and tillite from China (δ41K = −0.57 to −0.23‰) have heterogeneous K isotopic compositions while the 12 loess samples from around the world display more limited K isotopic variation (δ41K = −0.47 to −0.35‰). δ41K values display a smaller dispersion in loess compared to shales, which have comparatively more intense weathering and higher chemical index of alteration (CIA). δ41K correlates with Fe 2 O 3 /Al 2 O 3 and Fe 2 O 3 /K 2 O in shales and Al 2 O 3 /SiO 2 , K 2 O/Al 2 O 3 , and δ7Li in loess, suggesting that K isotopes are fractionated during chemical weathering. Overall, the upper continental crust has a heterogeneous K isotopic composition, ranging from −0.68 to −0.12‰ with an average δ41K of −0.44 ± 0.05‰ (2SD, n = 88), which is indistinguishable from the mantle. [ABSTRACT FROM AUTHOR]

The upper continental crust (UCC) is an important reservoir of Ba within the Earth. We report high precision (≤±0.05‰, 2SD) Ba isotopic data for 71 samples (including granites, granodiorites, loess, glacial diamictites, and river sediments) to constrain the Ba isotopic composition of the UCC. I-type granites from the Fogang batholith, Southeastern (SE) China, exhibit variable δ 137/134 Ba (−0.16‰ to 0.01‰), which may be due to isotopic fractionation during the latest stages of magmatic differentiation. The δ 137/134 Ba of S-type granites from Darongshan-Shiwandashan, SE China (−0.03‰ to 0.11‰) correlate with ɛ Nd (t), likely reflecting mixing of heterogeneous crustal source materials. Five A-type granites with high SiO 2 contents (∼76 wt%) from Nankunshan, SE China have remarkably low δ 137/134 Ba (−0.47‰ to −0.33‰), which possibly arose from magmatic differentiation or assimilation of crustal materials with light Ba isotopic compositions. Loess from northwestern China has homogeneous δ 137/134 Ba (−0.02‰ to 0.03‰) that shows no correlation with bulk compositions, sample locations, or degree of chemical weathering, suggesting that loess is representative of the average Ba isotopic composition of the UCC. Three river sediments from northern China have δ 137/134 Ba similar to that of loess. The δ 137/134 Ba of glacial diamictites vary with CIA values: those with high CIA (≥60) have heterogeneous δ 137/134 Ba (−0.19‰ to 0.35‰), while those with low CIA (<60) have δ 137/134 Ba around 0‰, identical to the values of loess and river sediments, suggesting that Ba isotopes can be fractionated during chemical weathering. In all, samples from the UCC have highly heterogeneous δ 137/134 Ba, ranging from −0.47‰ to 0.35‰. Using the weighted average of samples in this study, the δ 137/134 Ba of the UCC is estimated to be 0.00 ± 0.03‰ (2SD/ √ n , n  = 71), which is similar to the average Ba isotopic composition of the upper mantle, but significantly lower than δ 137/134 Ba of seawater. [ABSTRACT FROM AUTHOR]

To investigate the use of cerium (Ce) in the upper continental crust (UCC) for tracing oxidative weathering, we measured the Ce anomaly (Ce/Ce*), the Ce stable isotope composition (δ142Ce), and the average Ce valence state (Ce#) in twenty-four glacial diamictite composites with depositional ages ranging from ∼2.9 to 0.3 Ga. We observe no distinguishable secular changes in Ce/Ce* (Ce/Ce* = 0.87–0.97) or δ142Ce (−0.092 ± 0.033 to 0.028 ± 0.043‰, 2 S.D.) of the diamictites. Limited elemental partitioning and isotopic fractionation of Ce can be ascribed to the immobilization of REEs, including Ce, in the diamictites during oxidative weathering. We calculated the present-day UCC δ142Ce to be −0.025 (median) ± 0.018‰ (MAD) or −0.029 (mean) ± 0.058‰ (2 S.D.). We assigned average valence state data of Ce (Ce#) in the diamictites into three age groups relative to the timing of the Great Oxidation Event (GOE): pre-GOE (2.9–2.4 Ga; Ce# = 3.09 to 3.22; average of 3.14), syn -GOE (2.4–2.2 Ga; Ce# = 3.18 to 3.73; average of 3.29), and post-GOE ages (0.8–0.3 Ga; Ce# = 3.21 to 3.68; average of 3.36). Our results show distinguishable correlations between Ce# and common weathering (major element ratios and Li isotopes) and redox tracers (e.g., Mn, Mo, V and Th/U) due to increasingly oxidative weathering with time. These results suggest that the valence state of Ce is a sensitive indicator of redox, and advise against relying solely on element- or isotope-based proxies. Combining redox proxies with valence state information can strengthen and complement the interpretation of past redox conditions. [ABSTRACT FROM AUTHOR]

