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ABSTRACT Zoned dolomite crystals, characterized by their dirty core and clear outer rim, are common in most island dolostones. The conditions under which these dolostones formed, however, remains controversial. To explain the origin of island dolostones, here, in situ determinations of Mg isotopic compositions, major (Ca, Mg) and trace (Fe, Mn, Sr, Na) elemental concentrations are carried out for the cores and rims of zoned dolomite crystals for dolostone samples from the Sanya Formation (Lower Miocene) and Meishan Formation (Middle Miocene) of well XK‐1 drilled on Shidao Island, the Xisha Islands. For all of the dolomite crystals, both cores and rims are formed of high‐Ca calcian dolomite, but the cores have higher %Ca, Sr and Na concentrations than the rims. Moreover, the cores (−3.85 to −2.95‰) have ca 0.2 to 0.5‰ lower δ26Mg values than the rims (−3.34 to −2.60‰). The difference in δ26Mg values between the dolomite crystal core and the rim cannot be explained by the presence of calcite inclusions or dolomite recrystallization, but rather reflect the nature of Mg isotopic fractionation due to the growth of the dolomite crystals during different stages of replacement. For zoned dolomite crystals, the progressive decrease in Ca and trace element concentrations but increase in δ26Mg values from dirty core to clear rim demonstrate that: (i) the dolomite crystal cores grow via a diffusion‐limited process; and (ii) the rims form through an incremental process (interface‐controlled) whereby the zone of dissolution/dolomite precipitation was very thin and simply repeated many times until it had fully developed. This growth model of zoned dolomite crystals may be applied to dolostones that share similar zoned patterns in petrography and geochemistry throughout the world. [ABSTRACT FROM AUTHOR]

The widespread Cenozoic alkaline magmatism within and around the Tibetan Plateau offers a prime opportunity to probe the nature of the mantle at the depths where basalt magmas originate. The close temporal and spatial relationship between volcanism and regional strike‐slip fault systems also helps better understand the geodynamics of outward growth of the plateau in response to the continued India‐Asia convergence. We present a comprehensive study of the deeply sourced alkaline basalts formed along the Kunlun strike‐slip fault with the aim of understanding their petrogenesis and the composition of mantle sources beneath the northeastern Tibetan Plateau. High Nb/U and Ce/Pb ratios and relatively depleted bulk‐rock Sr‐Nd‐Pb isotope compositions corroborate the mantle origin of these alkaline basalts. Homogeneous and low 87Sr/86Sr of clinopyroxene indicates negligible crustal contamination during magmatic evolution. Low δ26Mg in the alkaline basalts and positive correlations with Hf/Sm and Ti/Ti* indicate that the basalts were derived from mantle that was metasomatized by melts derived from sedimentary carbonates during the Paleo‐Tethyan seafloor subduction. Based on 40Ar/39Ar dating results, it appears that the alkaline basaltic magmatism in the northeastern Tibetan Plateau occurred simultaneously with Kunlun strike‐slip faulting. These observations suggest that the India‐Asia convergence must have reactivated ancient subduction plate boundaries and resulted in strike‐slip faulting along these suture zones within and around the Tibetan Plateau. The eruption of low‐volume and deeply rooted alkaline basalts may have been controlled by fractures associated with the strike‐slip fault systems. Plain Language Summary: Small volcanoes that erupted basalt lava with alkaline compositions about 11.5 million years old occur along the northeast margin of the Tibetan Plateau and are close to a major fault system, the Kunlun Fault. Based on their chemical compositions, these basalts were melted from the upper mantle and provide information on the composition of the mantle beneath the plateau. Our chemical, geochronological, and isotopic analyses of the lava indicate that the sub‐plateau mantle was indeed affected by ancient oceanic slab subduction, including subduction of seafloor sediments that resulted in the addition of carbonate‐rich melts to the sub‐plateau mantle, making the mantle more susceptible to melting and more likely to produce alkaline basalt when it did melt. The age of the basalt lava overlaps the age of movement along the Kunlun fault, which suggests that the magma may have been produced from the same tectonic movements that produced the fault and risen through the crust along fractures associated with the fault. Key Points: The light Mg isotopes of the studied alkaline basalts highlight the occurrence of mantle carbonatite metasomatism via Paleo‐Tethyan oceanic slab subductionAlkaline basalts in northeastern Tibetan Plateau erupted coevally with Kunlun strike‐slip faulting that gave rise to mantle upwelling and decompression meltingLithospheric extrusion induced by India‐Asia collision reactivated pre‐existing lithospheric weakness and triggered mantle melting beneath the NE and SE plateau [ABSTRACT FROM AUTHOR]

Impact glasses are abundant in the lunar regolith, and Mg isotopes have the potential to trace components from various lunar crustal reservoirs, which have recently been shown to exhibit large Mg isotopic fractionations. However, it remains unclear whether Mg isotopic fractionation occurs during the formation of impact glasses. In this study, we report in situ Mg isotopic and elemental compositional data for agglutinatic glasses returned by the Chang'e 5 mission and obtained using the laser ablation split stream technique. Vesicular textures, Fe–Ni alloys, tiny Fe droplets, and high Ni contents suggest the studied agglutinatic glasses had an impact origin. The agglutinatic glasses exhibit large Mg isotopic fractionation, with δ26Mg values ranging from −1.36 ‰ to −0.01 ‰. The lack of correlations between δ26Mg values, Ni contents, and ratios between volatile and relatively refractory elements (K/La, Rb/Sr, and Ce/Pb) indicate the addition of a meteoritic component and evaporation was not the major process responsible for the measured Mg isotopic variations. In fact, the MgO profiles and correlations between δ26Mg and MgO, Na 2 O, Sc, Sr, CaO/Al 2 O 3 , and δEu reflect Mg isotopic fractionation caused by Mg diffusion from a region with high Mg contents (i.e., more melted pyroxene) to one with lower contents (i.e., more melted plagioclase). Diffusion modeling shows that the duration of diffusion was less than a fraction of a second. Our results indicate that chemical diffusion can produce large Mg isotopic fractionation in impact glasses on a scale of at least tens of microns, and that isotopic fractionation driven by chemical diffusion needs to be considered when the Mg isotopic compositions of impact glasses are used to identify different lunar rock reservoirs. [ABSTRACT FROM AUTHOR]

Investigating the carbonate preservation efficiency (CPE) of continental crust is crucial to understand the global carbon cycle, which requires constraints on initial carbonate abundances (ICAs) of crustal rocks. To link Mg isotopes to ICAs, we present elemental and Mg isotopic data for Himalayan carbonate‐bearing and carbonate‐free metasedimentary rocks. Given no evident melt extraction or external‐fluid infiltration, ICAs of these samples can be independently estimated by elemental data. Despite different carbonate species in the protoliths, all the samples show congruent relationship between their δ26Mg and ICAs, owing to the elevated carbonate δ26Mg and Mg/Ca in protoliths of calcite‐rich samples resulting from diagenetic processes. When collated with literature data, we suggest the observed correlation here can be applied to most carbonate‐bearing (meta‐)sedimentary rocks. Based on a steady state box‐model, we constrained the modern net carbonate accretion flux (9.50−5.56+9.50 ${9.50}_{-5.56}^{+9.50}$ Tmol/year) and the average time‐integrated CPE (∼80−43+20 ${80}_{-43}^{+20}$%) for continental crust. Plain Language Summary: Investigating the fate of carbonate preserved in continental crust is fundamental for understanding its role playing in the global carbon cycle, but is hindered by the lack of knowledge about the initial carbonate abundance of metasedimentary rocks prior to modification (e.g., anatexis). By analyzing Himalayan metasedimentary rocks, here we show a congruent relationship between their δ26Mg and initial carbonate abundances, irrespective of their protolith carbonate species, and suggest it is applicable to most carbonate‐bearing (meta‐)sedimentary rocks. Based on this relationship, the carbonate preservation in continental crust was simulated using a steady state box‐model. The results indicate a very high carbonate accretion influx to the continental crust, seven times higher than the C degassing flux in the mid‐ocean ridge. Considering explosive degassing of the accreted carbonates during episodic tectonomagmatic events, the continental crust could have been an important driving force for regulating the atmospheric CO2 during Earth's history. Key Points: A strong correlation between bulk δ26Mg of carbonate‐bearing metasedimentary rocks and their initial carbonate abundances was establishedDespite different carbonate species in protoliths, both the calcite‐rich and dolomite‐rich samples show nearly identical correlationsSteady state box‐model on the carbonate preservation efficiency of continent indicates it is a notable force to regulate atmospheric CO2 [ABSTRACT FROM AUTHOR]

