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Magnesium isotopic compositions of olivine, clinopyroxene, and ilmenite from the Baima intrusion, SW China, for the first time, are investigated to constrain the magnitude and mechanisms of Mg isotope fractionation among cumulus minerals in layered mafic intrusions and to evaluate their geological implications. Olivine and clinopyroxene have limited Mg isotope variations, with δ 26 Mg ranging from −0.33 to +0.05‰ and from −0.29 to −0.13‰, respectively, similar to those of mantle xenolithic peridotites. By contrast, ilmenites display extremely large Mg isotopic variation, with δ 26 Mg ranging from −0.50 to +1.90‰. The large inter-mineral fractionations of Mg isotopes between ilmenite and silicates may reflect both equilibrium and kinetic processes. A few ilmenites have lighter Mg isotopic compositions than coexisting silicates and contain high MgO contents without compositional zoning, indicating equilibrium fractionation. The implication is that the light Mg isotopic compositions of lunar high-Ti basalts may result from an isotopically light source enriched in cumulate ilmenites. On the other hand, most ilmenites have heavy Mg isotopic compositions, coupled with high MgO concentration and chemical zoning, which can be quantitatively modeled by kinetic Mg isotope fractionations induced by subsolidus Mg–Fe exchange between ilmenite and ferromagnesian silicates during the cooling of the Baima intrusion. The extensive occurrence of kinetic Mg isotope fractionation in ilmenites implies the possibility of widespread compositional disequilibrium among igneous minerals in magma chambers. Consequently, disequilibrium effects need to be considered in studies of basaltic magma evolution, magma chamber processes, and magmatic Fe–Ti oxide ore genesis. [ABSTRACT FROM AUTHOR]

To investigate the behaviour of Mg isotopes during carbonatite magmatism, we analyzed Mg isotopic compositions of natrocarbonatites and peralkaline silicate rocks from Oldoinyo Lengai, Tanzania. The olivine melilitites from the vicinity of Oldoinyo Lengai have homogeneous and mantle-like Mg isotopic compositions ( δ 26 Mg of −0.30 to −0.26‰), indicating limited Mg isotope fractionation during mantle melting. The highly evolved peralkaline silicate rocks not related to silicate–carbonatite liquid immiscibility, including phonolites from the unit Lengai I, combeite–wollastonite nephelinites (CWNs) from the unit Lengai II A and carbonated combeite–wollastonite–melilite nephelinites (carbCWMNs), have δ 26 Mg values (from −0.25 to −0.10‰) clustered around the mantle value. By contrast, the CWNs from the unit Lengai II B, which evolved from the silicate melts that were presumably generated by silicate–carbonatite liquid immiscibility, have heavier Mg isotopes ( δ 26 Mg of −0.06 to +0.09‰). Such a difference suggests Mg isotope fractionation during liquid immiscibility and implies, based on mass-balance calculations, that the original carbonatite melts at Lengai were isotopically light. The variable and positive δ 26 Mg values of natrocarbonatites (from +0.13 to +0.37‰) hence require a change of their Mg isotopic compositions subsequent to liquid immiscibility. The negative correlations between δ 26 Mg values and contents of alkali and alkaline earth metals of natrocarbonatites suggest Mg isotope fractionation during fractional crystallization of carbonatite melts, with heavy Mg isotopes enriched in the residual melts relative to fractionated carbonate minerals. Collectively, significant Mg isotope fractionation may occur during both silicate–carbonatite liquid immiscibility and fractional crystallization of carbonatite melts, making Mg isotopes a potentially useful tracer of these processes relevant to carbonatite petrogenesis. [ABSTRACT FROM AUTHOR]