Stable Ti isotopes have been applied in the detrital sediment record to reconstruct the bulk composition of Earth's continental crust due to the relationship between magmatic differentiation and Ti isotopic compositions. However, no study has systematically evaluated the influence of provenance, physical, and chemical weathering on the composition of sediments relative to the protolith they originated from. To test the influence of these processes on Ti isotopic compositions we investigate the Ti isotope composition of 82 surface samples including loess, volcaniclastic rocks, river sediment, and two separate weathering profiles through igneous rocks, collected from a broad geographical area and a range of environmental conditions. Limited but significant Ti isotope fractionation exists in samples subjected to extreme chemical weathering processes, potentially as a result of elemental mobilization. For example, the δ49Ti isotopic composition of bauxites developed on Columbia River basalt varies by up to 0.1‰, becoming isotopically heavier with increasing weathering intensity. However, negligible variation in δ49Ti was found in a second profile of saprolites developed on weathered diabase. Titanium isotope variations in loess do not correlate with chemical weathering intensity or size sorting, but may instead be related to the provenance of the sediment. We find that the δ49Ti of Amazon River sediments is correlated with the Al/Zr ratio, indicating that δ49Ti is impacted by sediment sorting. At our study sites, the river averaged offset between the isotopic composition of the bedload and the suspended sediment fraction is 0.051‰, with the largest offset being + 0.116‰. Our data suggest that during chemical weathering, heavy Ti isotopes are preferentially incorporated into secondary minerals producing higher δ49Ti in intensely weathered soils. During fluvial transport, the Ti isotopic composition of fine-grained sediment is heavier than that of its coarser counterpart. Crustal protolith composition and sorting during transport and sedimentation have a stronger effect on the Ti isotopic composition than chemical weathering. Our results have implications for studies that utilize the Ti elemental concentration to calculate relative enrichment or depletion during chemical weathering and physical transport processes in the Critical Zone and for studies using Ti isotopes in terrigenous sediments to infer the composition of their provenance. [ABSTRACT FROM AUTHOR]

Glacial diamictites, with ages ranging from ∼2900 to 0.01 Ma, record the changing composition of the upper continental crust through time ( Gaschnig et al., 2014 ). Li concentrations and isotopic compositions, combined with Pb isotopic compositions, chemical index of alteration (CIA) values and relative Sr concentrations are used here to assess the degree of chemical weathering recorded in these deposits and the origin of this signature. The δ 7 Li values of most of the diamictites (ranging from −3.9 to +3.5) are lower than those of mantle-derived basalts (+3.7 ± 2, 2 σ ), and the low δ 7 Li values are generally accompanied by high CIA and low Sr/Sr ∗ values (or Sr depletion factor, Sr/Sr ∗ = Sr/(Ce ∗ Nd) 0.5 ), reflecting a weathering signature that may have derived from pre-depositional, syn-depositional, and/or post-depositional weathering processes. Profiles through three glacial diamictites with relatively high CIA (a fresh road cut of the Neoproterozoic Nantuo Formation (CIA = 62–69), and drill cores through the Paleoproterozoic Timeball Hill (CIA = 66–75) and Duitschland Formations (CIA = 84–91)) do not show evidence of significant post-depositional weathering. High Th/U, reflecting loss of uranium during oxidative weathering, is seen in all Paleozoic and Neoproterozoic diamictites and a few Paleoproterozoic deposits. Pb isotopic systematics suggest that this signature was largely inherited from preexisting crust, although a subset of samples (the Neoproterozoic Konnarock, Paleozoic Dwyka, and several of the Paleoproterozoic Duitschland samples) appears to have experienced post-depositional U loss. Modern glaciomarine sediments record little weathering (CIA = 47, Sr/Sr ∗ = 0.7, δ 7 Li = +1.8), consistent with the cold temperatures accompanying glacial periods, and suggesting that limited syn-depositional weathering has occurred. Thus, the chemical weathering signature observed in ancient glacial diamictites appears to be largely inherited from the upper continental crust (UCC) over which the glaciers traversed. The strength of this weathering signature, based on the CIA, is greatest in the Mesoarchean and some of the Paleoproterozoic diamictites and is weaker in the Neoproterozoic and Phanerozoic glacial diamictites. Combining these data with data for Archean shales and other types of post-Paleoproterozoic sedimentary rocks (i.e., shales, mudstones, etc.), it appears that post-Paleoproterozoic upper continental crust experienced less intense chemical weathering, on average, than Archean and Paleoproterozoic upper continental crust. [ABSTRACT FROM AUTHOR]