The distribution of Mg isotopes in minerals is becoming increasingly relevant in Earth science. Usually, they are determined by dissolving mineral concentrates and, after purifying Mg with ion exchange resins, analysing the resulting solutions by TIMS or, most often, MC‐ICP‐MS. When applied to individual minerals, these methods are slow and prone to contamination from impurities in the concentrates, inconveniences that may be avoided using spot analysis techniques such as LA‐MC‐ICP‐MS or SIMS, albeit at the price of a large instrumental mass fractionation (IMF) and isobaric interferences, most prominent in the former. Here, we studied the potential of the multi‐collector SHRIMP II ion microprobe for measuring Mg isotopes in Fe‐Mg silicates and oxides. We found that, when corrected for the divergence of the Mg ion paths within the sample chamber caused by the Earth's magnetic field, the SHRIMP's IMF overwhelmingly depends on the mineral species, and the effects of variable chemical composition are negligible. We propose that the IMF is caused by the force constant difference, ∆F, between "hard" and "soft" bonds linking the ions of the studied element to the mineral lattice. Given that ∆F is a constant for each mineral species, we calculated IMF‐correction factors for the most common Mg‐bearing minerals. The thus‐calculated correction factors permit the analysis in the same session, and with reasonable accuracy (within ~ 0.3‰ of the δ26Mg determined by SN‐MC‐ICP‐MS analyses of concentrates), of samples from different mineral species, facilitating the application of Mg isotopes to terrestrial studies. [ABSTRACT FROM AUTHOR]

It is debated whether there was strong climate seasonality during the Eocene, which provides a close geological analogy for near‐future scenarios of greenhouse gas emissions. Lithological data suggest the existence of a broad arid zone centered around 30°N paleo‐latitude, while a humid climate was supported by palaeobotanic assemblages in East Asia. Here, we report the occurrence of massive primary lacustrine dolomite and magnesite in the central East Asia during the middle Eocene. We provide a novel perspective from magnesium isotopes to link the formation of Mg‐carbonates to seasonal dry‐wet cycles. Rapid magnesium input during the rainy season and intense evaporation in the dry season likely caused the formation of magnesium carbonates in an enclosed lake. These findings provide insights into hydroclimatic seasonality during the Eocene, contributing to our understanding of the hydrological cycle response to a greenhouse climate. Plain Language Summary: The Eocene epoch serves as a valuable analog for future climates. While geochemical reconstructions and model simulations have illuminated lower thermal latitudinal gradients and seasonal variations, our understanding of Eocene precipitation patterns lags, encompassing wet‐dry conditions and seasonal dynamics. To enhance our understanding of Eocene precipitation patterns, we investigated a 158‐m‐thick primary dolomite and magnesite deposition in the middle Eocene lacustrine succession of the Lushi Basin, central China. From a novel perspective, we provide evidence from magnesium isotopes to link the formation of Mg‐carbonates to climate seasonality. Clumped isotopes (∆47) and Mg isotopes provide evidence supporting the formation under specific hydroclimatic conditions. A surge in magnesium input during the rainy season, succeeded by intense evaporation in the dry season, likely led to the development of extensive Mg carbonate layers in an enclosed lake. The prevalence of seasonal variations in precipitation in the central East Asia during the middle Eocene is further substantiated by a compilation of the occurrence of Eocene lacustrine Mg‐carbonates in this region. Our findings suggest that while Eocene temperature seasonal variability was weak, significant precipitation seasonality could have coexisted. Key Points: Magnesium isotopes of Eocene lacustrine dolomites and magnesites provide insight into the presence of seasonal precipitation variationMg‐carbonate formation was linked to hydroclimatic seasonality characterized by alternation between heavy rainfall and strong evaporationWeak temperature seasonal variation and significant precipitation seasonality could have coexisted in central East Asia during the Eocene [ABSTRACT FROM AUTHOR]

The investigation of magnesium (Mg) isotopes in dolomite has mainly focused on marine dolomite environments, leaving a significant gap in the understanding of their dynamics within lacustrine settings, especially in saline lake basins. In this study, a total of 16 sediment core samples from Well BX-7 in the Qianjiang Depression were sequentially selected for scanning electron microscope observation, whole-rock analysis for major and minor elements, and isotopic measurements including δ18Ocarb, δ13Ccarb, δ26Mgdol, and δ26MgSi. In addition, two intact cores were subjected to detailed analysis on the centimeter scale. Sedimentation models were established to elucidate dolomite formation under contrasting climatic conditions, specifically humid climates with a significant riverine Mg input versus relatively dry conditions with a lower Mg input. Furthermore, a quantitative model was developed to assess the magnesium flux and isotopic mass balance within lacustrine systems, simulating the magnesium isotope variations in lake water under different climatic scenarios. The dolomite sample data at a smaller scale (sampling interval ≈ 3~5 mm) demonstrate a consistent trend with the established model, providing additional confirmation of its reliability. Dolomite precipitated under humid climatic conditions exhibits a lower and relatively stable δ26Mgdol, lower δ18O, and higher CIA, indicating higher river inputs and relatively stable Mg isotope values of lake water controlled by river input. Nevertheless, dolomite formed under relatively dry climatic conditions shows a relatively high δ26Mgdol, higher δ18O, and lower CIA, suggesting reduced river inputs and weathering intensity, as well as relatively high magnesium isotope values of the lake water controlled by dolomite precipitation. This study contributes to the understanding of magnesium isotopes in lacustrine dolomite systems. [ABSTRACT FROM AUTHOR]

Abstract The widespread Cenozoic alkaline magmatism within and around the Tibetan Plateau offers a prime opportunity to probe the nature of the mantle at the depths where basalt magmas originate. The close temporal and spatial relationship between volcanism and regional strike‐slip fault systems also helps better understand the geodynamics of outward growth of the plateau in response to the continued India‐Asia convergence. We present a comprehensive study of the deeply sourced alkaline basalts formed along the Kunlun strike‐slip fault with the aim of understanding their petrogenesis and the composition of mantle sources beneath the northeastern Tibetan Plateau. High Nb/U and Ce/Pb ratios and relatively depleted bulk‐rock Sr‐Nd‐Pb isotope compositions corroborate the mantle origin of these alkaline basalts. Homogeneous and low 87Sr/86Sr of clinopyroxene indicates negligible crustal contamination during magmatic evolution. Low δ26Mg in the alkaline basalts and positive correlations with Hf/Sm and Ti/Ti* indicate that the basalts were derived from mantle that was metasomatized by melts derived from sedimentary carbonates during the Paleo‐Tethyan seafloor subduction. Based on 40Ar/39Ar dating results, it appears that the alkaline basaltic magmatism in the northeastern Tibetan Plateau occurred simultaneously with Kunlun strike‐slip faulting. These observations suggest that the India‐Asia convergence must have reactivated ancient subduction plate boundaries and resulted in strike‐slip faulting along these suture zones within and around the Tibetan Plateau. The eruption of low‐volume and deeply rooted alkaline basalts may have been controlled by fractures associated with the strike‐slip fault systems.

