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Knowledge of the behavior of magnesium (Mg) isotopes during magmatic differentiation is a prerequisite for applying Mg isotopes as a tracer of crustal recycling and mantle heterogeneity. Crystal fractionation of mafic silicate minerals leads to limited Mg isotope fractionation; hence, Mg isotopic variations in basalts largely result from contributions of subducted crustal materials into their mantle sources. However, recent studies have revealed that the segregation of iron oxides, such as chromite during early-stage basaltic differentiation and titanomagnetite and ilmenite during late-stage differentiation, can potentially modify Mg isotopic compositions of mantle-derived lavas. Titanomagnetite is much more commonly observed in terrestrial basaltic lavas than chromite and ilmenite, and can also form during early-stage basaltic differentiation. To constrain the effects of titanomagnetite crystallization on Mg isotopic systematics of basaltic magma, we report Mg isotopic data for a suite of well-characterized alkaline basaltic lavas (MgO wt% = 4.99 to 6.52) from Gerba Guracha, western Ethiopian Plateau, which have undergone extensive titanomagnetite fractionation. Their δ26Mg values range from −0.29‰ to −0.17‰ and do not correlate with indicators of titanomagnetite fractionation such as TiO 2 and V. These observations suggest that the segregation of titanomagnetite produced limited Mg isotope fractionation, possibly because of its composition-dependent, highly variable Δ26Mg Ti-Mgt−melt (Δ26Mg Ti-Mgt−melt = δ26Mg Ti-Mgt − δ26Mg melt) values. Considering that most iron oxides occur as complex solid solutions that are controlled by the oxygen fugacity of magmas, the effects of oxide segregation on Mg isotopic systematics of basalts require further evaluation. • Alkaline basaltic lavas from Gerba Guracha display limited Mg isotopic variations. • The δ26Mg values do not correlate with indicators of titanomagnetite fractionation. • Titanomagnetite fractionation has produced limited Mg isotope fractionation. [ABSTRACT FROM AUTHOR]

Abstract The Mg isotope compositions of weathering profiles developed on granite and granodiorite bedrock have been analyzed to investigate Mg isotope behavior during the weathering of felsic rocks in a continental environment. δ26Mg generally exhibits a small increase from the bedrock values, which range from −0.17 ± 0.06‰ to +0.07 ± 0.06‰, until the upper two meters in which a decrease is observed, producing near surface values between −0.14 ± 0.06‰ and +0.09 ± 0.06‰. The small observed increase in isotopic composition below 2 m is likely due to Mg loss to the hydrosphere during biotite weathering and in situ illite production, while the decrease at depths <2 m is likely due to illite weathering near the surface, with a possible minor contribution from downslope deposition of less weathered material. One hydrothermally altered profile shows more extreme fractionation, reaching δ26Mg values as heavy as +0.55 ± 0.06‰ in the near surface component of the section. The fractionation associated with Mg loss during weathering can be modeled by Rayleigh distillation with fractionation factors between the residual rock and fluid ranging from α = 1.00001 to 1.00040. The Mg isotope composition is primarily controlled by the behavior of illite in the weathering profile, as Mg isotope composition is positively correlated with the illite fraction of major Mg-bearing minerals. A thorough understanding of Mg isotope systematics in these complex clay minerals may permit the use of Mg isotopes in paleosols as paleoclimate indicators. Highlights • Mg isotopes are primarily controlled by illite behavior during granite weathering. • Profiles varied from δ26Mg ≅ −0.17‰ in the bedrock up to +0.09‰ in the regolith. • One hydrothermal profile varied from δ26Mg ≅ −0.07‰ in the bedrock up to +0.55‰. • Mg isotopes could help to elucidate Mg cycling in continental environments. [ABSTRACT FROM AUTHOR]