The Archean Eon witnessed the production of early continental crust, the emergence of life, and fundamental changes to the atmosphere. The nature of the first continental crust, which was the interface between the surface and deep Earth, has been obscured by the weathering, erosion, and tectonism that followed its formation.We used Ni/Co and Cr/Zn ratios in Archean terrigenous sedimentary rocks and Archean igneous/metaigneous rocks to track the bulk MgO composition of the Archean upper continental crust. This crust evolved from a highly mafic bulk composition before 3.0 billion years ago to a felsic bulk composition by 2.5 billion years ago. This compositional change was attended by a fivefold increase in the mass of the upper continental crust due to addition of granitic rocks, suggesting the onset of global plate tectonics at ~3.0 billion years ago. [ABSTRACT FROM AUTHOR]

• Detrital zircon age and Hf isotope and whole-rock Nd isotope data are provided for Mesoarchean, Paleoproterozoic, Neoproterozoic, and Paleozoic glacial diamictites from around the world. • Hf and Nd isotope data record major episodes of crustal growth at 3.2, 2.8–2.7, 2.0–1.7, and 1.3–0.9 Ga, • Zircons older than 3.6 Ga are completely absent, as is Hf and Nd isotope evidence for reworking of greater than 3.6 Ga continental crust; such crust appears to have been rare. Diamictites produced by continental glaciations were deposited in the Mesoarchean (∼2.9 Ga), Paleoproterozoic (∼2.4–2.2 Ga), Neoproterozoic (∼0.75–0.58 Ga), and Paleozoic (∼0.3 Ga), and, with the exception of the Mesoarchean, occur on multiple modern continents. We examine the provenance of a selection of these diamictites from all four time periods and three continents via detrital zircon U-Pb geochronology and Hf isotope and whole-rock Nd isotope analyses. Mesoarchean samples from South Africa contain mostly Mesoarchean zircon, with a much smaller number of Paleoarchean grains. Zircon Hf and whole-rock Nd point towards their derivation from reworked Paleoarchean crust. Paleoproterozoic samples from North America share a similar unimodal zircon age distribution, with a peak at 2.7 Ga and a narrow range in Hf isotopes that is slightly superchondritic. Coupled with whole-rock Nd results, these data point to their derivation from reworked juvenile Mesoarchean crust. By contrast, the Nd isotope data and existing published detrital zircon U-Pb data for Paleoproterozoic diamictites from South Africa indicate a mix of Neoarchean and Mesoarchean sources. Neoproterozoic samples from Namibia share a Paleoproterozoic detrital zircon population with variable but subchondritic Hf isotope compositions, indicating derivation from reworked Neoarchean crust. Most samples also have Mesoproterozoic and Neoproterozoic zircon with both positive and negative ε Hf(i) , and all have Nd isotope compositions yielding Paleoproterozoic model ages. A Neoproterozoic sample from Canada contains mostly Neoproterozoic detrital zircon with variable Hf isotope compositions, indicating a mix of juvenile and reworked Mesoproterozoic crust. Samples from different parts of the Paleozoic Dwyka Group in South Africa show significantly different provenance, with a western locality containing only zircon older than 2.0 Ga and a Neoarchean Nd model age. By contrast, eastern samples have Paleoproterozoic model ages and published detrital zircon ages are largely Mesoproterozoic and younger. Paleozoic samples from Bolivia have a Paleoproterozoic Nd model age. Overall, the difference between zircon initial Hf isotope compositions and the depleted mantle is smallest for Archean zircons, indicating major episodes of crustal reworking followed shortly after crust formation (perhaps reflecting the last stage of cratonization). Hf and Nd data together record major episodes of juvenile crust formation at 3.2, 2.8–2.7, 2.0–1.7, and 1.3–0.9 Ga, consistent with previously documented global crust formation events. The diamictites provide no evidence for pre-3.6 Ga felsic crust in these regions. [ABSTRACT FROM AUTHOR]