This study is an investigation of the effect of the pairing strength using different types of pairing (surface, volume and mixed surface–volume pairing) on the ground state properties of even and odd Mg isotopes. The Hartree–Fock–Bogoliubov (HFB) theory and two types of Skyrme forces, SKI3 and SLY6, were employed in our calculations. The pairing strength values are modified at each execution of the HFBTHO code to obtain the experimental values of binding energy for the investigated even and odd isotopes separately. Then a new formula for pairing strength for each even and odd isotope independently for all the pairing, surface, volume and mixed surface–volume types is developed by fitting the pairing strength values to the mass number. Based on the newly generated formulas for pairing strength, some of the calculated ground state properties are found to depend strongly on the pairing strength values and the pairing type also plays an essential role in providing accurate results. [ABSTRACT FROM AUTHOR]

The magnesium isotopic composition (δ26Mg) of river water is a promising indicator of continental chemical weathering. While many studies have investigated the factors that influence riverine δ26Mg, the impact of hydrology remains unclear. In this study, we collected eighty-four samples of stream water with nearly diurnal resolution in 2018 from a well-monitored, carbonate-rich catchment on the Chinese Loess Plateau. Our results demonstrate that δ26Mg in stream water increases (0.14 ± 0.05 ‰) from dry to wet seasons, but decreases (0.21 ± 0.05 ‰) during rainfall events. These variations closely link to the dissolution and deposition of carbonates (i.e., source-related processes), and the adsorption and desorption of the exchangeable pool. Carbonate dissolution during rainfalls lowers the δ26Mg in stream water, while during rainfall-free periods in wet seasons carbonate deposition elevates the δ26Mg. Conversely, the exchangeable pool, reflecting carbonate weathering in the geological past, cannot be a source of Mg in stream waters, but act as a transfer Mg-pool. At an instantaneous picture, it releases the majority of Mg (>80 %) to stream water, and thereby has buffering effect on riverine Mg isotope. This highlights the significance of considering the buffering effects when studying riverine δ26Mg variations. Overall, our findings suggest that the response of δ26Mg to hydrology is typically associated with extreme hydrologic events and has important implications for tracing continental weathering. [ABSTRACT FROM AUTHOR]

In order to better constrain the Mg isotopic composition of the mantle, we have analysed twenty-eight samples of both oceanic and continental peridotite using a high-precision, critical mixture double spiking approach. The unaltered samples show no variability δ26Mg in outside analytical uncertainty and yield a value of −0.236 ± 0.006‰ (2 s.e.) for the accessible mantle, substantiating its non-chondritic composition. We have also determined inter-mineral Mg isotopic fractionations for a sub-set of samples. We document small but significant differences in δ26Mg between olivine and pyroxenes, Δ26/24Mg ol/cpx = −0.118 ± 0.018‰ and Δ26/24Mg ol/opx = −0.056 ± 0.018‰, in excellent agreement with ab initio calculations for temperatures ∼1000 °C, as recorded by mineral thermometry in the peridotites. The differences in δ26Mg between olivine and spinel (Δ26/24Mg ol/sp) are more variable and generally higher than theoretical calculations at corresponding temperatures, likely due to incomplete Fe-Mg diffusive exchange during post-eruptive cooling of the xenoliths. Using these data, together with a recently determined olivine-melt fractionation factor for Mg isotopes, we show that partial melting has a negligible influence on the δ26Mg of residual peridotites. This helps account for the minimal variability of δ26Mg in fresh, mantle peridotites. However, the δ26Mg of primary mantle melts are predicted to be discernibly higher than their sources (Δ26Mg ∼ 0.06‰ and ∼0.123‰ for representative partial melts of peridotitic and pyroxenitic sources respectively) across a wide range of melting conditions. Such elevated δ26Mg values are not generally observed in the current dataset of mantle derived melts. We propose that this inconsistency is likely a consequence of diffusive fractionation during partial re-equilibration between low Mg/Fe melts migrating through high Mg/Fe mantle en route to the surface. [ABSTRACT FROM AUTHOR]

The growth and morphology of stromatolites have been previously linked to diverse microbial activities. However, the role of microbial metabolisms on carbonate formation during stromatolite growth remains controversial. Magnesium isotopes have been proposed to serve as a tracer of microbial carbonate formation, implying a potential biological isotope fractionation. To further elucidate whether Mg isotope fractionation is modified during microbial carbonate formation, this study reports Mg isotope compositions of Holocene stromatolites and pore waters from Lagoa Salgada, a coastal ephemeral lake in Brazil. The stromatolitic carbonates are composed of high-Mg calcites characterized by extremely positive inorganic δ13C values, up to +20‰, and variable δ26Mg values, ranging from −2.98‰ to −0.68‰. Multiple pieces of evidence consistently demonstrate changes in microbial metabolism resulting from ecosystem fluid chemistry evolution. However, the direction of Mg isotope fractionation associated with carbonate precipitation remained invariable despite the changes in microbial activity. Mineralogical features indicate that the stromatolitic carbonates formed via 2-D nucleation. An isotopic mass balance calculation based on the observed variations in δ26Mg values and Mg concentrations of associated pore waters argues for an Mg isotopic equilibrium fractionation factor of −2.60‰ associating with the carbonate formation at Lagoa Salgada, well-matching experimental values for abiotic calcite precipitation. Thus, the observed δ26Mg variability of stromatolitic carbonates is primarily controlled by the changes of physicochemical conditions in the ambient fluid. We infer that during the formation of stromatolites, a consortium of different microbial communities produces extracellular polymeric substances, which serve as a substrate for carbonate nucleation from ambient fluid without significantly affecting the fractionation of Mg-isotopes. Our findings shed light on the effects of microbial processes on carbonate formation during stromatolite growth and improve the current understanding of the Mg isotope record in microbial carbonates. [ABSTRACT FROM AUTHOR]

Magnesium (Mg) isotopes have been utilized to constrain continental weathering; however, to date, little is known about the climate effects on Mg isotope fractionation during weathering. In this study, we measured δ26Mg values of bulk regolith and exchangeable fraction in two granite regolith profiles developed under temperate, semiarid and tropical, humid climate conditions, respectively. Combined with mineralogy and element composition, we aimed to investigate how climate influences fractionation patterns of Mg isotopes during chemical weathering. At the temperate site, δ26Mg values of regolith are slightly higher than that of the bedrock and negatively correlated with τMg,Th. Correspondingly, the exchangeable Mg is characterized by low δ26Mg values. These results can be explained by the formation of small number of clay minerals. For the tropical regolith profile, δ26Mg values decrease toward the surface, and the regolith has either lower δ26Mg values above −250 cm or higher δ26Mg values below −250 cm relative to the bedrock. The δ26Mg value of exchangeable Mg is markedly lower than that of the regolith and varies significantly. These results can be explained by the mixing of Mg from solid weathering products and atmospheric deposition. The Mg from rainwater and/or marine aerosol deposit on the regolith and some may enter the crystal structure of the illite. The deposited Mg can overprint the granitic Mg, and the δ26Mg value of shallow regolith samples will reflect mixing between granitic and atmospheric sources. The compilation of our and previously published Mg isotopic data reveals the potential control of climate on Mg isotope fractionation during continental weathering. Plain Language Summary: The isotopes of magnesium (Mg) can serve as a useful tracer in understanding the chemical weathering processes of silicate rocks. Previous studies have mainly focused on Mg isotope fractionation during silicate weathering by investigating single weathering profiles that have developed under constant climate conditions; however, the impact of climate on Mg isotope fractionation during silicate weathering remains poorly understood. Here, we analyzed Mg isotope compositions in two granite regolith profiles developed under temperate (semiarid) and tropical (humid) climate conditions. Our study suggests that the patterns of Mg mobilization and Mg isotope fractionation during granite weathering differ significantly in distinct climate zones. The compilation of our and previously published Mg isotopic data reveals that climate may have a huge impact on Mg isotope fractionation during terrestrial weathering, which is of significance to interpret the Mg isotope compositions of weathering products and river waters at a global scale. Key Points: Magnesium isotope fractionation differs significantly during granite weathering in temperate and tropical climate zonesThe formation of illite results in heavy magnesium isotope enrichment in solid weathering productsAtmospheric deposition contributes a mass of light magnesium isotopes to shallow regolith under extremely weathered conditions [ABSTRACT FROM AUTHOR]