Subducted sediments have been suggested to be a possible cause for local Mg isotopic variation in the overall homogeneous mantle based on anomalous Mg isotopic compositions detected in some mantle-derived rocks. Here we examine the Mg isotopic systematics of 77 bulk marine sediment samples from ten DSDP and ODP drill sites adjacent to the world's major subduction zones to assess their potential to cause mantle heterogeneity. In contrast to the limited mantle range (δ 26 Mg = − 0.25 ± 0.07‰), δ 26 Mg of these sediments varies widely from − 1.34‰ to + 0.46‰, which is similar to values measured in continental sediments and appears to be linked to sediment lithology. Calcareous oozes are generally isotopically lighter than seawater; however, the presence of detrital impurities can shift their δ 26 Mg to higher values. In comparison to calcareous oozes, Mg-rich aluminosilicate sediments typically have higher δ 26 Mg values, although they may be either isotopically heavier or lighter than the mantle. Detrital sediments are the dominant sediment type and display large variation (− 0.90‰ to + 0.18‰) that reflects provenance heterogeneity and textural/mineralogical sorting. Sediments derived from juvenile terranes have δ 26 Mg similar to mantle values while those derived from mature, and thus more strongly weathered sources, often have higher δ 26 Mg values. On the other hand, turbidites may have lower δ 26 Mg values if containing isotopically light dolomite and/or garnet. In addition, clay-size fractions in turbidites commonly have higher δ 26 Mg than the associated sands. Hydrogenetic clays have a relatively high and uniform δ 26 Mg value of + 0.02 ± 0.14‰ (2SD, n = 14), suggesting preferential uptake of heavy Mg isotopes during authigenic clay formation while hydrothermal clays have highly variable δ 26 Mg values (− 0.57‰ to + 0.46‰) that likely reflect variable exchange with seawater/hydrothermal fluids. Overall, considerable variations exist in both regional Mg mass flux and the weighted δ 26 Mg average of sedimentary sections subducting at different trenches. The flux-weighted δ 26 Mg of − 0.336‰ is estimated for Global Subducting Sediments (GLOSS-II), which is isotopically lighter than the mantle average because of the presence of thick turbidite sequences with relatively low δ 26 Mg values and local carbonate enrichment in a few trenches near the equator. Therefore, it appears that subducting marine sediments are a source of heterogeneous δ 26 Mg values in the mantle, and they may be manifested locally in volcanic rocks that form in regions having a high flux of sediment melts to the mantle. [ABSTRACT FROM AUTHOR]

Modern dolomite formation is conditional and restricted in specific environmental and geochemical conditions, accordingly cannot be used as the analog of ancient dolostones, which normally have platform-wide distribution. The key to understand ancient dolostone formation is to find a dolomitization process that is independent of physical/geochemical environments. It is well-known that ribbon rock (one type of calcareous rhythmites, sensu stricto referring to the alternating limestone and marlstone layers or the limestone-marl alternations) that are deposited in nearly all marine environments throughout the Earth's history, are exclusively, though normally partially, dolomitized. Thus, understanding ribbon rock dolomitization may provide valuable insight into the ‘dolomite problem’. In this study, we measured Mg isotopic compositions ( δ 26 Mg) of ribbon rock samples collected from the Ediacaran Doushantuo Formation in South China. The marlstone layers contain higher dolomite contents and are characterized by higher δ 26 Mg (ranging from − 2.68‰ to − 1.84‰) than the limestone layers (varying between − 3.21‰ and − 2.53‰). A numerical model calculation indicates that clay minerals cannot provide enough Mg for dolomitization, arguing against the traditional interpretation that dolomitization is resulted from diagenetic Mg release from clays. Instead, contemporaneous seawater might provide sufficient Mg for dolomitization, because both Mg isotopes and dolomite content can be simulated by the Diffusion-Advection-Reaction model. Contemporaneous seawater dolomitization requires kinetic barrier be overcome at shallow depth of sediments. We propose the ribbon rock dolomitization is attributed to differential diagenesis of the clay-rich and clay-poor sediment layers, in which Ca 2 + -Mg 2 + exchange elevates porewater Mg/Ca in clay-rich layers. [ABSTRACT FROM AUTHOR]

Magnesium isotopic compositions of igneous rocks could be potentially used to trace recycling of supracrustal materials. High-δ 26 Mg granitoids have been previously reported and explained to reflect the involvement of surface weathered materials in their sources. Low-δ 26 Mg granitoids, however, have not been reported. In this study, we report high-precision Mg isotopic analyses of Cenozoic alkaline syenites and syenogranites from the Kuzigan and Zankan plutons, northwest Xinjiang, China. The Kuzigan syenites were originated from the mantle metasomatized by recycled supracrustal materials, and the syenogranites are differentiated products of the syenites. Both syenites and syenogranites have δ 26 Mg values (− 0.46 to − 0.26‰ and − 0.41 to − 0.17‰, respectively) significantly lighter than the mantle (− 0.25 ± 0.07‰, 2SD). No correlation of δ 26 Mg with either SiO 2 or MgO is observed, indicating limited Mg isotope fractionation during alkaline magmatic differentiation. The low δ 26 Mg of the syenites and syenogranites thus reflects a light Mg isotopic source. This, combined with high 87 Sr/ 86 Sr ratios (0.70814 to 0.71105) and negative correlation between δ 26 Mg and δ 18 O, suggests that the magma source contains recycled marine carbonates. Modeling of the Mg-O-Sr isotopic data indicates that the recycled carbonate is mainly limestone with minor dolostone, suggesting that the metasomatism occurred at depths shallower than 60 to 120 km. Given that the plutons are located at the India–Eurasia collision zone, the carbonate recycling was most likely derived from the subducted Tethyan oceanic crust during the Mesozoic–Cenozoic. Our study suggests that the combined Mg, O, and Sr isotopic studies are powerful for tracing recycled carbonates and identifying their species in mantle sources. [ABSTRACT FROM AUTHOR]

