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• Zircon and titanite in xenoliths document Eocene metasomatism of the Wyoming craton lower crust. • Metasomatism by hydrous and carbonated fluids linked to subduction of the Shatsky conjugate and Farallon slab beneath the northern margin of the craton. • Metasomatism, slab foundering, and heating triggered partial loss of the Wyoming craton lithosphere. New zircon and titanite U-Pb and trace-element data from minette- and kimberlite-hosted lower crustal xenoliths from near the northern margin of the Wyoming craton document multiple periods of metasomatism from 62 to 50 Ma. Metasomatism was either coincident or preceded eruption of the host magmas by up to 14 Myr (eruption dated at 52 Ma and 48 Ma based on new zircon U-Th/He data). Thus, fluid/melt interactions were not solely related to magmatic entrainment and eruption. Accessory mineral trace element data reveal metasomatism by fluids from multiple sources, including water and carbonatite potentially sourced from the subducted Farallon slab. These metasomatic episodes are interpreted to reflect pan-lithospheric hydration events that weakened the deep cratonic lithosphere and facilitated partial removal following rollback and foundering of the Farallon plate. The new data corroborate previous suggestions that subduction-related metasomatism may be a key requisite for the destruction of otherwise stable cratons. [ABSTRACT FROM AUTHOR]

Micron- to submicron-scale observations of Li distribution and Li isotope composition profiles can be used to infer the mechanisms of Li diffusion in natural zircon. Extreme fractionation (20–30‰) within each single crystal studied here confirms that Li diffusion commonly occurs in zircon. Sharp Li concentration gradients frequently seen in zircons suggest that the effective diffusivity of Li is significantly slower than experimentally determined ( Cherniak and Watson, 2010; Trail et al., 2016 ), otherwise the crystallization/metamorphic heating of these zircons would have to be unrealistically fast (years to tens of years). Charge coupling with REE and Y has been suggested as a mechanism that may considerably reduce Li diffusivity in zircon ( Ushikubo et al., 2008; Bouvier et al., 2012 ). We show that Li diffused in the direction of decreasing Li/Y ratio and increasing Li concentration (uphill diffusion) in one of the zircons, demonstrating charge coupling with REE and Y. Quantitative modeling reveals that Li may diffuse in at least two modes in natural zircons: one being slow and possibly coupled with REE+Y, and the other one being fast and not coupled with REE+Y. The partitioning of Li between these two modes during its diffusion may depend on the pre-diffusion substitution mechanism of REE and Y in the zircon lattice. Based on our results, sharp Li concentration gradients are not indicative of limited diffusion, and can be preserved at temperatures >700 °C on geologic timescales. Finally, large δ 7 Li variations observed in the Hadean Jack Hills zircons may record kinetic fractionation, rather than a record of ancient intense weathering in the granite source materials. [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]

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]

Glacial diamictites deposited in the Mesoarchean, Paleoproterozoic, Neoproterozoic, and Paleozoic eras record temporal variations in their average compositions that reflect the changing composition of the upper continental crust (UCC). Twenty six of the 27 units studied show elevated chemical index of alternation (CIA) and low Sr abundances, regardless of their age, documenting pervasive weathering of the average UCC. Lower abundances of transition metals reflect a shift towards more felsic crustal compositions after the Archean. Superimposed on this chemical difference is the signal of the rise of oxidative weathering of the continents, recorded by changes in the absolute and relative abundances of the redox sensitive elements Mo and V. Neoproterozoic and Paleozoic diamictites show pervasive depletion in Mo and V, reflecting their loss from the continents due to increasing intensity of oxidative weathering, as also recorded in some of the Paleoproterozoic diamictites. A few of the Paleoproterozoic diamictites deposited after the Great Oxidation Event show no depletion in Mo and V (e.g., Gowganda), but such signatures could be inherited from their provenance. In contrast, the pre-GOE Duitschland diamictite (ca. 2.3–2.5 Ga) from South Africa reveals evidence of intense oxidative weathering (i.e., large depletions in Mo), supporting a growing body of observations showing the presence of measurable atmospheric oxygen prior to permanent loss of the mass independent fractionation signal in sulfur isotopes. [ABSTRACT FROM AUTHOR]