Diffusion-driven isotopic fractionation during crystal growth and crystal-melt interaction has important implications for identifying diffusive processes and estimating the timescales of magmatic processes. Especially, Fe-Ti-Cr oxides are common crystalline phases in both relatively unfractionated and highly evolved basaltic magmas and may record the evolution history of the host magmas. High-precision Zn and Mg isotopic compositions for a set of Fe-Ti-Cr oxides and their host lavas in three types of Cenozoic basalts (sodic, transitional and potassic) from northeast China are reported in this study. These oxide crystals exhibit a wide range of chemical compositions (e.g., FeO T = 20.6–49.1 wt%, TiO 2 = 1.38–8.70 wt%, Cr 2 O 3 = 16.2–43.1 wt%), reflecting substantial chemical disequilibrium with their host magmas. A large range of δ66Zn JMC-Lyon from −1.12‰ to 0.24‰ and δ26Mg DSM-3 from 0.04‰ to 0.80‰ is observed in oxides (n = 17) from all three lava types. Compared with the host basalts, the oxides are enriched in lighter Zn with △66Zn oxide-melt (δ66Zn oxide –δ66Zn melt) from −1.43‰ to −0.19‰ and heavier Mg with △26Mg oxide-melt (δ26Mg oxide –δ26Mg melt) from 0.43‰ to 1.13‰. Comparison with existing theoretical work indicates that these isotopic offsets, even if compositional effects of oxides are considered, are too large to be in equilibrium. There are negative correlations between tetrahedral-site Fe2+ + Zn2+ and Mg2+ (R2 = 0.97) and between δ66Zn and δ26Mg (R2 = 0.66) for the oxides, which are best attributed to isotopic fractionation induced by Zn–Mg inter-diffusion, supported by the core-to-rim increase of Zn contents and decline of Mg contents in zoned oxides. Our results thus suggest that inter-diffusion between Mg and Zn can occur during crystal growth and reaction with host magmas, which is commonly observed between Mg and Fe, and this process is accompanied by strong Zn and Mg isotope fractionations. Such fractionations can be utilized to estimate the timescales for the formation of zoned minerals. Numerical simulation yields a short (∼35 days) diffusion interval for the observed Zn and Mg isotopic variations in the investigated oxide crystals, which indicates rapid cooling of the host lavas. Our results also indicate that fractional crystallization of oxides would lead to slight δ66Zn increase and δ26Mg decrease in the residual melts. Such fractionation should be considered while characterizing the Zn and Mg isotopic compositions of evolved magmas with low MgO and Zn contents. [ABSTRACT FROM AUTHOR]

Basalts formed during the early development of an arc are usually buried, but they record critical information relating to the mantle source of the overriding plate prior to the addition of subducting slab components. Basalts recovered at Site U1438 of International Ocean Discovery Program (IODP) Expedition 351, in the Amami Sankaku Basin (ASB), formed during the transition from forearc basalt (FAB) and boninite eruptions to stratovolcano developments in the Izu-Bonin-Mariana (IBM) arc. In this study, we present magnesium (Mg) isotopic data (δ26Mg) for the ASB basalt core samples, formed at arc inception in the absence of down-going slab components, and report the implications of these data for the characteristics of the mantle sources underlying the proto-IBM arc. The δ26Mg values of the ASB basalts range from −0.21‰ to +0.08‰, with an average value of −0.13 ± 0.07‰. These values are systematically higher than those of mid-ocean ridge basalts (MORBs) (−0.25 ± 0.06‰). No obvious effects of post-eruptive alteration, fractional crystallization, partial melting, or subduction component addition can be identified in the Mg isotopic compositions either of the ASB basalts or their mantle source, given the absence of correlations between the δ26Mg values and the proxies of these processes (e.g., K/Nb, Nb/Zr, Ba/La, Nb/Y, etc.). We conclude that the high δ26Mg values of the ASB basalts are inherited from their mantle source that was enriched in heavy Mg isotopes. Melting of an ultra-depleted, refractory spinel peridotite source containing talc-bearing serpentinite components can explain the origin of the heavy Mg isotopic compositions in the ASB basalts. The presence of water-rich serpentinite components in the mantle provides a new perspective on how a refractory mantle source can undergo partial melting at temperatures and pressures similar to the formation of MORBs without the influence of subducted materials, which has important implications for the initiation of plate subduction. Furthermore, the Mg isotopic contributions of subducted additions and mantle source materials should be carefully discussed in future studies due to the heterogeneity of Mg isotopes in the mantle. [ABSTRACT FROM AUTHOR]

The Mg ( δ 26 M g), Ca ( δ 44 C a), and Sr ( 87 S r / 86 S r) isotopic compositions of pore fluids, bulk carbonates, planktonic foraminiferal tests, and bulk clays from ODP Site 762 Hole B are presented, as are pore fluid and bulk carbonate δ 26 M g and 87 S r / 86 S r from ODP Site 806 Hole B and pore fluid δ 26 M g from ODP Site 807 Hole A. The primary objective of the study is to elucidate the major processes controlling marine pore fluid δ 26 M g , specifically the effects of calcite recrystallization and authigenic clay precipitation in sedimentary sections with relatively high carbonate contents. Such studies are critical for evaluating the potential of pore fluids in carbonate section to drive diagenetic alteration, which can compromise applications of geochemical proxies to the past. Pore fluid δ 26 M g values at all three sites range from −0.83 to −0.13‰ and exhibit a systematic increase with depth. Bulk carbonate δ 26 M g generally decrease with depth, ranging from −3.60 to −5.27‰, at Sites 762 and 806, while mixed species foraminiferal tests (∼250–500 μm) from Site 762 range between −5.08 and −4.36‰. Residual siliciclastics at depths of ∼105 to 145 mbsf at Site 762 have δ 26 M g values (−0.09 to 0.27‰) that are markedly higher than carbonate and pore fluid δ 26 M g values. Simple 1-D reactive transport modeling suggests that the general increase in pore fluid δ 26 M g with depth, accompanied by a decrease in carbonate δ 26 M g , is a result of calcite recrystallization (assuming an isotopic fractionation factor of ∼0.9955). However, subtle but significant deviations from the carbonate recrystallization-only scenario suggest that another process impacts δ 26 M g at all three sites. Scanning electron microscope images document clay particles embedded in nannofossils and foraminiferal tests at Site 762, which suggest that clay authigenesis is active in carbonate sediments and could affect pore fluid δ 26 M g. The formation of secondary clays preferentially sequesters isotopically heavy Mg (α clay - M g 2 + ≈ 1.0005), driving pore fluid δ 26 M g to lower values. An increase in carbonate δ 26 M g within the clay-rich layer at Site 762 and an increase in bulk carbonate Na/Ca supports the hypothesis that clay authigenesis also impacts the preservation of proxy archives. Multi-component reactive transport modeling suggests that authigenic rates of ∼1·10−13 mol/m3/s (∼3.15 µmol/m3/a; assuming that the authigenic clay is sepiolite) can generate deviations from the carbonate recrystallization-only case by several tenths of a permil, indicating that carbonate sediment-associated clay authigenesis (CSCA) may be more relevant in deep-sea carbonate sections than has been previously considered. [ABSTRACT FROM AUTHOR]

To assess how chemical weathering processes in large high-relief river systems respond to climatic variability, we studied seasonal changes in radiogenic strontium (87Sr/86Sr) and stable calcium (δ44/40Ca) and magnesium (δ26Mg) isotopes in the Jinsha and Yalong rivers, which drain the southeastern Tibetan Plateau. During the low-runoff season, with discharge (Q) < 2000 m3/s, the river waters reflect the Sr, Ca, and Mg isotope signatures of recharge meltwaters, with additional isotope fractionation signals for Ca and Mg related to secondary mineral precipitation, which might imply that meltwater flushes soil solutions from the soil. During medium-runoff intervals (2000 m3/s < Q < 4000 m3/s), the Sr, Ca, and Mg isotope signatures in the Jinsha river waters are similar to those of the headwaters, which are influenced by evaporite dissolution, while the Yalong is affected by greater carbonate weathering relative to silicate weathering. In both rivers, bedrock dissolution governs the chemical composition of the river waters. During the high-runoff season (Q > 4000 m3/s), storms generate rapid overland flow, which transfers large volumes of soil into the rivers, such that soil weathering plays an important role in regulating riverine chemical compositions. At these times, the riverine Ca and Sr isotope evolution is influenced by secondary mineral dissolution and sediment–water cation exchange. Overall, this study highlights the potential of combining multiple isotope systems (Sr, Ca, Mg) to trace the dynamics of water–rock interaction under variable hydrological conditions. [ABSTRACT FROM AUTHOR]