Elemental exchange during seafloor alteration exerts substantial control on the chemical compositions of seawater and oceanic crust, as well as the global elemental cycling. However, detailed processes and mechanisms for element transport and isotopic exchange remain unclear. Here, we report Li and K isotopic compositions across an altered MORB-like pillow basalt from Mariana Trench. We find large and systematic isotopic variations on a centimeter scale, with δ7Li decreasing from +9.07‰ to +6.21‰ and δ41K increasing from −0.12‰ to +0.33‰ from the rim to the core. The heavier-than-mantle Li and K isotopic compositions in the core likely inherit from an enriched mantle source rather than post-magmatic alteration processes. The low K and high Li isotopic compositions at the outmost rims result from adsorption during seawater-rock interactions. The basin-shaped elemental and isotopic variations were subsequently formed by chemical diffusion and spontaneous seawater penetration from the weathered rind to the interior of pillow basalt sample. This study uses a small-scale sampling approach and provides the first evidence for multi-mode chemical exchange in oceanic basalts and improves our understanding of the seafloor alteration processes. [ABSTRACT FROM AUTHOR]

Magnesium isotopic ratios have been used as a natural tracer to study weathering processes and biogeochemical pathways in surficial environments, but few have focused on the mechanisms that control Mg isotope fractionation during shale weathering. In this study we focus on understanding Mg isotope fractionation in the Shale Hills catchment in central Pennsylvania. Mg isotope ratios were measured systematically in weathering products, along geochemical pathways of Mg during shale weathering: from bedrock to soils and soil pore water on a planar hillslope, and to sediments, stream water, and groundwater on a valley floor. Significant variations of Mg isotopic values were observed: δ 26 Mg values (− 0.6‰ to − 0.1‰) of stream and soil pore waters are about ~ 0.5‰ to 1‰ lighter than the shale bedrock δ( 26 Mg values of + 0.4‰), consistent with previous observations that lighter Mg isotopes are preferentially released to water during silicate weathering. Dissolution of the carbonate mineral ankerite, depleted in the shallow soils but present in bedrock at greater depths, produced higher Mg 2 + concentrations but lower δ 26 Mg values (− 1.1‰) in groundwater, ~ 1.5‰ lighter than the bedrock. δ 26 Mg values (+ 0.2‰ to + 0.4‰) of soil samples on the planar hillslope are either similar or up to ~ 0.2‰ lighter than the bedrock. Hence a heavy Mg isotope reservoir – complementary to the lighter Mg isotopes in soil pore water and stream water – is missing from the residual soils on the hillslope. In addition, soil samples show a slight but systematic decreasing trend in δ 26 Mg values with increasing weathering duration towards the surface. We suggest that the accumulation of light Mg isotopes in surface soils at Shale Hills is due to a combined effect of i) sequestration of isotopically light Mg from soil water during clay dissolution–precipitation reactions; and ii) loss of isotopically heavy particulate Mg in micron-sized particles from the hillslope as suspended sediments. This latter mechanism is somewhat surprising in that most researchers do not consider physical removal or particles to be a likely mechanism of isotopic fractionation. Stream sediments (δ 26 Mg values of + 0.3‰ to + 0.5‰) accumulated on the valley floor are ~ 0.2‰ heavier than the bedrock, and are thus consistent with that mobile particulates are the heavy Mg isotope reservoir. Our study provides the first field evidence that changes in clay mineralogy lead to accumulation of lighter Mg isotopes in residual bulk soils. This example also demonstrates that transport of isotopically distinct fine particles from clay-rich systems could be a new and important mechanism to drive the Mg isotope compositions of silicate weathering residuals. This mechanism drives fractionation in an opposite direction as might be expected from previous studies, i.e. residual soils are driven to lighter Mg values and sediments become isotopically heavier. [ABSTRACT FROM AUTHOR]