Abstract: To investigate the behavior of Li during low-grade metamorphism and fluid flux in an accretionary prism we measured the Li concentrations ([Li]) and isotopic compositions (δ7Li) of sub-greenschist and greenschist-facies Otago Schist composites, as well as cross-cutting quartz veins, which are interpreted to have precipitated from slab-derived fluids. The average [Li] of sub-greenschist facies composites (41±13μg/g, 2σ) is statistically distinct (97% confidence level, student t test) to that of greenschist facies composites (34±9μg/g, 2σ), which have experienced mass addition of silica in the form of quartz veins having [Li] between 0.4–2.3μg/g. A linear regression of the correlation between [Li] and calculated mass additions suggests that the depletion of [Li] in greenschist facies composites is due to both dilution from the addition of the quartz veins, as well as metamorphic dehydration. The [Li] of both groups of composites correlates with their CIA (Chemical Index of Alteration) values (50–58), which are low, consistent with the inferred graywacke protolith of the Otago Schist. The δ7Li of sub-greenschist and greenschist facies composites are remarkably constant, with an average δ7Li of 0.2±1.7 (2σ) and −0.5±1.9 (2σ), respectively, and comparable to that of the average upper continental crust. Thus, metamorphism has had no discernable effect on δ7Li in these samples. The Li isotopic signature of the schists is similar to that seen in pelitic sedimentary rocks and likely reflects the δ7Li of the protoliths. The surprisingly light δ7Li of the quartz veins (−2.8 to −1.4) likely records kinetic fractionation associated with Li ingress into the veins from surrounding wallrock. An isotopic equilibrium fluid flow model indicates that: 1) if the [Li] of slab-derived fluids is less than a few μg/g, the δ7Li of the overlying lithologies (i.e., the schists) is not significantly influenced by the fluid flux, regardless of the δ7Li of the fluids, 2) the slab-derived fluids will have heavy δ7Li of > +10 after reacting with the prism sediments during their ascent, and 3) the [Li] of the slab-derived fluids is likely in the range of 0<[Li]≤41(μg/g). Thus, isotopically heavy slab-derived fluids that traverse sediments in accretionary prisms may leave little trace in the rocks and their surface compositional characteristics will reflect the net result of their interaction with the sediments of the prism. [Copyright &y& Elsevier]

Abstract: Xenolithic peridotites having a similar range of major element compositions from two nearby localities in the Trans-North China Orogen, North China Craton, provide a rare opportunity to explore effects resulting from both primary partial melting and secondary processes on Os isotopes and highly siderophile element (HSE) abundances. HSE patterns of peridotites from Hannuoba are similar to those of orogenic peridotite massifs worldwide, but are rare for xenolithic peridotites. These patterns can be explained by relatively low degrees of melt depletion, coupled with long-term preservation of sulfides. By contrast, peridotites from Yangyuan have major element compositions similar to or slightly more depleted than Hannuoba xenoliths, but are characterized by distinct, highly fractionated HSE patterns with lower total HSE abundances and Os, Pd and Re depletions relative to Ir. Some of the latter HSE characteristics must reflect secondary processes. The low S and Se contents of Yangyuan peridotites, coupled with scarcity of observable sulfides, suggest that they experienced sulfide breakdown, possibly as a result of interaction with a S-undersaturated melt/fluid. This may have occurred under oxidizing conditions, as suggested by the somewhat higher ƒO2 recorded in the Yangyuan peridotites compared to the Hannuoba peridotites, as well as the metal-deficient composition of rare, mono-sulfide-solid solution (mss) sulfides within the Yangyuan peridotites. We speculate that under such conditions, Os, Pd, and possibly Re, more strongly partition into a sulfide liquid, or the oxidizing medium (melt or fluid), than Ir and Pt and, thus, become depleted. These effects would have been imposed on original patterns that were similar to those in the Hannuoba suite. The good correlation between 187Os/188Os and major element indices of melt depletion in the Yangyuan rocks, coupled with the poor correlation between 187Os/188Os and 187Re/188Os, suggests that the S, Os, Pd and Re removal was recent. Hence, the long-term Re–Os isotopic systematics of these rocks would not have been affected, and Re depletion model ages, based on Os isotopes, remain viable to constrain the timing of melt deletion in these peridotites. The similarity of model age distributions between Yangyuan and Hannuoba peridotites (TRD =0 to 1.7 and 0 to 1.5Ga, respectively) is consistent with this, and further indicates that these peridotites formed in the Paleoproterozoic. [Copyright &y& Elsevier]

