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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]

Carbonatite metasomatism plays an important role in modifying the composition of Earth's mantle, however, its effect on mantle Mg isotopic composition is poorly constrained. Here, we report high-precision mineral Mg isotope data for three suites of mantle peridotite xenoliths that experienced variable degrees of carbonatite metasomatism. The δ 26 Mg values of minerals in these xenoliths are variable and range from −0.32 to −0.11‰ in olivine, from −0.28 to −0.09‰ in orthopyroxene, from −0.27 to −0.05‰ in clinopyroxene, from 0.06 to 0.44‰ in spinel and from −0.61 to −0.37‰ in garnet. Calculated bulk-rock δ 26 Mg values of the peridotites vary from −0.27 to −0.10‰, falling within and slightly higher than the normal mantle range (−0.25 ± 0.07‰). The coexisting minerals are in isotopic equilibrium, with clinopyroxene δ 26 Mg values correlated with the carbonatite metasomatic indices such as MgO and Na 2 O in orthopyroxene. These results suggest that carbonatite metasomatism does not produce light Mg isotopic signature in mantle peridotites as previously suggested, instead it might slightly elevate their δ 26 Mg values. Therefore, carbonatite-metasomatized peridotites in the mantle cannot be the primary source rocks of low- δ 26 Mg mantle-derived magmas. Instead, fractional crystallization and accumulation of chromite during ascent of the basaltic magmas may explain the isotopically light basalts, as supported by the covariations of δ 26 Mg with chemical indices of chromite crystallization (e.g., Cr, V, Fe and Ti). Consequently, chromite crystallization may significantly influence the physiochemical processes on the genesis of basalts, which would require comprehensive evaluation in future studies. • First report of Mg isotopic compositions of mantle carbonatite-metasomatized peridotites. • Overall normal-mantle Mg isotopic compositions of major constituent minerals. • No relation of carbonatite metasomatized peridotites with low- δ 26 Mg mantle-derived magmas. • Significant effects of chromite crystallization on Mg isotopic compositions and genesis of continental basalts. [ABSTRACT FROM AUTHOR]

Carbonate-silicate interaction within subduction channels influences the chemical and physical properties of subducting carbon and is crucial to understanding the fate of carbonates in subduction zones. However, geochemical evidence for such interaction is still limited. Here we investigate zinc isotopes (expressed as δ 66 Zn JMC-Lyon) to decipher potential carbonate-silicate interaction in subducting slabs given the remarkable Zn isotopic difference between silicate and carbonate components. A suite of high- to ultrahigh-pressure metasediments containing variable amounts of carbonates from the Schistes Lustrés nappe (western Alps) record subduction processes to various depths (∼15–90 km) in a cool geothermal condition (∼8 °C/km). Despite a broad range of metamorphic grades (0.8–2.9 GPa and 300–630 °C) and carbonate contents (0–49 wt%), the bulk metasediments have nearly constant δ 66 Zn values (av. 0.21 ± 0.05 ‰ , 2sd, n = 12), which are indistinguishable from their putative protoliths (0.25 ± 0.07 ‰ , 2sd, n = 5). The lack of systematic variation of bulk sediment δ 66 Zn with metamorphic grades implies a limited transfer of Zn during prograde metamorphism. Therefore, sediments will preserve their original Zn isotopic compositions after subduction into the mantle. By contrast, the carbonate components in sediments obtained by leaching exhibit a progressive decline in δ 66 Zn from 0.82 ± 0.01 ‰ to 0.20 ± 0.04 ‰ and a modest increase in Zn concentration with pressure. These variations are best interpreted as a result of prolonged closed-system Zn isotopic exchange between isotopically heavy carbonates and light silicate components in the subducting slab. Our results demonstrate that a considerable amount of carbonates in subducting sediments can be retained at least down to the depths of the sub-arc mantle (>90 km). The strong pressure-dependent Zn isotopic fractionation (R 2 = 0.87) during carbonate-silicate interaction makes Zn isotopes a novel proxy for the storage depth of subducting carbonates. • A systematic zinc isotope study on western Alpine metasediments. • Prograde metamorphism has limited impact on Zn isotopes of bulk subducting sediments. • Variation of δ 66 Zn in carbonates reflects interaction with silicates in a closed system. • Zn isotopes are an indicator for the storage depth of subducting carbonates. [ABSTRACT FROM AUTHOR]

• 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]