We present high-precision Fe and Mg isotopic data for the Purang ophiolite, southwestern Tibet, representing the first combined Fe and Mg isotopic study of the oceanic lithosphere hitherto. The δ 56 Fe and δ 26 Mg values of the ophiolitic peridotite, dunite and gabbro vary from −0.209 to 0.187‰ and from −0.28 to − 0.14 ‰ , respectively. The average δ 56 Fe of the peridotites is − 0.030 ± 0.143 ‰ (2SD, n = 17 ), a value indistinguishable from abyssal peridotites and chondrites, and lower than oceanic basalts. The average δ 26 Mg value of the peridotites is − 0.20 ± 0.10 ‰ , a value slightly higher than both chondrites and oceanic basalts. Correlations between δ 56 Fe and indices of partial melting indicate fractionation of 0.323‰ in δ 56 Fe between the oceanic lithospheric mantle and the overlying mafic crust during an early episode of partial melting, presumably beneath a spreading centre. Subsequent metasomatism in a supra-subduction zone caused elevated oxygen fugacity and heavy Fe isotopic compositions in the oceanic lithospheric mantle. The dunite with high Ba/La, a proxy for oxygen fugacity, and high δ 56 Fe values was likely formed during this process of sub-arc mantle-melt interaction. The negatively coupled Fe–Mg isotopic variations of the Purang ophiolite indicate that Mg isotope fractionation may also occur during high-temperature mantle processes. The observed isotopic variations among different lithologies in the ophiolite may satisfactorily account for the isotopic differences between arc lavas and mantle peridotites with respect to oceanic basalts, thus providing implications for crust–mantle differentiation. [ABSTRACT FROM AUTHOR]

Abstract: Loess deposits serve as important continental archives for studying Quaternary climatic variations and for estimating the average chemical composition of the upper continental crust. Here, we report high-precision Mg isotopic data for 19 loess samples from China, Argentina and Europe, which were previously used to estimate the composition of the upper continental crust. The results show that global loess samples have heterogeneous Mg isotopic compositions, with δ26Mg (per mil deviation of the 26Mg/24Mg ratio from the DSM3 standard) values ranging from −1.64‰ to +0.25‰ and a weighted average of −0.89‰, which is lighter than both crust and mantle silicates. MgO content and δ26Mg of loess positively correlate with CaO/Al2O3 ratio, suggesting a two-component mixing between carbonates and secondary silicate minerals. The large variation in Mg isotopic composition of loess results from a combination of factors, including source heterogeneity, eolian sorting during transport of the loess and chemical weathering during formation of the loess deposit. Our results suggest that Mg isotopic composition of loess has potential to be a proxy indicator to characterize the paleoclimatic change, but may not represent the average Mg isotopic composition of the upper continental crust due to mixing, sorting of isotopically distinct components and isotope fractionation during loess deposit formation. [Copyright &y& Elsevier]

Abstract: Cratonic eclogites play an important role in the formation and dynamic evolution of the subcontinental lithospheric mantle. However, their origin, whether as fragments of subducted oceanic crust or high-pressure mantle cumulates, remains controversial. Here, we report Mg isotopic compositions (δ26Mg) for cratonic eclogites from Kaalvallei and Bellsbank kimberlite pipes, South Africa. We find that clinopyroxene is 0.375±0.069 to 0.676±0.075‰ heavier than coexisting garnet, which reflects equilibrium isotope fractionation between these phases, primarily driven by the difference in Mg coordination between clinopyroxene and garnet. Bulk eclogites have strikingly variable Mg isotopic compositions, which range from −0.797±0.075 to −0.139±0.061‰, values that are significantly lighter than the range displayed by global mantle peridotites to date (−0.25±0.07‰, 2SD). As significant Mg isotope fractionation is only known to occur during low-temperature water–rock interaction, our results provide further evidence for the derivation of cratonic eclogites from subducted altered oceanic crust. In addition, the lack of correlation between Δ26Mg and Δ57Fe provides evidence for redox control on equilibrium inter-mineral Fe isotope fractionation. [Copyright &y& Elsevier]