Abstract: To explore the possibility of tracing recycled carbonate by using Mg isotopes and to evaluate the effects of the western Pacific oceanic subduction on the upper mantle evolution of the North China craton, Mg isotopic compositions of the Mesozoic–Cenozoic basalts and basaltic andesites from the craton have been investigated. The samples studied here come from a broad area in the craton with variable ages of 125–6Ma, and can be divided into two groups based on geochemical features: the >120Ma Yixian basalts and basaltic andesites, and the <110Ma Fuxin and Taihang basalts. Our results indicate that these two groups have distinct Mg isotopic compositions. The >120Ma Yixian basalts and basaltic andesites, with low Ce/Pb, Nb/U ratios and lower-crust like Sr-Nd-Pb isotopic compositions, have a mantle-like Mg isotopic composition, with δ26Mg values ranging from −0.31‰ to −0.25‰ and an average of −0.27±0.05‰ (2SD, n =5). This suggests that continental crust contamination may strongly modify their Ce/Pb, Nb/U ratios and Sr-Nd-Pb isotopic compositions but do not influence their Mg isotopic compositions. By contrast, the <110Ma basalts from Fuxin and Taihang exhibit lower δ26Mg values of −0.60‰ to −0.42‰, with an average of −0.46±0.10‰ (2SD). Since these basalts still preserve mantle-like Sr-Nd-Pb isotopic compositions and Ce/Pb, Nb/U ratios and have high U/Pb and Th/Pb ratios similar to those of HIMU basalts, the light Mg isotopic composition is most likely derived from interaction of their mantle source with isotopically light recycled carbonate melt. Since the Tethys and Mongolia oceanic subductions from south and north toward the North China craton were terminated in the Triassic and light Mg isotopic signature in basalts did not appear before 120Ma, the subducted Pacific oceanic crust could be the major source of the recycled carbonate. Therefore, this study not only presents an example to trace recycled carbonate using Mg isotopes but also confirms the important role of the western Pacific oceanic subduction in generating the <110Ma basalts in the North China craton. [Copyright &y& Elsevier]

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

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

Abstract: The major, trace element, and Li isotopic compositions of granodiorite and metapelites from the Onawa pluton and surrounding contact aureole have been measured to document the behavior of Li and its isotopes during progressive metamorphic dehydration. Major and trace element concentrations in metapelites of the contact aureole change little, while loss on ignition (LOI) decreases with increasing metamorphic grade, which ranges from regional chlorite-zone metamorphism far removed from the pluton to partially melted rocks adjacent to the pluton. Lithium concentrations in metapelites from all zones correlate with LOI and abundances of the main Li-bearing minerals, chlorite + biotite + muscovite, decreasing by a factor of two (from 130 ppm far removed from the pluton to 64 ppm adjacent to the pluton). In contrast, Li isotopic compositions remain relatively unchanged across the aureole (δ 7Li=−3.5 to +1.0), and are comparable to the range observed in schists and unmetamorphosed shales. Compared to the surrounding metasediments, the granodiorite has a lower Li content (45 ppm) and a comparable δ 7Li value of −0.2, similar to those of other granites and the average upper crust (0±2). These observations are consistent with the removal of Li from metapelites via Rayleigh distillation during progressive metamorphic dehydration, with isotopic fractionation factors between fluids and rocks ranging between 1.001 and 1.004. These values are similar to those of a recent experimental study and produce isotopic fractionation that is barely beyond analytical uncertainty (±1‰) for the observed degree of Li depletion. The good correlations between Li concentrations and LOI with mineralogical abundances reflect the importance of mineralogy in controlling Li concentrations and isotopic compositions of metamorphic rocks. Comparison of these results with those from a study of a contact halo around the Tin Mountain pegmatite suggests that the nature of the intrusion plays a critical role in controlling the behavior of Li in country rocks. Large, diffusive-driven isotopic fractionation over a small scale is expected to occur when rocks are infiltrated by Li-rich magmatic fluids, while metamorphic devolatilization results in minor isotopic fractionation on the aureole scale. [Copyright &y& Elsevier]