Dolomite genesis is a century-old mystery in sedimentology. To reveal the mechanism of dolomite genesis, two core problems need to be addressed. The first is the origin and migration mechanism of Mg2+-rich fluids during the dolomitization process. The second is the kinetic barrier caused by Mg2+ hydration during dolomite precipitation at low temperatures. To address these problems, our study, based on detailed petrological, sedimentological, geochemical (major and trace elements), and isotopic (C-O-Mg) analysis, clarified the source and migration of Mg2+-rich fluids and the kinetic barrier mechanism of low-temperature dolomite precipitation in the Upper Sinian Qigebulake Formation and the Lower Cambrian Xiaoerbulake Formation in the Tarim Basin. First, we found that the Mg2+-rich fluids required for the dolomitization of dolomite in the Xiaoerbulake Formation were primarily derived from the Early Cambrian marine fluid. At the interface of the sedimentary cycle, δ26Mg values fluctuated considerably, indicating that the sequence interface was the starting point and channel for the migration of dolomitized fluids. Sea level variation plays a major role in controlling the dolomitization process of the Xiaoerbulake Formation. Second, the Qigebulake Formation contains low-temperature dolomite with Mg2+-rich fluids supplied by seawater, microorganisms, and sedimentary organic matter. Comprehensive analysis shows that the dolomite of the Qigebulake Formation was formed by microbial induction by anaerobic methane bacteria. Finally, the properties and sources of dolomitization fluids and the formation process of dolomite were the reasons for the difference in the Mg isotope composition of dolomite during the Sinian-Cambrian transition. This study reveals the genetic mechanism of the Sinian-Cambrian dolomite in the Tarim Basin and establishes a new method to explain the genesis of microbial dolomite by C-O-Mg isotopes, providing a reference for the reconstruction of the formation and evolution of dolomites. [ABSTRACT FROM AUTHOR]

The formation of orogenic ultrapotassic magmatism in a subduction setting is still unclear as it requires various co-processing mechanisms. To address this problem, especially to investigate the role of carbonate metasomatism in the mantle source, whole-rock Mg-Sr-Nd isotopic compositions have been measured for ultrapotassic magmatic rocks from the Central Pontides arc, northern Turkey, one of the rare examples of subduction-related ultrapotassic magmatism. These ultrapotassic rocks have relatively low (87Sr/86Sr) i (0.70461–0.70687) and ε Nd (t) (0.3 to 2.6) values and display much lower δ26Mg values (−0.73 ‰ to −0.19 ‰) than the normal mantle. The petrological and geochemical characteristics of the Pontide Ultrapotassic rocks (PUR) indicate that the low δ26Mg values do not result from surficial weathering, diffusion, or magmatic differentiation, but rather reflect the presence of distinct metasomatizing agent in their mantle source. The low Hf/Hf*, Ti/Ti*, Fe/Mn and Ti/Eu ratios, high Ca/Al and (Na 2 O + K 2 O)/TiO 2 ratios, and lack of significant correlations between δ26Mg and Fe/Mn or (Gd/Yb) N suggest that the low δ26Mg values cannot be reconciled with the contribution of recycled carbonated eclogites, but are induced by the addition of carbonate-rich sediments to their mantle source. This interpretation is also supported by the significant (La/Yb) N vs. Ti/Eu and Hf/Sm vs. CaO/Al 2 O 3 correlations for the ultrapotassic rocks. Using two end-member mixing calculations of Mg Sr isotopic compositions, the mantle source of PUR is constrained to contain varying proportions of carbonates, mostly of dolomite composition. The addition of carbonate-rich sediments to the mantle may have occurred at depths of 100–150 km as indicated by the trace element compositions of PUR. In this regard, the enrichment of K and other large ion lithophile elements in the ultrapotassic rocks can be induced by the contribution of silicic sediments. Meanwhile, carbonatitic melts can be formed through the partial melting of carbonate-rich sediments in the subducting oceanic slab, which then reacts with the overlying mantle to form carbonated peridotite that serves as the source of ultrapotassic rocks with low-δ26Mg, low-silica, and high LREEs. This study reports for the first time the Mg isotope data of ultrapotassic rocks formed in a continental arc setting and documents that carbonate-rich sediments play an important role in creating such rocks. Future work is needed to test whether this process is common in both oceanic arc and continental arc settings. • The ultrapotassic rocks from the Central Pontides arc of northern Turkey have very low δ26Mg values. • Such low δ26Mg values were formed through carbonate melt metasomatism of the mantle source. • Mg isotopes constrain the formation of ultrapotassic rocks at a continental arc setting. [ABSTRACT FROM AUTHOR]

Evaporites have recently been suggested as a potential archive for recording the Mg isotope compositions (δ26Mg) of coeval seawater. However, episodic dolomitization during the deposition of massive evaporites could cause considerable Mg removal and isotopic fractionation. To constrain the hydrological changes and influence of dolomitization on ambient brine δ26Mg, we present petrographic and mineralogical features, as well as Mg-C-O-Sr isotope data extracted from carbonate phases of a middle Triassic (ca. 247 Myr) marine anhydrite-dolostone sequence from a drill core in eastern China. The drilled lithologies are characterized by massive dolostone layers in the lower part, followed by an upward decline in dolomite contents accompanied by a rise in anhydrite. Multiple lines of evidence consistently point to a syn -depositional origin for the dolostones in a marginal basin of the Tethys Ocean and a lack of diagenetic alteration since deposition. We reconstructed the dynamic changes of δ26Mg values of basin waters based on Mg isotope compositions in dolomite leachates. According to our findings, the δ26Mg values of the basin waters were remarkably high (about 0.38 ± 0.05‰) at the onset of evaporation, indicating a significant Mg sink of the massive dolomitization. The δ26Mg of brine in the basin then changed towards the value of coeval seawater (about −0.32 ± 0.05‰) starting with the deposition of evaporites. Concurrently, 87Sr/86Sr ratios of dolomites shift from radiogenic values towards contemporaneous seawater composition. Our results demonstrate that the evaporite basin was not strictly restricted, and water exchange with the open ocean never ceased. Modeling calculation reveals that, even when the seawater exchange rate is far below the average ocean circulation, δ26Mg of brine in the basin will reach the value of open ocean within 1 Myr, completely removing the influence of early dolomitization. We suggested that massive marine evaporite sequences have the potential to record the long-term evolution of seawater Mg isotopes. • Seawater recharge is the pre-requirement accounting for the formation of massive evaporites on the basin scale. • The δ26Mg of brine in the basin will shift towards the value of coeval seawater since the deposition of evaporate. • The massive marine evaporites have the potential to record the long-term evolution of seawater Mg isotopes. [ABSTRACT FROM AUTHOR]

Serpentinite plays a crucial role in element mobility during seawater hydrothermal alteration of mid-ocean ridge peridotite on the seafloor and fluid metasomatism of peridotite in the subduction zone. However, element mobility and Mg isotope fractionation during serpentinization of peridotites are still uncertain. In order to address this issue, we present a geochemical study of serpentinites and their associated peridotites in the Xigaze ophiolite from southern Tibet. The serpentinites have lower MgO contents and Mg/Si ratios than the peridotites. The low Mg/Si ratios in serpentinites, along with the Isocon calculation results, indicate the Mg loss during serpentinization. In terms of trace elements, the serpentinites have higher contents of Ba, U, Sr and Pb than the peridotites, though both types of ultramafic rocks exhibit similar REE distribution patterns and HFSE contents. The serpentine-magnetite O isotope thermometry yields a relatively low serpentinization temperatures of 200–300 °C. The trace element composition of the serpentinites is distinct from that of forearc serpentinites but consistent with that of mid-ocean ridge ones. The peridotites show mantle-like δ26Mg values of –0.27 to –0.23‰ with an average of –0.25 ± 0.03‰ (2SD, n = 6). In contrast, the serpentinites have systematically higher δ26Mg values of –0.21 to –0.09‰ with an average of –0.16 ± 0.09‰ (2SD, n = 8). The Mg isotope composition of serpentinites shows no correlations with either the chemical weathering proxy or the talc modal content, indicating that the Mg isotope fractionation between serpentinite and peridotite is not caused by chemical weathering of the serpentinites. Instead, it probably resulted from the serpentinization of peridotite on the seafloor. Thus, there is the loss of not only element Mg but also isotopically light Mg during the serpentinization of peridotites. The isotopically heavy Mg serpentinites can be carried to oceanic subduction zones, giving rise to higher δ26Mg mantle sources for various basalts. The isotopically light Mg can be released into the seawater, regulating the Mg isotope composition of seawater in geological history. Taking into account the sources and sinks of Mg in subduction zone fluids, the formation of isotopically heavy Mg serpentinites during seafloor serpentinization can be probably an important process in changing the Mg isotope composition of mantle sources. [ABSTRACT FROM AUTHOR]