Abstract: Magnesium isotopic compositions of a set of clay-rich saprolites developed on the Neogene tholeiitic basalt from Hainan Island in southern China have been measured in order to document the behavior of Mg isotopes during continental weathering. Compared with unaltered basalts (δ 26Mg=−0.36‰), the overlying saprolites are strongly depleted in Mg (i.e., τ Th,Mg=−99.1% to −92.9%), and display highly variable δ 26Mg, ranging from −0.49‰ to +0.40‰. Magnesium concentration and δ 26Mg value of the saprolites display a general increasing trend upwards in the lower part of the profile, but a decreasing trend towards the surface in the upper part. The variations of Mg concentration and isotopic composition in this weathering profile can be explained through adsorption and desorption processes: (1) adsorption of Mg to kaolin minerals (kaolinite and halloysite), with preferential uptake of heavy Mg isotopes onto kaolin minerals; and (2) desorption of Mg through cation exchange of Mg with the relatively lower hydration energy cations in the upper profile. Evidence for adsorption is supported by the positive correlation between δ 26Mg and the modal abundance of kaolin minerals in saprolite of the lower profile, while negative correlations between δ 26Mg and concentrations of lower hydration energy cations (e.g., Sr and Cs) in the upper profile support the desorption process. Our results highlight that adsorption and desorption of Mg on clay minerals play an important role in behavior of Mg isotopes during extreme weathering, which may help to explain the large variation in Mg isotopic composition of river waters. [Copyright &y& Elsevier]

Abstract: Diffusion plays an important role in Earth sciences to estimate the timescales of geological processes such as erosion, sediment burial, and magma cooling. In igneous systems, these diffusive processes are recorded in the form of crystal zoning. However, meaningful interpretation of these signatures is often hampered by the fact that they cannot be unambiguously ascribed to a single process (e.g., magmatic fractionation, diffusion limited transport in the crystal or in the liquid). Here we show that Mg and Fe isotope fractionations in olivine crystals can be used to trace diffusive processes in magmatic systems. Over sixty olivine fragments from Hawaiian basalts show isotopically fractionated Mg and Fe relative to basalts worldwide, with up to 0.4‰ variation in 26Mg/24Mg ratios and 1.6‰ variation in 56Fe/54Fe ratios. The linearly and negatively correlated Mg and Fe isotopic compositions [i.e., δ56Fe=(−3.3±0.3)×δ26Mg], co-variations of Mg and Fe isotopic compositions with Fe/Mg ratios of olivine fragments, and modeling results based on Mg and Fe elemental profiles demonstrate the coupled Mg and Fe isotope fractionation to be a manifestation of Mg–Fe inter-diffusion in zoned olivines during magmatic differentiation. This characteristic can be used to constrain the nature of mineral zoning in igneous and metamorphic rocks, and hence determine the residence times of crystals in magmas, the composition of primary melts, and the duration of metamorphic events. With improvements in methodology, in situ isotope mapping will become an essential tool of petrology to identify diffusion in crystals. [Copyright &y& Elsevier]

Abstract: To investigate the magnitude and mechanism of inter-mineral Mg isotope fractionation at mantle temperatures, we measured Mg isotopic compositions of coexisting olivine (Ol), orthopyroxene (Opx), clinopyroxene (Cpx), phlogopite (Phl) and spinel (Spl) from harzburgite, lherzolite and clinopyroxenite xenoliths in the North China craton. These xenoliths are well-characterized and formed over a wide temperature range from ~800 to 1150°C. The coexisting Opx and Ol have constant and indistinguishable Mg isotopic compositions, with δ26Mg ranging from −0.29 to −0.22‰ in Ol and from −0.28 to −0.22‰ in Opx (Δ26MgOpx–Ol =δ26MgOpx −δ26MgOl =−0.04 to +0.04‰; n=11). By contrast, Mg isotopic compositions of Cpx and Phl are variable and slightly heavier than coexisting Ol (Δ26MgCpx–Ol =0 to +0.13‰, n=13; Δ26MgPhl-Ol =+0.11 to +0.20‰, n=3). Isotope fractionations between coexisting Cpx and Ol are correlated with temperatures, implying equilibrium isotope fractionation. The degree and direction of isotope fractionations among these mantle silicates agree with theoretical predictions, suggesting that inter-mineral Mg isotope fractionation is primarily controlled by the Mgrength, with stronger bonds favoring heavier Mg isotopes. Cpx and Phl have shorter and thus stronger Mgand hence are isotopically heavier than coexisting Ol. Compared with coexisting silicates, δ26Mg values of spinels are more variable and much heavier, ranging from +0.03 to +0.28‰. The Δ26MgSpl–Ol values vary significantly from +0.25 to +0.55‰ (n=10) and show an excellent, positive, linear correlation with 106/T2(K) [Δ26MgSpl–Ol =0.63(±0.12)×106/T2(K)−0.03(±0.08)], indicating equilibrium Spl–Ol isotope fractionation. The absence of intra-mineral isotopic variation and quantitative diffusion calculations further confirm isotope exchange equilibrium between coexisting Spl and Ol. Our results demonstrate the existence of measurable Mg isotope fractionation between mantle minerals and suggest that the large high-temperature equilibrium Spl–Ol Mg isotope fractionation in peridotite xenoliths can potentially be used as a geothermometer in mantle geochemistry. [Copyright &y& Elsevier]