Abstract: Early Cretaceous alkaline picrites and high-magnesium basalts from the North China craton provide evidence for recycling of continental lithosphere by density foundering. Both the picrites and basalts contain xenocrystic olivines with high Fo92–93 and low CaO (<0.10%), consistent with the lavas'' derivation from, or interaction with Archean mantle lithosphere. Most importantly, both the picritic and basaltic lavas contain unusual, reversely zoned clinopyroxene phenocrysts whose cores have low MgO, high Na2O (up to 2.4 wt.%, or 17.3 mol% Jd), and frequently contain ilmenite exsolution lamellae, consistent with their crystallization from an eclogite-derived melt (tonalite or trondhjemite). In contrast, the clinopyroxene exteriors have low Na2O (<0.92 wt.%, or <6.5 mol% Jd) and are lamellae-free, suggesting crystallization from a mantle-derived melt (picrite or basalt). Both the cores and exteriors have high Al2O3 contents (up to 6.9 wt.%). These features reflect crystallization of the cpx from an aluminous melt at mantle depths, with the cores forming at a significantly greater depth (≥2.5 GPa) than the surrounding cpx (≥1.5 GPa). Calculated primary melt compositions further constrain the magmas'' formation at 3–4 GPa, in the presence of garnet. The unusually low CaO, high Ni/MgO and low 100Mn/Fe of primary melts indicate derivation of both the picritic and basaltic lavas from pyroxenite sources containing limited or no olivine. High Sr/Y, La N /Yb N and Th/U and low Lu/Hf, together with radiogenic initial 87Sr/86Sr and 187Os/188Os ratios and negative ε Nd values implicate contributions from melts derived from foundered eclogitic lower continental crust. Modelling suggests that the basalt source region contained a variable proportion (30–40%) of eclogite-derived component whereas the source of the picrites, on average, likely contained a generally higher proportion (60–70%) of a different eclogite-derived component. Collectively, these results suggest that both the basaltic and picritic lavas originated by partial melting of Archean lithospheric mantle that was variably hybridised by melts derived from foundered lower crustal eclogite. Together with previous studies, these findings provide new evidence that thinning of the North China craton was caused by the removal of the lower lithosphere (mantle and lower crust). Recycling and melting of eclogitic lower crust may contribute more to mantle heterogeneity than has previously been recognized. [Copyright &y& Elsevier]

Abstract: Lithium concentrations and isotopic compositions of coexisting olivine and clinopyroxene (cpx) in well-characterized peridotite xenoliths from Tok (SE Siberian craton) and samples from two other far-east Russian localities reveal strong elemental and isotopic disequilibria, which correlates with bulk rock composition. Lithium concentrations in cpx from Tok (1–12 ppm) are equal to or significantly greater than those in coexisting olivines (1–5 ppm). The Li-rich cpx show core to rim zoning, indicative of Li infiltration from the grain boundaries. Olivines are generally unzoned, although Li concentrations can vary significantly from grain to grain. ol/cpx D varies from 0.2 to 1.0, which is lower than that expected for equilibrium partitioning (ol/cpx D eq =1.1 to 2.0), and reflects preferential Li enrichment in cpx. The Li isotopic compositions of both minerals range far beyond normal mantle δ 7Li of ∼ +4±2. δ 7Licpx (−0.8 to −14.6) is systematically lighter than δ 7Li of coexisting olivine (−1.7 to +11.9), and Δ7Liol-cpx varies from 2.8 to 22.9‰. The greatest elemental and isotopic disequilibria occur in the most fertile samples (lherzolites) and may reflect longer equilibration times and/or enhanced melt permeability in the more refractory samples. Collectively, these observations suggest that the peridotite minerals experienced Li addition via diffusion from a grain boundary melt or fluid shortly before or coincident with their entrainment into the host basalt (i.e., within tens of thousands of years, based on published diffusion coefficients for Li in cpx at the temperatures of equilibration). This diffusional ingress of Li generated large kinetic isotopic fractionation, leading to unusually light cpx and heavier olivines. Thus, low δ 7Licpx do not reflect the influence of an exotic mantle component related to crustal recycling. [Copyright &y& Elsevier]