It is controversial how S-type granites get higher maficity (MgO + FeO t) than initial melts derived from metasedimentary rocks. Different models, including source-controlled processes (residual/peritectic mineral assemblage entrainment), mixing of magmas derived from different source rocks, and fractional crystallization, have been proposed. However, whole-rock major-trace element and radiogenic isotope data generally provide ambiguous constraints to these models, and the entrained minerals can be later modified by host magmas to have re-equilibrated compositions or replaced by new mineral phases. We address this issue by means of whole-rock Mg isotope and zircon trace element and O isotope analyses on the ∼250 Ma Jiuzhou S-type granite pluton from South China. δ26Mg values of the granites exhibit a negative relationship with increasing maficity, varying from −0.14‰ at 3.00 wt% to −0.43‰ at 8.72 wt%. Zircon yields high and variable δ18O values of 10.0–15.2‰, suggesting that the dominant source rocks are metasedimentary rocks with heterogeneous compositions. No relationship between zircon δ18O value versus whole-rock δ26Mg value or maficity is observed, indicating that magma mixing has played a negligible role in the origin of the granites. Meanwhile, neither Hf nor Ti concentration in zircon shows correlation with whole-rock maficity, precluding fractional crystallization as the dominant mechanism for the compositional variation of the granites. Instead, zircon from granite samples with different maficities seems to have grown from magmas with different compositions but rather similar initial magma temperatures of ca. 810–850 °C as suggested by the upper limit values of the Ti-in-zircon thermometer. Consequently, the negative relationship between δ26Mg value and maficity is best explained as melt entraining a solid assemblage from the melting source, with garnet as the main ferromagnesian phase because this phase is enriched in lighter Mg isotopes than other ferromagnesian phases. This conclusion is supported by the negative relationship between δ26Mg and (Yb/Dy) N values, since garnet is also enriched in heavy rare earth elements (HREE). Furthermore, phase equilibrium modeling using the sample with the lowest δ26Mg value and highest maficity indicates that garnet is always the dominant ferromagnesian phase (exceeding 50% to nearly 100% among the ferromagnesian phases) in the solid assemblage, which equilibrates with melt across a variety of P–T–H 2 O initial magma conditions (6–8 kbar/800–900 °C/1–5 wt% H 2 O) at the melting sources. Simple mixing calculations also confirm that the mixing between melt and a solid assemblage consisting mainly of garnet can account for the compositional variation of Mg isotopes with maficity. This study highlights the advantage of integrated geochemical analyses, particularly Mg isotopes, in placing important constraints on the petrogenetic processes that produce high-maficity S-type granites. [ABSTRACT FROM AUTHOR]

The Mg isotope system helps constrain a variety of geochemical processes in the hydrosphere, lithosphere and biosphere. DSM3 is commonly used as delta‐zero material for relative Mg isotopic measurements. In this study, we have characterised the Mg isotopic composition of a new pure Mg solution (Alfa‐Mg). This solution could potentially be used as a secondary delta‐zero reference material, or as an alternative primary reference material when DSM3 is depleted. The Alfa‐Mg solution has been checked for homogeneity and stability and is available free of charge upon request. The certified δ26/24Mg value relative to DSM3 of the Alfa‐Mg solution is −1.40‰, with an expanded (k = 2) uncertainty of 0.05‰. We also characterised a range of existing Earth‐surface reference materials including soils, riverine sediments and plants. The δ26/24Mg values (relative to DSM‐3) of plant materials range between −1.06 ± 0.03‰ (2s, n = 12) and −0.75 ± 0.03‰ (2s, n = 12). The δ26/24Mg values of soil materials vary from −0.72 ± 0.11‰ (2s, n = 12) to 0.17 ± 0.05‰ (2s, n = 12). The δ26/24Mg values of riverine sediment materials range between −0.66 ± 0.01‰ (2s, n = 12) and −0.17 ± 0.03‰ (2s, n = 12). These data could serve as a reference for inter‐laboratory calibration and quality control, as well as for potential applications in geological and biological fields. [ABSTRACT FROM AUTHOR]

The Capitan Reef Complex in West Texas is famous for its high prevalence of early marine cements, unusual for a Phanerozoic platform, leading some to suggest that Precambrian styles of carbonate sedimentation enjoyed a Permian encore. Here, we use patterns of stable Ca, Mg, C and S isotopes to better understand the environmental driver(s) of the enigmatic cementation. We find that calcite that is the most enriched in 44Ca has δ34S values that approach the inferred composition of Permian seawater sulfate. Microbial sulfate reduction in pore fluids must have been spatially and temporally coincident with recrystallization of primary carbonate phases, such that substantial 34S-enriched sulfate was incorporated into diagenetic calcite under relatively closed-system conditions. Moreover, the magnitude of 34S-enrichment of carbonates relative to seawater was strongly influenced by local diagenetic conditions, with fluid-buffered early marine cements, shelf, reef, and upper slope preserving more seawater-like S isotope ratios than the more sediment-buffered lower slope. Some samples are far more 34S-enriched relative to seawater than those from modern sites in similar depositional environments, possibly responding to specific combinations of sedimentary parameters (e.g., grain size, porosity, organic matter rain rate). Additionally, the sulfate concentration in the Delaware Basin might have been slightly lower than modern levels, leading to more extensive isotopic evolution of sulfate in pore waters during carbonate recrystallization. Based on the data and a numerical model of carbonate recrystallization, we suggest that one driver of the extensive seafloor cement precipitation in the Capitan Reef Complex was a Permian water column [Ca2+]:[SO 4 2−] ratio somewhere between 1 and modern seawater. [ABSTRACT FROM AUTHOR]

The Moldanubian Zone of the Bohemian Massif was intruded by three Variscan (c. 354–335 Ma) subduction-related plutonic suites. Their mantle sources evolved from CHUR-like (low-K calc-alkaline suite), through slightly enriched ( ε Nd 346 ~ − 3; high-K calc-alkaline suite—HKCA) to strongly enriched ( ε Nd 337 < − 7.5; (ultra-)potassic suite—UK). This evolution has been previously interpreted in terms of Andean-type subduction passing to deep subduction/relamination of the Saxothuringian continental crust, metasomatizing the mantle source of the HKCA and, more significantly, of the UK suite. This is in accord with the heterogeneity of lithospheric mantle fragments (spinel/garnet peridotites, garnet pyroxenites, eclogites, glimmerites...) sampled by the high-grade Moldanubian orogenic root. Newly acquired Mg isotopic compositions (δ26Mg = − 0.12 to − 0.53‰) vary over similarly broad intervals within each suite. Majority of the most magnesian samples fall within the range of local orogenic mantle peridotites (− 0.33 to − 0.29‰) or close to the global mantle average (− 0.25‰). This implies that the δ26Mg of the progressively metasomatized harzburgitic mantle was mostly buffered by the mantle end-member, while its mantle-incompatible elemental/related isotopic (Sr–Nd–Pb) signal was swamped by the crustally derived contribution. The crustal contaminant, source of the metasomatic fluids, had to be dominated by Mg-poor, felsic metaigneous > > clastic metasedimentary material. Subducted carbonates were of limited importance, as recorded by lowered δ26Mg values in several mafic UK samples and in a glimmerite vein cutting the peridotites (− 0.44‰). The Mg isotopic variation in less magnesian potassic rock types reflects a complex interplay between source heterogeneity, equilibrium fractionation during fractional crystallization, kinetic fractionation due to chemical diffusion during magma mixing and/or AFC-style contamination by carbonate-derived fluids. [ABSTRACT FROM AUTHOR]