Abstract: To investigate the magnitude and mechanism of inter-mineral Mg isotope fractionation at high temperatures, we report high-precision analyses of Mg isotopes for 10 whole rocks and 13 mineral separates for a set of eclogites from Bixiling in the Dabie orogen, China. Magnesium isotopic compositions of whole rocks (δ26Mg of −0.44 to −0.26‰) are similar to the estimated δ26Mg values of the mantle, suggesting Mg isotopic inheritance from a gabbroic protolith with limited Mg isotope fractionation during eclogite-facies metamorphism. By contrast, mineral separates are highly heterogeneous, with δ26Mg values ranging from +0.30 to +0.60‰ in phengite, from +0.16 to +0.40‰ in omphacite and from −0.95 to −0.74‰ in garnet. Phengite and omphacite are >1‰ heavier in δ26Mg than coexisting garnet, indicating large high-temperature inter-mineral Mg isotope fractionations. The constant Δ26Mgomphacite–garnet (=δ26Mgomphacite −δ26Mggarnet) value (1.14±0.04‰), together with homogeneous mineral chemistry and equilibrium oxygen isotopic partitioning between omphacite and garnet, suggests an equilibrium Mg isotope fractionation, controlled by the difference in coordination number of Mg between omphacite (six) and garnet (eight). The 1.14‰ fractionation is the largest high-temperature equilibrium inter-mineral Mg isotope fractionation observed so far and makes the omphacite-garnet Mg isotope fractionation a potential geothermometer. By contrast, Mg isotope fractionations between phengite and garnet and between phengite and omphacite vary from 1.25 to 1.47‰ and from 0.14 to 0.32‰, respectively. This implies Mg isotopic disequilibria between phengite and garnet/omphacite, which might result from the Mg isotopic variation in phengites due to Mg isotope exchange between phengites and retrograde fluids in Bixiling eclogites. [Copyright &y& Elsevier]

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: High-precision Mg isotopic analysis was performed on a suite of well-characterized I-type granitoids and associated hornblende and biotite minerals from the Dabie Orogen in central China, to address the behavior of Mg isotopes during granite differentiation. Although these granitoids formed through different degrees of partial melting and fractional crystallization, with large variations in elemental and mineral compositions, their δ 26Mg values vary from −0.26 to −0.14 and are indistinguishable within our analytical precision (±0.07‰; 2SD). Coexisting hornblendes and biotites in these granitoids display similar Mg isotopic composition, with δ 26Mg ranging from −0.31 to −0.14 in hornblendes and −0.23 to −0.12 in biotites. The inter-mineral fractionation factors (Δ26MgHbl-Bt = δ 26MgHbl − δ 26MgBt) vary from −0.10 to −0.02, with an average=−0.06±0.08 (2SD). The limited inter-mineral fractionation agrees with the theoretic prediction that Mg cations in both hornblende and biotite are octahedrally coordinated with oxygen, which restricts the magnitude of equilibrium isotope fractionation. Overall, data from both bulk granitoids and associated mineral separates suggest that Mg isotope fractionation during I-type granite differentiation is limited. Collectively, granitoids studied here have Mg isotopic composition similar to that of terrestrial basalts and peridotites (δ 26Mg=−0.21±0.07 vs. −0.25±0.07; 2SD), confirming that magmatic processes do not significantly fractionate Mg isotopes. The continental crust in the Dabie Orogen, as sampled by these I-type granitoids, has a mantle-like Mg isotopic composition. Given that significant Mg isotope fractionation occurs during chemical weathering processes, Mg isotopes may potentially be used for tracing granite genesis, in particular, if sedimentary materials are involved in granite sources. [Copyright &y& Elsevier]