Re–Os data for peridotite xenoliths carried in Paleozoic kimberlites and Tertiary alkali basalts confirm previous suggestions that the refractory and chemically buoyant lithospheric keel present beneath the eastern block of the North China craton (and sampled by Paleozoic kimberlites) is indeed Archean in age and was replaced by more fertile lithospheric mantle sometime after the Paleozoic. Moreover, lithospheric mantle beneath the central portion of the craton (west of the major gravity lineament) formed during the last major Precambrian orogeny, around 1900 Ma ago. This age is significantly younger than the overlying crust (2700 Ma), suggesting that the original Archean lithosphere was replaced in the Proterozoic. The timing of lithospheric replacement in the eastern block of the North China Craton is constrained only to the Phanerozoic by the Re–Os results. Circumstantial geologic evidence suggests this new lithosphere is Jurassic or Cretaceous in age and formed after collision of the Yangtze and North China cratons in the Triassic, an event that was also responsible for the subduction and uplift of ultrahigh-pressure metamorphic rocks. Collectively, these data suggest that lithosphere replacement occurred in response to two continent collisional events widely separated in time (∼1900 and ∼220 Ma). Coupled with observations from other Archean cratons we suggest that wholesale replacement of lithospheric mantle (±lower crust) may require large-scale continental collision. [Copyright &y& Elsevier]

• 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]

We evaluate the factors influencing the abundance, [Li], and isotopic composition of riverine Li delivered to the oceans through analyses and modeling of [Li] and δ 7 Li in streams and groundwaters draining a single continental lithology, the Columbia River Basalts (CRBs). The streams were sampled in different climate zones that lie east (dry), and west (wet) of the Cascades Mountains, and during two different seasons (summer and late winter) in order to evaluate climatic and seasonal influences on Li isotopes in rivers. Dissolved Li ( δ Li dis 7 = + 9.3 to +30.4) is systematically heavier than that of fresh or weathered CRBs (−4.7 to +6.0, Liu et al., 2013 ), suspended loads (−5.9 to −0.3), and shallow groundwaters (+6.7 to +9.4), consistent with previous studies showing that Li isotope fractionation is affected by equilibration between stream water and secondary minerals. However, the lack of correlation between δ 7 Li dis and climate zone, the uniform secondary minerals and bedrock, coupled with the highly variable ( > 20 ‰ ) δ Li dis 7 indicate that other factors exert a strong control on δ 7 Li dis . In particular, the heavier Li in streams compared to the shallow groundwaters that feed them indicates that continued isotopic fractionation between stream water and suspended and/or bed loads has a major influence on riverine δ 7 Li. Seasonal δ 7 Li variation is observed only for streams west of the Cascades, where the difference in precipitation rate between the dry and wet seasons is greatest. Reactive transport model simulations reveal that riverine δ 7 Li is strongly controlled by subsurface residence times and the Li isotope fractionation occurring within rivers. The latter explains why there is no positive correlation between δ 7 Li and traditional weathering proxies such as Si or normalized Si in rivers, as riverine Li isotope fractionation drives δ 7 Li to higher values during transport, whereas the concentrations of major cations and anions are diluted. The varying residence time for groundwaters feeding the western streams in summer (long residence times, higher δ 7 Li, greater weathering) and winter (short residence times, lower δ 7 Li, less weathering) explains the observed seasonal variations. A global, negative correlation between δ 7 Li and Li/Na for streams and rivers draining basaltic catchments reflects the overall transport time, hence the amount of silicate weathering. Based on our results, the increase of δ 7 Li in seawater during the Cenozoic is unlikely related to changing climate, but may reflect mountain building giving rise to increased silicate weathering. [ABSTRACT FROM AUTHOR]