To better understand the influence of subduction on geochemical heterogeneity of the mantle, Mg isotope ratios were measured in a suite of continental basalts from East Asia. In contrast to global oceanic basalts (δ 26 Mg = − 0.25 ± 0.07‰), these continental basalts display considerable Mg isotope variation that can be roughly related to the time and place of eruption. The Cenozoic basalts have slightly lower δ 26 Mg values (n = 32; − 0.46 ± 0.05 to − 0.25 ± 0.06‰; average − 0.38‰) than the Mesozoic basalts (n = 12; − 0.41 ± 0.08 to − 0.23 ± 0.05‰; average − 0.30‰) and display a decreasing δ 26 Mg trend with increasing distance from the present subduction trench, particularly for the basalts from South China Craton. Such variations may be related to westward subduction of Pacific oceanic plate beneath East Asia. Some outliers of Mg isotopic data in the North China Craton probably involve additional contributions from earlier subduction and collision from the south (Tethyan ocean and Yangtze Craton) and north (Paleo-Asian ocean). The correlations between δ 26 Mg and element indices (e.g., TiO 2 , Li/Y) of contributions of slab melt indicate input of a component from the slab that is progressively dehydrating in the source region. Highly variable Sr, Nd and Pb isotope ratios for a given δ 26 Mg value suggest continuous interaction rather than a simple binary mixing of the mantle with subduction components. Intra-continental magmatism in East Asian is thus derived from a mantle wedge affected by the ongoing subduction of the Pacific Plate. Mixing with the uniform Mg isotopic compositions of MORB and OIB sources, global mantle convection likely erases the enriched components, whereas mantle wedges might be able to retain some Mg isotope heterogeneity. [ABSTRACT FROM AUTHOR]

The element, Mg and Sr isotope ratios of a weathering profile from Hubei, China have been measured in order to document the behavior of Mg isotopes during dolostone weathering. The profile is developed in the joint system, and the weathering intensity increases from the least weathered dolostone towards the joint plane. According to the element and isotope ratios, the weathering profile can be divided into the weakly and intensely weathered zones. In the weakly weathered zone, Mg/Al (atomic) ratios decrease from 4.99 to 1.75 with increasing weathering intensity, while Ca/Al ratios slightly decrease from 13.70 to 9.02. These suggest massive loss of Mg and simultaneous conservation of Ca. δ 26 Mg values in the weakly weathered zone slightly decrease from − 1.90‰ to − 2.22‰, lighter than the corresponding protolith (− 1.90‰). These element and isotope variations are likely caused by the cooccurrence of dolomite dissolution and calcite re-precipitation, suggesting that the fluids co-existing with the dolostone are saturated with respect to calcite while not saturated with respect to dolomite. By contrast, both Mg/Al and Ca/Al ratios of intensely weathered samples display a decreasing trend towards the joint plane, ranging from 1.62 to 0.16 and 10.24 to 2.05, respectively. The significant loss of Mg and Ca indicates the considerable dissolution of both dolomite and calcite. δ 26 Mg and 87 Sr/ 86 Sr values increase from − 2.22‰ to − 0.41‰ and from 0.71128 to 0.71368, respectively. These observations demonstrate that the isotope compositions of the intensely weathered residues are dominantly controlled by the increasing silicate fractions after carbonate dissolution, given that silicates have considerably high 87 Sr/ 86 Sr and δ 26 Mg values relative to the carbonates. Overall, our study reveals large Mg isotope fractionation during dolostone weathering. The direction and extent of Mg isotope fractionation are mainly governed by the dissolution and re-precipitation of carbonate minerals, which are further controlled by the fluid's saturation with respect to carbonate minerals (calcite and dolomite). Therefore, cautions should be taken when using whole rock Mg isotope data of dolostones to trace paleoclimate change and dolostone genesis. Primary dolomite could be partially reserved in weakly weathered dolostones and step-leaching experiments may help to reveal its bearing Mg isotope signals. [ABSTRACT FROM AUTHOR]

The Triassic-Jurassic boundary is one of the most important geological boundaries in the Earth's evolutionary history. Previous paleoclimate studies focused primarily on Tethys realm at low and middle latitudes, while the northern hemisphere has yet to be studied. Here, we report Mg-Zn-Cu isotopic data for a continuous terrestrial sedimentary section in the southern margin of the Junggar Basin to constrain chemical weathering intensity and climatic change during this critical interval at high latitude. This section is enriched in heavy Mg isotopes with δ26Mg ranging from 0.08‰ to 0.43‰. It displays a decreasing Zn isotopic composition with δ66Zn ranging from −0.05‰ to 0.24‰, as well as limited Cu isotope fractionation (δ65Cu = −0.06‰ to 0.09‰). These isotopic results, combined with major and trace elemental variations, suggest a reduced sedimentary environment and increasingly intensive chemical weathering across the boundary, responding to a warmer and more humid paleoclimate. Compared with other places located around Tethys, this regional climatic change in high latitude areas can be linked to known global climate change during this period. Our study provides new insights from high-latitude Asia to reconstruct the global pattern of climate change at the Triassic-Jurassic boundary. [ABSTRACT FROM AUTHOR]