Abstract: In order to further investigate inter-mineral Mg isotope fractionation at mantle temperatures and to better constrain the Mg isotopic composition of the terrestrial mantle, we have studied a set of well-characterized mantle peridotite xenoliths from Sanyitang and Beiyan, North China craton. The Sanyitang and Beiyan peridotites, which have diverse origins with different mineralogy, chemical composition and degree of partial melting and metasomatism, display a small variation in Mg isotopic composition, with δ26Mg varying from −0.48 to −0.12 and an average value of −0.29±0.19 (2SD, n =21) in olivines, from −0.27 to −0.10 and an average value of −0.21±0.09 (2SD, n =12) in orthopyroxenes and from −0.35 to −0.08 and an average value of −0.22±0.14 (2SD, n =15) in clinopyroxenes. The Mg isotopic compositions of the coexisting olivine, orthopyroxene and clinopyroxene in all peridotites are identical within our external precision (~±0.1‰, 2SD), suggesting that Mg isotope fractionation between olivine and pyroxenes at the temperatures of >900°C is insignificant. These results are thus consistent with the absence of Mg isotope fractionation during basalt differentiation. The ~0.4‰ Mg isotopic variations in these peridotite samples, which are larger than our external precision, might result from thermal diffusion-driven isotope fractionation or melt-rock interactions. Overall, the δ26Mg of the mantle, based on peridotite minerals analyzed here, is estimated to be −0.26±0.16 (2SD). This value is in excellent agreement with recent studies based on peridotites and oceanic basalts (Handler et al., 2009; Teng et al., 2007). It is also similar to δ26Mg values of two additional dunite standards (DTS-1 and DTS-2) and two carbonaceous chondrites (Allende and Murchison) analyzed in this study as well as δ26Mg of all published chondrites (~−0.3). The silicate Earth thus has a chondritic Mg isotopic composition. [Copyright &y& Elsevier]

Abstract: To investigate whether magnesium isotopes are fractionated during basalt differentiation, we have performed high-precision Mg isotopic analyses by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) on a set of well-characterized samples from Kilauea Iki lava lake, Hawaii, USA. Samples from the Kilauea Iki lava lake, produced by closed-system crystal-melt fractionation, range from olivine-rich cumulates to highly differentiated basalts with MgO content ranging from 2.37 to 26.87 wt.%. Our results demonstrate that although these basalts have diverse chemical compositions, mineralogies, crystallization temperatures and degrees of differentiation, their Mg isotopic compositions display no measurable variation within the limits of our external precision (average δ 26Mg=−0.36±0.10 and δ 25Mg=−0.20±0.07; uncertainties are 2SD). This indicates that Mg isotopic fractionation during crystal-melt fractionation at temperatures of ≥1055 °C is undetectable at the level of precision of the current investigation. Calculations based on our data suggest that at near-magmatic temperatures the maximum fractionation in the 26Mg/24Mg ratio between olivine and melt is 0.07‰. Two additional oceanic basalts, two continental basalts (BCR-1 and BCR-2), and two primitive carbonaceous chondrites (Allende and Murchison) analyzed in this study have Mg isotopic compositions similar to the Kilauea Iki lava lake samples. In contrast to a recent report [U. Wiechert, A.N. Halliday, Non-chondritic magnesium and the origins of the inner terrestrial planets, Earth and Planetary Science Letters 256 (2007) 360–371], the results presented here suggest that the Bulk Silicate Earth has a chondritic Mg isotopic composition. [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]