Abstract: Common and radiogenic Pb isotopic compositions of plagioclase and anti-perthitic feldspars from granulite-facies lower crustal xenoliths from the Labait Volcano on the eastern margin of the Tanzanian Craton have been measured via laser ablation MC-ICP-MS. Common Pb in plagioclase and a single stage Pb evolution model indicate that the lower crust of the Tanzanian Craton was extracted from mantle having a 238U/204Pb of 8.1±0.3 and a 232Th/238U of 4.3±0.1 at 2.71±0.09Ga (all uncertainties are 2σ). Since 2.4Ga, some orthoclase domains within anti-perthites have evolved with a maximum 238U/204Pb of 6 and 232Th/238U of 4.3. The spread in Pb isotopic composition in the anti-perthitic feldspars yields single crystal Pb–Pb isochrons of ∼2.4Ga, within uncertainty of U–Pb zircon ages from the same sample suite. The Pb isotopic heterogeneities imply that these granulites resided at temperatures <600°C in the lower crust of the Tanzanian Craton from ca. 2.4Ga to the present. In concert with the chemistry of surface samples, mantle xenoliths, and lower crustal xenoliths, our data imply that the cratonic lithosphere in Tanzania formed ca. ∼2.7Ga, in a convergent margin setting, and has remained undisturbed since 2.7Ga. [ABSTRACT FROM AUTHOR]

Abstract: Magnesium isotopic compositions of a profile through saprolites developed on a diabase dike from South Carolina have been measured in order to study the behavior of Mg isotopes during continental weathering. As weathering progresses, Mg isotopes are greatly fractionated and are correlated with Mg concentration, clay mineral proportions and density of the saprolites. δ26Mg values increase from −0.22 in the unweathered diabase to +0.65 in the most weathered saprolite. These observations are consistent with the release of light Mg to the hydrosphere and formation of isotopically heavy Mg in the weathered products. The loss of Mg during weathering can be modeled by Rayleigh distillation with an apparent fractionation factor between the saprolite and fluid (α) of 1.00005 to 1.0004, i.e., up to 0.4‰ fractionation in the 26Mg/24Mg ratio between the saprolite and fluid. The large variation in α value reflects a mineralogical control on Mg isotope fractionation during primary dissolution of Mg-rich minerals and formation of secondary minerals during continental weathering. Like Mg isotopes, Li isotopes in the saprolite profile are also greatly fractionated, with δ7Li values ranging from −6.7 down to −20. The large Li isotope fractionation and variation in Li concentration, as well as irregularities in the δ7Li profile with depth, however, cannot be explained by Li loss during weathering alone. Instead, Li can be modeled by a two-step process: (1) equilibrium isotope fractionation during continental weathering, which lowered δ7Li and Li concentrations and produced a Li concentration gradient in the saprolites like that seen in Mg, and (2) subsequent kinetic isotope fractionation produced by diffusion of Li in the saprolites, possibly across a paleo-water table. The results presented here suggest that continental weathering will shift the Mg isotopic composition of the continental crust to values higher than the mantle value, whereas crustal recycling over the history of the Earth will have no discernible effect on the Mg isotopic composition of the mantle. [Copyright &y& Elsevier]