Abstract: High-temperature isotopic variations documented in natural materials span a narrow range and call for precise and accurate isotopic analyses. Precisions better than ~0.03‰ (95% confidence interval) for δ 56Fe have been obtained by High-Resolution Multi-Collector Inductively Coupled Plasma Mass Spectrometry (HR-MC-ICPMS). Whether these uncertainties encompass all sources of error has not been carefully evaluated. The current study examines all parameters that can affect accuracy and precision of Fe isotopic measurements by HR-MC-ICPMS. It is demonstrated that accurate δ 56Fe measurements can be routinely achieved within the quoted uncertainties. [Copyright &y& Elsevier]

Abstract: Recent developments documenting high temperature isotope fractionations of a variety of elements (Li, Mg, Si, Ca, Fe, Ni) by a variety of processes (evaporation, chemical diffusion and thermal diffusion) are reviewed along with some recent applications of these fractionations to Earth and meteoritic problems. [Copyright &y& Elsevier]

Zinc isotopes have been increasingly utilized as a proxy for (bio)geochemical cycles of nutrient Zn in the ecosystem like soils, but the behavior and mechanism of Zn isotope fractionation during weathering and soil formation remain to be constrained. Here we reported zinc contents and isotope data for saprolites and laterites from two well-characterized mafic-rock weathering profiles, formed under a subtropical climate (South Carolina, USA) and a tropical climate (Hainan Island, China), respectively. Leaching experiments with basalt and andesite as the starting materials were also performed for comparison. The leaching results revealed no resolvable Zn isotope fractionation during dissolution of silicate minerals in the absence of organic ligands in solutions. The Hainan laterites show strong Zn depletion and have lower δ66Zn Lyon values (0.10‰ to 0.27‰) relative to the parent rock (0.31 ± 0.04‰), which are coupled with light copper isotope compositions in the same samples. These observations suggest that Zn mobilization was achieved via metal-organic complexation and Fe-colloids transportation. By contrast, saprolites from the South Carolina are extremely Zn-rich (ten-fold higher concentration than the parent rock) and most samples have higher δ66Zn values (0.10‰ to 0.65‰) compared with the parent rock (0.24 ± 0.04‰). Zinc isotope ratios of the South Carolina profile are positively correlated with iron isotope ratios (δ56Fe) and Fe3+/Fe Total , the latter of which is related to secondary Fe (hydr)oxide formation. These relationships link the high Zn contents and δ66Zn values of saprolites from the South Carolina profile to the substantial adsorption of Zn onto or isomorphic substitution into Fe (hydr)oxides that prefers heavier Zn isotopes. Our study highlights that Fe (hydr)oxides exert a strong influence on the re-distribution of nutrient Zn in natural soils and the (bio)geochemical cycling in natural soil systems. [ABSTRACT FROM AUTHOR]

There is a renewed interest in K isotope geochemistry driven by the advances in analytical precision and it is emerging as a useful tracer in a variety of disciplines ranging from Earth sciences to biology. However, high-quality K isotopic data for reference materials are still limited but highly needed. Here, we report high-precision stable K isotopic compositions (δ41K) for 23 commercially available international reference materials, including igneous, sedimentary, and metamorphic rocks, as well as an in-house seawater standard. Potassium in digested samples was separated by cation-exchange chromatography with Bio-Rad AG50W-X8 (200–400 mesh) resin in 0.5 N HNO 3 media. Potassium isotopes were measured on a Nu Plasma II high-resolution MC-ICPMS. The reproducibility of K isotopic analysis, based on over one year of measurements of pure K solutions and rock standards, was ≤0.06‰ (95% confidence interval). Synthetic solutions made by mixing single element standards to represent various rock matrices confirmed the accuracy of our methods. The 23 reference materials have δ41K values ranging from −0.562‰ to −0.253‰ and the seawater standard has a much higher δ41K value of 0.143‰. The comprehensive dataset presented here provides a reference for quality control and inter-laboratory comparison of high-precision K isotopic analyses for future studies. [ABSTRACT FROM AUTHOR]