• Comprehensive study of Mg isotopic behavior in progressive weathering. • Phyllosilicates control the Mg isotopic fractionation during weathering of granite. • δ 26 Mg can substantially increase since the onset of incipient weathering. • Incorporation of saprock could form high- δ 26 Mg aeolian sedimentary rocks. • Interaction with saprock can significantly lower δ 26 Mg in riverine system. Continental weathering is a fundamental process in releasing magnesium (Mg) from crystalline rocks to the hydrosphere and biosphere. Mg isotopes can be substantially mobilized, re-distributed, and fractionated during weathering, and therefore can be used as a powerful tool to trace the biogeochemical cycle of Mg. Causes of significant Mg isotopic fractionation and behaviors during silicate weathering are still not well understood, hindering further application of the Mg isotopes to probe different geological processes. In this study, we demonstrate that dissolution and formation of phyllosilicates are the main control of Mg isotopic fractionation during sub-tropical weathering of granite. Furthermore, different formation and dissolution mechanisms for the same mineral phase could also cause variations in magnitude and directionality of fractionation. In incipient weathering, supergene phyllosilicates form mainly through topotactic transformation. Vermiculitization of parental chlorite tends to release 24Mg and causes significant 26Mg enrichment in the saprock. In an advanced stage of weathering, Mg isotopic compositions of supergene phyllosilicates are more influenced by the interaction with the soil solutions. Minerals formed mainly through a dissolution-precipitation mechanism with Mg in neoformed phyllosilicates dominantly sourced from the contemporary soil solutions. 26Mg would be firstly incorporated into neoformed phyllosilicates, such as vermiculite, interstratified biotite/vermiculite and chlorite/vermiculite. Therefore, soil solutions became more enriched in 24Mg with depth in the pedolith, from which relatively 24Mg-rich phyllosilicates would form. However, in the saprolite, precipitation of illite may have preferentially scavenged 24Mg, enriching the soil solutions with 26Mg. Varying relative abundances of different phyllosilicate minerals along the profile could cause large variations in the Mg isotopic compositions of regolith. Our study shows that Mg isotopic composition of the slightly weathered materials could be significantly heavy. Hence, entrainment of 26Mg-rich but slightly weathered materials could be an alternative to explain the high δ 26 Mg as recorded in some sedimentary rocks, especially of aeolian source. Whereas low δ 26 Mg widely archived in groundwater and river water could be alternatively explained by interaction with the saprock and 26Mg scavenging during phyllosilicate transformation, instead of severe depletion of 26Mg in soil solutions due to intense weathering and vast formation of secondary minerals, as previously suggested. Comprehensive characterization of the weathering processes and the resultant products is essential to interpret the observed Mg isotopic fractionation and trace the biogeochemical cycle of Mg. [ABSTRACT FROM AUTHOR]

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]

Magnesium isotope compositions are reported for a suite of Martian meteorites that span the range of petrological and geochemical types recognized to date for Mars, including crustal breccia Northwest Africa (NWA) 7034. The δ 26 Mg values (per mil units relative to DSM-3 reference material) range from −0.32 to −0.11‰; basaltic shergottites and nakhlites lie to the heavier end of the Mg isotope range whereas olivine-phyric, olivine–orthopyroxene-phyric and lherzolitic shergottites, and chassignites have slightly lighter Mg isotope compositions, attesting to modest correlation of Mg isotopes and petrology of the samples. Slightly heavier Mg isotope compositions found for surface-related materials (NWA 7034, black glass fraction of the Tissint shergottite fall; δ 26 Mg > −0.17‰) indicate measurable Mg isotope difference between the Martian mantle and crust but the true extent of Mg isotope fractionation for Martian surface materials remains unconstrained. The range of δ 26 Mg values from −0.19 to −0.11‰ in nakhlites is most likely due to accumulation of clinopyroxene during petrogenesis rather than garnet fractionation in the source or assimilation of surface material modified at low temperatures. The rather restricted range in Mg isotope compositions between spatially and temporally distinct mantle-derived samples supports the idea of inefficient/absent major tectonic cycles on Mars, which would include plate tectonics and large-scale recycling of isotopically fractionated surface materials back into the Martian mantle. The cumulative δ 26 Mg value of Martian samples, which are not influenced by late-stage alteration processes and/or crust–mantle interactions, is − 0.271 ± 0.040 ‰ (2SD) and is considered to reflect δ 26 Mg value of the Bulk Silicate Mars. This value is robust taking into account the range of lithologies involved in this estimate. It also attests to the lack of the Mg isotope variability reported for the inner Solar System bodies at current analytical precision, also noted for several other major elements. [ABSTRACT FROM AUTHOR]