Twenty-four composite samples of the fine-grained matrix of glacial diamictites deposited from the Mesoarchaean to Palaeozoic have been analysed for their silicon isotope composition and used to establish, for the first time, the long-term secular Si isotope record of the compositional evolution of upper continental crust (UCC). Diamictites with Archaean and Palaeoproterozoic Nd model ages show greater silicon isotope heterogeneity than those with younger model ages (irrespective of depositional age). We attribute the anomalously light Si isotope compositions of some diamictites with Archaean model ages to the presence of glacially milled banded iron formation (BIF), substantiated by the high iron content and Ge/Si in these samples. We infer that relatively heavy Si isotope signatures in some Palaeoproterozoic diamictites (all of which have Archaean Nd model ages) are due to contribution from tonalite-trondhjemite-granodiorites (TTGs), evidenced by the abundance of TTG clasts. By the Neoproterozoic (with model ages ranging from 2.3 to 1.8 Ga), diamictite Si isotope compositions exhibit a range comparable to modern UCC. This reduced variability through time is interpreted as reflecting the decreasing importance of BIF and TTG in post-Archaean continental crust. The secular evolution of Si isotopes in the diamictites offers an independent test of models for the emergence of stable cratons and the onset of horizontal mobile-lid tectonism. The early Archaean UCC was heterogeneous and incorporated significant amounts of isotopically light BIF, but following the late Archaean stabilisation of cratons, coupled with the oxygenation of the atmosphere that led to the reduced neoformation of BIF and diminishing quantities of TTGs, the UCC became increasingly homogeneous. This homogenisation likely occurred via reworking of preexisting crust, as evidenced by Archaean Nd model ages recorded in younger diamictites. • Ancient glacial diamictites record a secular trend in the average silicon isotope composition of the upper continental crust. • 2.90-2.30 Ga diamictites are heterogeneous for δ 30 Si compared to modern crust, due to BIF and TTG in their source. • Neoproterozoic and Palaeozoic diamictites approximate the δ 30 Si of modern upper continental crust. • Secular diamictite Si isotope trends support reworking of cratons that progressively homogenised Si isotopes in the crust. • Diamictite Si isotopes align with models supporting lithosphere stabilisation and crustal reworking after ∼3.0 Ga. [ABSTRACT FROM AUTHOR]

Abstract: High-precision Mg isotopic data are reported for ∼100 well-characterized samples (granites, loess, shales and upper crustal composites) that were previously used to estimate the upper continental crust composition. Magnesium isotopic compositions display limited variation in eight I-type granites from southeastern Australia (δ26Mg=−0.25 to −0.15) and in 15 granitoid composites from eastern China (δ26Mg=−0.35 to −0.16) and do not correlate with SiO2 contents, indicating the absence of significant Mg isotope fractionation during differentiation of granitic magma. Similarly, the two S-type granites, which represent the two end-members of the S-type granite spectrum from southeastern Australia, have Mg isotopic composition (δ26Mg=−0.23 and −0.14) within the range of their potential source rocks (δ26Mg=−0.20 and +0.15) and I-type granites, suggesting that Mg isotope fractionation during crustal anatexis is also insignificant. By contrast, δ26Mg varies significantly in 19 A-type granites from northeastern China (−0.28 to +0.34) and may reflect source heterogeneity. Compared to I-type and S-type granites, sedimentary rocks have highly heterogeneous and, in most cases, heavier Mg isotopic compositions, with δ26Mg ranging from −0.32 to +0.05 in nine loess from New Zealand and the USA, from −0.27 to +0.49 in 20 post-Archean Australian shales (PAAS), and from −0.52 to +0.92 in 20 sedimentary composites from eastern China. With increasing chemical weathering, as measured by the chemical index of alternation (CIA), δ26Mg values show a larger dispersion in shales than loess. Furthermore, δ26Mg correlates negatively with δ7Li in loess. These characteristics suggest that chemical weathering significantly fractionates Mg isotopes and plays an important role in producing the highly variable Mg isotopic composition of sedimentary rocks. Based on the estimated proportions of major rock units within the upper continental crust and their average MgO contents, a weighted average δ26Mg value of −0.22 is derived for the average upper continental crust. Our studies indicate that Mg isotopic composition of the upper crust is, on average, mantle-like but highly heterogeneous, with δ26Mg ranging from −0.52 to +0.92. Such large isotopic variation mainly results from chemical weathering, during which light Mg isotopes are lost to the hydrosphere, leaving weathered products (e.g., sedimentary rocks) with heavy Mg isotopes. [Copyright &y& Elsevier]