This study presents a method for high-precision stable potassium (K) isotope analysis using High-Resolution Multi-Collector Inductively Coupled Plasma Mass Spectrometry (HR-MC-ICPMS). Dry and cold plasma is used to suppress the formation of argon hydrides that are direct isobaric interferences on K isotopes. The residual 40 Ar 1 H + beam is resolved from 41 K + by using pseudo-high resolution mode, providing interference-free shoulders of 39 K + and 41 K + for isotopic measurements. Precision of better than 0.06‰ (95% confidence interval) has been routinely achieved based on multiple measurements of pure K standard solutions and rock standards over a six-month period. Mismatches in K concentration (up to 20%) and acid molarity (up to 67%) between samples and bracketing standards, as well as the presence of substantial amount of matrix element Rb, do not significantly affect the analytical precision and accuracy. Our study documents that high-precision K isotope analysis can be routinely achieved for a wide variety of natural samples by HR-MC-ICPMS, which makes it possible for using K isotopes to trace major geological processes. [ABSTRACT FROM AUTHOR]

The radiogenic 87 Rb- 87 Sr system has been widely applied to the study of geological and planetary processes. In contrast, the stable Sr isotopic composition of the bulk silicate Earth (BSE) and the effects of igneous differentiation on stable Sr isotopes are not well-established. Here we report the stable Sr isotope ( 88 Sr/ 86 Sr, reported as δ 88/86 Sr, in parts per mil relative to NIST SRM 987) compositions for ocean islands basalts (OIB), mid-ocean ridge basalts (MORB) and komatiites from a variety of locations. Stable Sr isotopes display limited fractionation in a OIB sample suite from the Kilauea Iki lava lake suggesting that igneous processes have limited effect on stable Sr isotope fractionation (±0.12‰ over 20% MgO variation; 2sd). In addition, OIB (δ 88/86 Sr = 0.16–0.46‰; average 0.28 ± 0.17‰), MORB (δ 88/86 Sr = 0.27–0.34‰; average 0.31 ± 0.05‰) and komatiites (δ 88/86 Sr = 0.20–0.97‰; average 0.41 ± 0.16‰) from global localities exhibit broadly similar Sr stable isotopic compositions. Heavy stable Sr isotope compositions (δ 88/86 Sr > 0.5‰) in some Barberton Greenstone belt komatiites may reflect Archean seawater alteration or metamorphic processes and preferential removal of the lighter isotopes of Sr. To first order, the similarity among OIBs from three different ocean basins suggests homogeneity of stable Sr isotopes in the mantle. Earth's mantle stable Sr isotopic composition is established from the data on OIB, MORB and komatiites to be δ 88/86 Sr = 0.30 ± 0.02‰ (2sd). The BSE δ 88/86 Sr value is identical, within uncertainties, to the composition of carbonaceous chondrites (δ 88/86 Sr = 0.29 ± 0.06‰; 2sd) measured in this study. [ABSTRACT FROM AUTHOR]

We report a new method for precise analysis of gallium (Ga) isotopic composition in geological samples. The purification of Ga is achieved by a three-step ion exchange chromatography to remove matrix and interfering elements. The 71 Ga/ 69 Ga ratios are analyzed by multi-collector inductively-coupled-plasma mass-spectrometer (MC-ICP-MS). The external reproducibility of the measurements (0.05‰, 2 s.d.) was assessed by replicate analyses of the USGS BCR-2 and BHVO-2 standards. This newly developed technique was then used to investigate the extent of Ga isotopic fractionation during igneous processes by analyzing well-characterized samples from the Kilauea Iki lava lake, USA. These samples were formed in a closed system and have MgO contents ranging from 26.9 to 2.4 wt.%. We found that igneous processes do not fractionate Ga isotopes within the analytical uncertainty and that the Ga isotopic composition of mafic-ultramafic lavas can be used to estimate the composition of their mantle source. Twelve ocean island basalts, two mid-ocean-ridge basalts, one continental flood basalt and one komatiite have homogeneous and nearly identical Ga isotopic compositions within analytical uncertainties averaging 0.00 ± 0.06‰ (2 s.d.). This value represents the best estimate for the Ga isotopic composition of the bulk silicate Earth [ABSTRACT FROM AUTHOR]