Abstract: The zinc stable isotope system has been successfully applied to many and varied fields in geochemistry, but to date it is still not completely clear how this isotope system is affected by igneous processes. In order to evaluate the potential application of Zn isotopes as a proxy for planetary differentiation and volatile history, it is important to constrain the magnitude of Zn isotopic fractionation induced by magmatic differentiation. In this study we present high-precision Zn isotope analyses of two sets of chemically diverse, cogenetic samples from Kilauea Iki lava lake, Hawaii, and Hekla volcano, Iceland, which both show clear evidence of having undergone variable and significant degrees of magmatic differentiation. The Kilauea Iki samples display small but resolvable variations in Zn isotope composition (0.26‰<δ66Zn<0.36‰; δ66Zn defined as the per mille deviation of a sample's 66Zn/64Zn compositional ratio from the JMC-Lyon standard), with the most differentiated lithologies exhibiting more positive δ66Zn values. This fractionation is likely a result of the crystallization of olivine and/or Fe–Ti oxides, which can both host Zn in their crystal structures. Samples from Hekla have a similar range of isotopic variation (0.22‰<δ66Zn<0.33‰), however, the degree of fractionation caused by magmatic differentiation is less significant (only 0.07‰) and no correlation between isotope composition and degree of differentiation is seen. We conclude that high temperature magmatic differentiation can cause Zn isotope fractionation that is resolvable at current levels of precision, but only in compositionally-evolved lithologies. With regards to primitive (ultramafic and basaltic) material, this signifies that the terrestrial mantle is essentially homogeneous with respect to Zn isotopes. Utilizing basaltic and ultramafic sample analyses, from different geologic settings, we estimate that the average Zn isotopic composition of Bulk Silicate Earth is δ66Zn=0.28±0.05‰ (2s.d.). [Copyright &y& Elsevier]

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]

Abstract Cycling of elements with multiple valences (e.g., Fe, C, and S) through subduction and magmatism may dictate the redox evolution of the deep mantle and atmosphere. To investigate the potential of Fe isotopes as a tracer of such cycles, here we report Fe isotopic compositions of thirty-seven Cenozoic basalts from eastern China. A nephelinitic melt component with δ 56 Fe up to 0.29 has been identified, which cannot be explained by weathering, alteration, magma differentiation, or chemical diffusion. Its low Fe/Mn ∼58, relatively low TiO 2 and high Na 2 O + K 2 O argue against a significant contribution of pyroxenite melting. Instead, the heavy Fe component requires enhanced isotope fractionation during partial melting of a peridotitic source with Fe3+/ΣFe ≥ 0.15. Low Ba/Th ∼ 50 and depleted 87Sr/86Sr(i) and ε Nd (t) suggest that the source was insignificantly affected by hydrous fluids and recycled terrigenous sediments. The heavy Fe component is known to be unique in its low δ 26 Mg and high δ 66 Zn and indicates hybridization by recycled carbonates. The source Fe3+/ΣFe was most likely enhanced at cost of reduction of recycled carbonates to diamonds in a mantle depth ≥300 km. The origin of the heavy Fe component illustrates a pathway with net transportation of oxidizer back to Earth's surface: CO 2 (in carbonates) → C (as diamond frozen in the deep mantle) + O 2 (ferric Fe being scavenged by melt extraction). Secular cooling of global subduction zones may have stepwisely increased the efficiency of this carbon driven deep oxygen cycle in the past, providing an alternative explanation for the rise of atmospheric O 2. Highlights • Three magma components are identified in Cenozoic basalts from eastern China. • A nephelinitic and heavy Fe component has δ 56 Fe up to 0.29. • Two tholeiitic components have δ 56 Fe typical of MORB and most OIB. • The heavy Fe component from a peridotitic source oxidized by recycled carbonates. • Illustration of a deep oxygen cycle induced by subducted carbonates. [ABSTRACT FROM AUTHOR]