Abstract: In situ Li analyses of 4348 to 3362 Ma detrital zircons from the Jack Hills, Western Australia by SIMS reveal that the Li abundances (typically 10 to 60 ppm) are commonly over 10,000 times higher than in zircons crystallized from mantle-derived magmas and in mantle-derived zircon megacrysts (typically <2 ppb). High Li concentrations in zircons (10 to 250 ppm) have also been found in igneous zircons from three continental parent rocks: granites, Li-rich pegmatites, and migmatites in pelitic metasediment. The substitution of trivalent cations (REEs and Y) in zircon correlates with Li+1 and P+5, suggesting that an interstitial site for Li, as well as the xenotime substitution for P, provides charge balance for REEs. Li is thus fixed in the zircon structure by coupled substitutions, and diffusive changes in [Li] composition are rate-limited by slow diffusion of REEs. The Jack Hills zircons also have fractionated lithium isotope ratios (δ 7Li=−19 to+13‰) about five times more variable than those recorded in primitive ocean floor basalts (2 to 8‰), but similar to continental crust and its weathering products. Values of δ 7Li below −10‰ are found in zircons that formed as early as 4300 Ma. The high Li compositions indicate that primitive magmas were not the source of Jack Hills zircons and the fractionated values of δ 7Li suggest that highly weathered regolith was sampled by these early Archean magmas. These new Li data provide evidence that the parent magmas of ancient zircons from Jack Hills incorporated materials from the surface of the Earth that interacted at low temperature with liquid water. These data support the hypothesis that continental-type crust and oceans existed by 4300 Ma, within 250 million years of the formation of Earth and the low values of δ 7Li suggest that weathering was extensive in the early Archean. [Copyright &y& Elsevier]

• First estimate of the δ 94 / 90 Zr IPGP-Zr of the UCC using diverse sedimentary samples. • No measurable Zr isotope fractionation due to chemical weathering and mineral sorting. • No evidence for Zr isotope fractionation of the UCC through space and time. The stable isotopic composition of insoluble, refractory elements such as titanium (Ti) or zirconium (Zr), which are modified by magmatic differentiation but, a priori , are poorly affected by weathering or diagenesis, serve as powerful potential proxies to reconstruct the compositional evolution of the continental crust. Here we present the evolution of the Zr stable isotopic compositions (δ 94 / 90 Zr IPGP-Zr , per mille deviation of 94Zr/90Zr from IPGP-Zr standard) of the continental crust through time, using 38 sedimentary samples from the upper continental crust (UCC), including 12 Holocene loesses from the Chinese Loess Plateau and Xinjiang, three oceanic sediments from the sea floor outboard of the Lesser Antilles island arc and 23 glacial diamictite composites with depositional ages ranging from ∼ 2.9 Ga to 0.3 Ga from South Africa, South America, Canada, USA and China. The samples show limited Zr isotopic variations with δ 94 / 90 Zr IPGP-Zr values of 0.043‰ - 0.109‰ for loess; 0.069‰ - 0.083‰ for oceanic sediments and 0.031‰ - 0.118‰ for glacial diamictites; their Zr-weighted average values are, 0.081 ± 0.044‰ (2SD, n = 12), 0.073 ± 0.015‰ (2SD, n = 3) and 0.078 ± 0.047‰ (2SD, n = 23), respectively. The isotopic similarity among loess, oceanic sediments and glacial diamictites, suggests that zircon enrichment effects previously documented in some sedimentary samples have not biased the Zr isotope compositions of these sedimentary rocks from their source rocks. Two groups with or without Zr enrichment have similar average δ 94 / 90 Zr IPGP-Zr values (0.075 ± 0.040‰ and 0.080 ± 0.046‰). There is no correlation between Zr isotope compositions and any proxy of chemical weathering (e.g., Al 2 O 3 /SiO 2 , Fe 2 O 3 /SiO 2 , CIA, K 2 O/Al 2 O 3 and δ 7 Li). The δ 94 / 90 Zr IPGP-Zr values are quite constant for these sedimentary samples regardless of their depositional ages and locations. Therefore, the UCC appears to have had a constant Zr isotopic composition between 3 Ga and present, and is homogeneous at a large scale. Combining data for sedimentary reference materials from the literature and the sedimentary rocks in this study, we suggest a Zr-weighted δ 94 / 90 Zr IPGP-Zr value of 0.077 ± 0.058‰ (2SD, n = 44) for the UCC, which is statistically distinct (t test, p value = 2.88 × 10−10) and higher than that of the mantle (0.040 ± 0.044‰, n = 72). Combining the δ 94 / 90 Zr IPGP-Zr values of different terrestrial reservoirs, the δ 94 / 90 Zr IPGP-Zr of the BSE and bulk Earth is constrained to be 0.041 ± 0.041‰. [ABSTRACT FROM AUTHOR]