The discovery of spherical accumulations of iron oxide mineralization on Mars has prompted renewed interest in widespread but enigmatic accumulations of iron oxide on Earth. The rinded iron oxide concretions found in the Navajo Sandstone (Utah, USA) have been among the most extensively studied of these occurrences. At least four different models have been proposed for the formation of iron oxide concretions in the Navajo Sandstone. These include 1) mixing of reduced Fe(II)-bearing and oxygenated groundwaters, 2) neutralization of oxidized, acidic waters by calcite concretions, 3) neutralization of reduced, acidic waters by calcite concretions, and 4) oxidation of siderite by microbes during invasion of the Navajo aquifer by oxygenated groundwaters. We measured δ56Fe values of iron-bearing minerals from whole-rock Navajo Sandstone, concretionary iron oxide cements, iron oxide stains that are oriented in the direction of paleo-groundwater flow, iron oxide stains on the interior of rinded concretions, and ferroan carbonate cements. We have then combined these data with published measurements to evaluate each of the four published models. Whole rock Navajo Sandstone from the Kolob Plateau has been interpreted to be unaltered by bleaching fluids and has a δ56Fe value of 0.21‰. Iron in the Kolob Plateau sample comprises a mixture of hematite grain coatings (δ56Fe = 0.12‰) and a magnetic mineral fraction (interpreted to comprise largely detrital magnetite) that has a higher δ56Fe value (0.45‰). Iron oxide cements have δ56Fe values that are typically negative and as low as −0.74‰. In contrast, iron oxide-stained Navajo Sandstone in close proximity to a rinded iron oxide concretion has δ56Fe values that are 0.9 to 1‰ greater than the δ56Fe value of the associated iron oxide concretion. Finally, ferroan carbonates from the Navajo Sandstone yield δ56Fe values as low as −0.86‰. These values are in good agreement with measurements published previously for lithologies for which comparable measurements have been made. The presence of ferroan carbonates with negative δ56Fe values is evidence that aqueous Fe(II) was transported through the Navajo Sandstone aquifer and produced carbonate mineralization with negative δ56Fe values. The difference in δ56Fe values between the iron oxide concretions and associated iron oxide staining is more consistent with equilibrium fractionation between aqueous Fe(II) and Fe(II) adsorbed on quartz grains than it is with equilibrium fractionation between aqueous Fe(II) and any reduced or oxidized iron oxide mineral species. The negative δ56Fe values of the iron oxide concretions are best explained by oxidation of a reduced iron phase in a closed system from which no aqueous Fe(II) could escape. We interpret the iron isotope data as evidence that iron was stripped from Navajo sand grains by reducing fluids, reprecipitated in the Navajo aquifer as siderite, and oxidized by groundwaters under closed conditions during incision of the Colorado Plateau. • Iron oxide concretions with negative δ56Fe values occur in Navajo Sandstone • Ferroan carbonate in the Navajo Sandstone also has low δ56Fe values • The Navajo Sandstone aquifer contained reduced iron and carbonate-bearing waters • Low δ56Fe values of Navajo concretions unlikely if primary precipitate was iron oxide • Low δ56Fe values are evidence that iron oxide concretions formed from siderite [ABSTRACT FROM AUTHOR]

Mass-dependent K isotopic fractionations can be used to trace cosmochemical, geological, and biological processes such as evaporation/condensation, core formation, magmatic processes, weathering, and cellular metabolism. However, the application of stable K isotopes has been limited by major isobaric interferences from Ar, common on conventional multi-collector inductively-coupled-plasma mass-spectrometer (MC-ICP-MS), particularly for the low-K samples. Here, we present a set of high-precision K isotopic data acquired on terrestrial rocks, seawater, as well as a lunar meteorite using the recently released Nu Sapphire™ MC-ICP-MS that utilizes a collision cell to minimize Ar based interferences while maintaining remarkably high K sensitivity (≈ 2000 V/ppm). The influence of several parameters on the precision and accuracy of the K isotopic data has been evaluated, including total K concentration, K intensity mismatch between sample and standard, HNO 3 molarity mismatch between sample and standard, and the presence of matrix elements. We found that the Nu Sapphire™ can be used to acquire precise and accurate data using as little as 125 ng of K, which represents an improvement by a factor 10 compared to what has been done on previous instruments. We present data for 23 previously analyzed samples; these data are highly consistent with literature values. On the other hand, accurate measurements are conditioned 1) to the close matching of sample and standard K intensities (a 1% mismatch creates a 0.02‰ offset on the 41K/39K ratio), and 2) to the absence of Ca (a Ca/K ratio of 1% creates a 0.069‰ offset on the 41K/39K ratio). In addition, Rb/K, Na/K, Ti/K and Cr/K ratio should also be maintained under 2.5% to avoid isotopic offset. We confirm the existence of significant mass-dependent K isotopic variations in terrestrial samples and that lunar rocks are isotopically heavier than terrestrial rocks. The incomparable sensitivity offered by the Nu Sapphire opens the possibility for high-precision K isotopic measurements across a wide range of samples for diverse applications. [Display omitted] [ABSTRACT FROM AUTHOR]