Knowledge of the evolution of the East Asian Summer Monsoon (EASM) provides a valuable opportunity to uncover the dynamic interactions of land-ocean-atmosphere system in the late Cenozoic. However, the evolutionary history of EASM remains debatable, mainly due to the difficulty in separating EASM precipitation and temperature signals. In this study, precipitation proxies of Sr/Ca ratio and δ26Mg value in loess secondary calcite (carbonate nodule (NC) and fine carbonate (FC) (grain size < 4 μm)) are further verified, with high precipitation corresponding to high δ26Mg FC but low δ26Mg NC and Sr/Ca NC ratio, and vice versa. Then, these proxies are investigated in well-researched Chinese loess eolian deposits since ∼22.5 Ma. Results suggests that the EASM precipitation was staged enhancement in the mid-Miocene (∼16.5–14.0 Ma) and the Quaternary (∼2.6–0 Ma). The mid-Miocene enhancement can be compared with the strong EASM intensity and the warmest global temperature since the Miocene. Global warming thus is considered as the dominant force for this enhancement via an expansion in the latitudinal extent of the Indo-Pacific Intertropical Convergence Zone (ITCZ). In contrast, the Quaternary enhancement was accompanied by decreases in global temperature. We propose their linkage via the reinforcement of the Pacific Walker Circulation and a northward shift of the subtropical ridge in the western Pacific. Our study provides a new insight into EASM evolution and its dynamic linkage with global climate changes. [ABSTRACT FROM AUTHOR]

The magnesium (Mg) isotopic composition of marine authigenic carbonates is considered as promising archive of ancient seawater geochemistry and paleoenvironments. Previous experimental and theoretical work has shown that Mg isotope fractionation during carbonate mineral formation is a function of mineralogy and precipitation rate. However, information on Mg isotope fractionation is limited for well-defined precipitation rates in natural settings. Here, we investigate pore waters from sediments of an area of active methane seepage in the South China Sea. Low δ13C values (< −48.3‰ VPDB) of dissolved inorganic carbon (DIC) near the sulfate-methane transition zone (SMTZ) indicate that sulfate-driven anaerobic oxidation of methane (SD-AOM) is the predominant biogeochemical process. Pore water composition of dissolved Mg, calcium (Ca), and strontium (Sr) agrees with aragonite as the dominant carbonate mineral at the site ROV1, and high Mg-calcite at sites ROV2 and ROV4. Calculated carbonate precipitation rates are 0.92 μmol cm−2 yr−1 for site ROV2 and 1.24 μmol cm−2 yr−1 for site ROV4; these estimates are similar to previous calculations for seeps from other areas. The pore water δ26Mg values (−0.88‰ to −0.71‰) obtained for the three study sites are similar to those of seawater, in accord with a minor effect of Rayleigh fractionation due to abundant supply of Mg from seawater and insignificant consumption of Mg during carbonate precipitation. The modeled Mg isotope fractionation (ϵ = −2.0‰ to −1.0‰ for core ROV2; ϵ = −1.3‰ to −0.3‰ for core ROV4) can be explained by kinetic isotope fractionation during carbonate precipitation. The calculated carbonate precipitation rates and the degree of fractionation of Mg isotopes support the notion that fractionation is small at high precipitation rates. However, the carbonate precipitation rates calculated for the studied seep environments are much smaller than those in laboratory experiments, documenting a discrepancy of isotopic fractionation between carbonate authigenesis in laboratory experiments and natural environments. These results, including the modeled precipitation rates, provide new constraints for Mg isotope fractionation in natural settings.

• δ26Mg variation in lavas from the EPR is resolved within 0.03‰ uncertainty (tSE). • Mg isotopes can be significantly fractionated during fractional crystallization. • Δ26Mg Ol-melt , Δ26Mg Cpx-melt , and Δ26Mg Ti − Mgt-melt are estimated. • δ26Mg deviation should be considered when tracing mantle source with evolved MORBs. The magnesium (Mg) isotope composition of oceanic basalts has provided useful constraints on the evolution of the upper mantle and crust-mantle interactions. However, the behavior of Mg isotopes during oceanic magma differentiation is still unclear because of the small range in Mg isotope values in typical normal mid-ocean ridge basalts (N-MORB). Here, we present high-precision Mg isotope data on a well-characterized suite of mid-ocean ridge (MOR) lavas from the 9–10°N segment of the East Pacific Rise. These samples range from relatively primitive basalt to evolved dacite with MgO contents decreasing from 8.62 to 0.80 wt.%, and display a resolvable variation in δ26Mg from -0.27 to -0.17‰. The less-evolved samples (MgO > 7.0 wt.%) that have experienced olivine and plagioclase fractional crystallization have δ26Mg values ranging from -0.23 to -0.17‰ that are negatively correlated with MgO content. Samples containing MgO of 3.5–7.0 wt.%, that have experienced significant crystallization of clinopyroxene, together with plagioclase, show a limited variation of δ26Mg (-0.20 to -0.18‰). For those highly evolved samples (MgO < 3.5 wt.%) saturated with Fe–Ti oxides, the δ26Mg vary from -0.27 to -0.20‰ and are positively correlated with MgO content. The variation of δ26Mg in the MOR lavas is consistent with three stages of magma differentiation, beginning with fractional crystallization of olivine and plagioclase followed by increasing amounts of clinopyroxene and finally joined by Fe-Ti oxides. Quantitative modeling of the Mg isotopic variation in EPR samples, shows that the variation of δ26Mg in MOR lavas is primarily a consequence of fractional crystallization of the Mg-bearing minerals when Δ26Mg Ol-melt ∼ -0.10‰, Δ26Mg Cpx-melt ∼ 0.00‰, and Δ26Mg Ti − Mgt-melt ∼ 0.20‰. This study provides evidence that shallow level crystal fractionation can produce significant and predicable variations in Mg isotopic compositions of MOR lavas and that this process should be carefully considered when using evolved lavas to trace mantle source compositions. [ABSTRACT FROM AUTHOR]

The occurrence of fault-controlled hydrothermal dolomitization (HTD) is ubiquitous across stratigraphic records and has been extensively studied due to its association with economic resources. The origin of HTD is often evaluated by combining carbonate geochemistry, fluid inclusion thermometry and reactive transport modelling. However, multiple diagenetic overprinting events can obscure original geochemical signatures. Here, we demonstrate the applicability of two combined isotope systems, magnesium and carbonate clumped isotopes (δ26Mg - ∆ 47), to trace the source of fluid and magnesium in basin-scale HTD from the Western Canadian Sedimentary Basin (WCSB) and Southern China. Extensive studies in these regions, providing tectono-stratigraphic information and dolomitization models, furnish a robust scenario to evaluate the new isotopic data. Our findings reveal that while the previous geochemical data (δ18O fluid and 87Sr/86Sr) are partly compatible with dolomitization from seawater, the elevated δ26Mg and δ18O fluid values (up to -0.3‰ and +11‰, respectively) and the high temperatures (up to 320 °C) are not consistent with dolomitization from seawater alone. Considering this, the uniform mean δ26Mg values of silicate rocks (-0.25‰) and the occurrence of basement-rooted faults in the study areas, the hypothesis from prior work that dolomitization was initially driven by seawater that progressively mixed with crustal fluids sourced from underlying basement rocks, was supported. As the δ26Mg values may reflect the isotopic signature of the fluid and the leached host rocks, when combined with ∆ 47 -derived temperatures , it results in an effective tracer of fluid and Mg sources during dolomitization in tectonically complex basins that are otherwise challenging to unravel. A comparison of our results paints a global picture of isotopic imprints in hydrothermal dolomite and further demonstrate the ability of Mg isotopes to differentiate between dolomitization from seawater versus evaporated brines and also through Mg-rich crustal fluids. [ABSTRACT FROM AUTHOR]