Abstract: Carbonatites are mantle-derived, intraplate magmas that provide a means of documenting isotopic variations of the Earth''s mantle through time. To investigate the secular Li isotopic evolution of the mantle and to test whether Li isotopes document systematic recycling of material processed at or near the Earth''s surface into the mantle, we analyzed the Li isotopic compositions of carbonatites and spatially associated mafic silicate rocks. The Li isotopic compositions of Archean (2.7 Ga) to Recent carbonatites (δ 7Li=4.1±1.3 (n =23, 1σ)) overlap the range typical for modern mantle-derived rocks, and do not change with time, despite ongoing crustal recycling. Thus, the average Li isotopic composition of recycled crustal components has not deviated greatly from the mantle value (~+4) and/or Li diffusion is sufficiently fast to attenuate significant heterogeneities over timescales of 108 years. Modeling of Li diffusion at mantle temperatures suggests that limited δ 7Li variation in the mantle through time reflects the more effective homogenization of Li in the mantle compared to radiogenic isotope systems. The real (but limited) variations in δ 7Li that exist in modern mantle-derived magmas as well as carbonatites studied here may reflect isotopic fractionation associated with shallow-level processes, such as crustal assimilation and diffusive isotopic fractionation in magmatic systems, with some of the scatter possibly related to low-temperature alteration. [Copyright &y& Elsevier]

Abstract: Lithium concentrations and Li isotope compositions are reported for natrocarbonatites and silicate lavas from Oldoinyo Lengai, Tanzania. Natrocarbonatites are characterized by very high Li contents (211–294 ppm) and a narrow range of δ 7Li values between +3.3 and +5.1. These Li isotope compositions overlap with those reported for MORB and OIB and suggest that the natrocarbonatites reflect the Li isotopic composition of their mantle source. Co-genetic silicate lavas, covering a wide compositional spectrum, show no obvious isotopic fractionation as a function of igneous differentiation or liquid immiscibility. Primitive olivine melilitites (Mg#=58–70), considered to be parental magmas, contain 14–23 ppm Li and have δ 7Li values of +2.4 to +4.4. A highly differentiated, peralkaline nephelinite (Mg#=12), likely to be related to the natrocarbonatites by liquid immiscibility, has about twice as much Li as the melilitite (57 ppm), but a similar isotopic composition (δ 7Li=+3). In contrast, a phonolite with 15 ppm Li has a lighter Li isotope composition (δ 7Li=−0.4), which may reflect assimilation of isotopically light lower crustal mafic granulites, a conclusion supported by radiogenic isotope data. Clinopyroxene and olivine separates from the silicate lavas have uniformly lower Li concentrations (3–15 ppm) and lower δ 7Li values (δ 7Li=−2.9 to −0.5) than the respective whole-rocks, with Δ 7Liwhole-rock-mineral between 1.4 and 6.3. This difference between whole-rock and mineral data is interpreted to reflect diffusion-driven isotopic fractionation. [Copyright &y& Elsevier]

Abstract: Lithium concentrations and isotopic compositions in the country rocks (amphibolites and schists) of the Tin Mountain pegmatite show systematic changes with distance to the contact. Both Li and δ 7Li decrease dramatically along a ∼10m traverse from the pegmatite into amphibolite, with Li concentration decreasing from 471 to 68ppm and δ 7Li decreasing from +7.6 to −19.9. Rubidium and Cs also decrease from the pegmatite contact into the country rock, but only within the first 2m of the contact, after which their concentrations remain constant. Neither mixing between pegmatite fluids and amphibolite, nor Li isotope fractionation by Rayleigh distillation during fluid infiltration is a likely explanation of these observations, due to the extremely light isotopic composition required for the amphibolite end-member in the mixing model (δ 7Li=−20) and the similarly extreme isotopic fractionation required in a Rayleigh distillation model. Rather, these variations are likely due to isotopic fractionation accompanying Li diffusion from the Li-rich pegmatite (Li=450 to 730ppm) into amphibolites (Li=20ppm). The fact that other alkali element concentrations vary only within 2m of the contact reflects the orders of magnitude faster diffusion of Li relative to heavier elements. Quartz mica schists in contact with the pegmatite also show large variations in both Li and δ 7Li as a function of distance from contact (∼1wt.% to ∼70ppm and +10.8 to −18.6, respectively), but over a longer distance of >30m. Lithium concentrations of the schist decrease from ∼1wt.% adjacent to the contact to ∼70ppm 300m from the contact; the latter is a typical concentration in metapelites. The nature of the δ 7Li variations in the schists is different than in the amphibolites. Schists within the first 2m of the contact have nearly identical δ 7Li of +10, which mimics that of the estimated bulk pegmatite (+8 to +11). At a distance of 30m the δ 7Li reaches the lowest value in the schists of −18.6 (similar to the lowest amphibolite measured). At a distance of 300m the δ 7Li climbs back to +2.5, which is within the range of δ 7Li of other schists in the region and metapelites worldwide. The behavior of Li in the schists can also be modeled by Li diffusion, with the effective diffusion coefficient in the schist being ∼10 times greater than that in the amphibolite. The effective diffusion coefficients of Li in the amphibolite and schist are >2 orders of magnitude greater that those in minerals, which implicates the importance of fluid-assisted grain-boundary diffusion over solid-state diffusion in transporting Li through these rocks. [Copyright &y& Elsevier]