Highlights • A comprehensive study on the Maowu garnet clinopyroxenites from the Dabie–Sulu orogen. • The first observation of Mg isotopically light metasomatized mantle wedge. • Sedimentary Mg-rich carbonate could be dissolved in subduction supercritical fluid. Abstract Recent studies have concluded that subducted calcium (Ca-) rich carbonates could be dissolved in slab-derived aqueous fluids and transported upwards into the shallow mantle wedge (75–120 km), while magnesium (Mg-) rich carbonates could be delivered to a greater depth (i.e., the mantle transition zone, ∼410 km), melted, and recycled into the convective upper mantle. However, it remains unknown whether or not Mg-rich carbonates can be transferred to the deep mantle wedge (>∼120 km) by subduction-zone fluids, which, if true, is important for tracing deep carbon. In this paper, we report a comprehensive mineralogical, geochemical, stable (Mg and O) and radiogenic isotopic (zircon U–Pb) study of garnet clinopyroxenites from the Maowu ultra-mafic massif (a slice of the mantle wedge) in the Dabie orogeny, Central China. Whole-rock and mineral trace elemental features and zircon U–Pb ages reveal evidence of mantle wedge metasomatism by a slab-derived melt or supercritical fluid from the subducted rutile-bearing eclogitic Paleo-Tethys oceanic crust, in addition to subsequent metamorphism occurring during the Triassic collision between the South and North China blocks. Combined with the results of previous works, the high Th/U ratios of both whole rocks and metasomatized zircons with no oscillatory zoning lead us to infer that a supercritical liquid rather than a melt was the metasomatic agent during oceanic subduction at peak conditions (5.3–6.3 GPa and ∼800 °C, 160 – 190 km). Abundant carbonate mineral inclusions (including calcite, dolomite and magnesite) and the high δ 18 O VSMOW values of the metasomatized zircons (up to 12.2‰) indicate that sedimentary carbonates were leached by the supercritical fluid. Furthermore, whole-rock δ 26 Mg values (−0.99‰ to −0.65‰) that are lower than normal mantle values (− 0.25 ± 0.07 ‰) imply that the incorporated carbonates contain not only calcites but also a certain amount of dolomites (approximately 1–10 wt.% of the metasomatic supercritical liquid). The dissolved Mg-rich carbonates in the slab-derived supercritical liquid could effectively modify the Mg isotope composition of the deep mantle wedge. Our study represents a critical step towards achieving a broad understanding of the behaviours of recycled carbonate during slab subduction. [ABSTRACT FROM AUTHOR]

Mantle-derived magmas at convergent plate boundaries provide unique insights into the nature of materials subducted to and recycled from depths. Here we present a study of Os–Sr–Mg isotopes on the Oligocene–Miocene ultrapotassic rocks aimed at better understanding sediment subduction and recycling beneath southern Tibet. New isotopic data confirm that ultrapotassic rocks in southern Tibet are of mantle origin, but underwent crustal contamination as evidenced by the variably high 187 Os/ 188 Os that obviously deviates from normal mantle reservoir. Still some samples with mantle-like 187 Os/ 188 Os exhibit δ 26 Mg significantly lower than mantle and crustal lithologies, suggesting that the isotopically light Mg may not result from crustal contamination but retain specific fingerprint of carbonate-related metasomatism in mantle sources. Mantle carbonatite metasomatism is manifested by the inverse δ 26 Mg– 87 Sr/ 86 Sr correlations, as well as the depletion of high field strength elements relative to rare earth elements and the enrichment of CaO in ultrapotassic rocks. The positive co-variations between δ 26 Mg and Hf/Sm defined by those low- 187 Os/ 188 Os ultrapotassic rocks provide evidence for the potential of recycled dolomites to modify mantle Mg isotopic composition. The correlated spatial variations of δ 26 Mg and Hf/Sm are interpreted to reflect carbonatitic metasomatism associated with the northward subduction of the Neo-Tethyan oceanic slab and its profound influence on postcollisional ultrapotassic magmatism. [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]