Eclogites from Trescolmen, Switzerland, derive from basaltic protoliths that experienced variable degrees of low-T seafloor alteration prior to high pressure metamorphism. δ7Li of the eclogites (−11 to +5‰) ranges to dramatically lower values than observed in fresh MORB, or altered MORB (+4.5 to +14‰ for low-T and −2 to +8‰ for high-T altered MORB). These low values cannot be explained by fluid interaction with surrounding garnet mica schist, which is generally isotopically heavier (δ7Li of +2.4 to +3.8‰). The low δ7Li values were likely produced by isotope fractionation through Rayleigh distillation during dehydration of clays and/or chlorite at early stages of metamorphism. These data are consistent with isotopically heavy Li being released into the forearc mantle wedge in subduction zones, while an isotopically light component is subducted deeply, and may form a distinct mantle reservoir that could be sampled by plume-related magmas. [Copyright &y& Elsevier]

• Zn isotopes in cratonic eclogites support their origin as carbonated oceanic crust. • Isotopic exchange of AOC protoliths with carbonates results in high- δ 66 Zn eclogites. • High-MgO eclogites were products of metasomatic overprinting of low-MgO eclogites. • Carbonated eclogites act as a potential source of high- δ 66 Zn intraplate basalts. Zinc isotopic compositions (δ 66 Zn JMC-Lyon) of low-MgO (<13 wt.%) and high-MgO (>16 wt.%) eclogites from the Koidu kimberlite complex, Sierra Leone, West African Craton, help constrain the origins of cratonic eclogites. The δ 66 Zn of low-MgO eclogites range from MORB-like to significantly higher values (0.21‰ to 0.75‰), and correlate inversely with Zn concentrations. Since marine carbonates are characterized by higher δ 66 Zn and lower Zn concentration than basaltic rocks, the low-MgO eclogites are suggested to originate from altered oceanic crustal protoliths that underwent isotopic exchange with carbonates within the crust during subduction. Compared to low-MgO eclogites, all but one of the high-MgO eclogites also have high δ 66 Zn (0.35‰ to 0.95‰), but they have lower Zn concentrations and Zn/Fe ratios, both of which are negatively correlated with MgO contents. These features point to formation of high-MgO eclogites via metasomatic overprinting of low-MgO eclogites through addition of secondary clinopyroxenes crystallized from infiltrating ultramafic melts. Thus, both low-MgO and high-MgO eclogites bear the imprint of subducted carbonate-bearing oceanic crust. Our study shows that the distinctively high- δ 66 Zn signatures of marine carbonates can be retained in deeply subducted oceanic crust that may contribute to mantle sources of intraplate alkali basalts with elevated δ 66 Zn and Zn/Fe. Therefore, Zn isotopes provide a viable means to trace carbonate recycling in the mantle. [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: 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]