Numerous studies have argued that the andesitic model (i.e., the arc directly produces andesitic magmas) rather than the basaltic-input model (i.e., andesitic magmas were derived by fractionation from basaltic magmas) better explains the composition of continental crust. However, it remains controversial as to how andesitic magmas are directly produced in subduction zones, and this weakens the basis of the andesitic model. Recently, the mélange-diapir model has been increasingly considered to be an important mechanism for the direct generation of arc andesites, and would thus support the andesitic model for crustal growth. Moreover, melting experiments on mélange rocks show that varying the composition of mélange rocks (e.g., serpentinite-dominated and sediment-dominated mélange) can produce a range of primary andesitic arc magmas, from tholeiitic, calc-alkaline, to high-K calc-alkaline and shoshonitic magma types. However, it remains challenging to determine the different mélange sources for low-K to high-K arc andesites. Here we verify these experimental results using data from early Cretaceous dioritic porphyries in the southern Qiangtang block of central Tibetan plateau. The generation of these dioritic rocks cannot be linked to any coeval basaltic rocks in this region and so they may represent primary andesitic magmas. These dioritic porphyries are divided into low-K type-1 and high-K type-2. Type-1 has higher δ26Mg values (−0.16 to −0.08‰) than MORBs (mid-ocean ridge basalt, δ26Mg = −0.25 ± 0.06‰), which can be ascribed to the contribution of subducted bulk/un-differentiated serpentinite in their source. Combined with their low K contents and La/Sm ratios, and high Nd Hf isotope ratios (εNd(t) = 3.36–5.03; εHf(t) = 12.88–13.76), we suggest a serpentine-dominated mélange for their origin. The type-2 and type-1 rocks have similar trace-element distribution patterns, the same ages (∼124 Ma), and are indistinguishable in field outcrops, indicating their common petrogenesis. However, compared to type-1, type-2 has higher K and Th contents, and La/Sm and Th/Nd ratios with lower Nd Hf isotope values (εNd(t) = −1.61 to +0.30; εHf(t) = 3.78–4.39), and mantle-like δ26Mg values (−0.28 to −0.17‰). This likely indicates derivation from a sediment-dominated mélange for type-2. This study thus verifies the experimental results of melting of different mélange from natural rock record and shows that the mélange model provides an important mechanism for generating variable andesitic arc magmas and so the formation of the andesitic continental crust. • Gerze ∼124 Ma dioritic porphyry dykes in Tibetan plateau represent primary andesitic magmas and are divided into two types. • Low-K and high-K rocks originated from serpentinite-dominated mélange and sediment-dominated mélange, respectively. • This study verifies experimental results of melting of different mélange rocks, arguing for andesitic crustal growth. [ABSTRACT FROM AUTHOR]

Given the importance of permafrost regions in providing the major elements to the Arctic Ocean, and the vulnerability of these regions to ongoing climate change due to permafrost thaw and landscape modifications, assessing the magnitude and characterizing the factors governing isotope compositions in river waters across seasons and space is of high priority. Towards identifying possible environmental controlling factors (climate, vegetation, rock lithology) and quantifying annual fluxes of dissolved Mg isotopic signatures in large Arctic rivers located in the continuous permafrost zone, here we measured hydrochemical and Mg isotope compositions of the Lena River main channel and its various tributaries over different seasons of the year. In general, we did not find evidence of any statistically significant (p < 0.05) correlation between δ26Mg and main physico-geographical parameters of the river watersheds (temperature, permafrost, type of vegetation). The exception is the decrease in riverine δ26Mg values that was accompanied by an increase in the carbonate rock proportion in the watershed, whereas an opposite trend was observed for the abundance of terrigenous and silicate rocks. An incontestable control of dolomite rocks on Mg isotope signature in the Lena River was also supported by decreasing δ26Mg values with increasing Dissolved Inorganic Carbon (DIC) concentration, as also observed for other Artic River basins. This relationship was mostly pronounced during the high-flow period, when secondary Mg silicate formation in soils and underground waters and so the terrestrial uptake of Mg were the lowest. In the Lena River main channel, the lowest δ26Mg values (−1.5 ± 0.05 ‰) were observed during autumn-winter (September to March), probably reflecting a dominant role of underground reservoirs and subsurface soil weathering of isotopically light Precambrian dolomites. The mean δ26Mg values of the Lena River during two months of spring (−1.02 to −1.28 ‰), 4 months of summer baseflow (−1.30 ‰) and 6 months of winter yields a mean discharge-weighted annual value of −1.26 ± 0.05 ‰. This number coincides with the value of summer baseflow reported in earlier works. Therefore, like in the recently studied Yenisey River basin, the Mg isotope signature of the Lena River water likely stems from a mixture of carbonate (−2.0 ≥ δ26Mg ≥ −2.5 ‰) and silicate (0 ≥ δ26Mg ≥ −0.2 ‰) rocks in its watershed. We also argue that the discharge-weighted mean annual isotopic signature of an element in the Artic rivers can be reasonably approximated by a single sampling campaign during summer baseflow. This allows cost-effective assessment of mean riverine isotopic signatures of other elements in both small and large rivers to the Arctic Ocean. [ABSTRACT FROM AUTHOR]

In the last decade, much attention has been devoted to Mg isotopic behavior during various weathering processes. Nonetheless, whether Mg isotopes in terrigenous siliciclastic sediments in modern oceans can reflect continental weathering regimes remains unresolved. Understanding the major controls of Mg isotope fractionation in siliciclastic sediments is thus a prerequisite for decoding past Mg isotope records for tracing continental weathering history. In this study, we present mineralogical, elemental, and Mg isotopic compositions of terrigenous siliciclastic sediments in the Changjiang (Yangtze River) estuary and the adjacent continental shelf of the East China Sea. δ26Mg values of the clay-sized sediments vary from −0.15‰ to 0.00‰ and lack of the correlation with the proportion of different clay minerals. Our results suggest that the incipient to intermediate weathering processes are characterized by the progressive leaching of isotopically light Mg from silicates, leading to the enrichment of 26Mg in secondary clay minerals. Therefore, Mg isotopic variability of clay-sized sediments is primarily determined by catchment weathering, rather than by provenance lithology or mixing of clay minerals. An isotopic mass balance calculation indicates that the δ26Mg value of silicate weathering flux on a continental scale is ca. -0.60‰ to −0.40‰. The new constraint on the δ26Mg value of silicate weathering flux provides a new opportunity to better understand the Mg cycle. The Mg flux from silicate weathering in the Changjiang catchment is estimated as ∼15–17 × 1010 mol/yr, accounting for ca. 50% to 60% of the total Mg influx of Changjiang River into the ocean. In addition, numerical modeling with the up-to-date flux and isotope fractionation data have indicated that a slight enhancement of silicate weathering can potentially drive the rapid rise of Mg/Ca in seawater since late Cenozoic, but maintain the constancy of seawater δ26Mg as reported by literature. Our study demonstrates that Mg isotopes of clay-sized sediments can be used to constrain the relevant δ26Mg value of silicate weathering flux on a continental scale, which is a key parameter to trace the Mg cycle on Earth's surface. • Mg isotopes of clay-sized sediments in marginal sea can potentially trace silicate weathering. • δ26Mg of silicate weathering flux on a continental scale is ca. -0.6‰ to −0.4‰. • The enhancement of silicate weathering can potentially drive the rapid rise of Mg/Ca in seawater since late Cenozoic. [ABSTRACT FROM AUTHOR]

Carbonatites are rare carbonate-rich igneous rocks derived from carbon and carbonate-rich regions of Earth's mantle. Although a number of igneous processes are recognized to have controlled their compositions, the origin of the carbonate-rich nature of these magmas remains debated and has been linked to various mantle-related processes, including subduction and plume–lithosphere interaction. High-precision isotope measurements can provide insights into carbonatite petrogenesis, including the identification of subducted crustal material in their source region. In particular, combining mass-dependent and kinetically corrected Mg isotope data, also known as the triple-isotope approach, provides insights into mass fractionation processes driving mass dependent fractionation, which in turn allows to distinguish between equilibrium and kinetic processes. In this work, we report high-precision Mg stable isotope data for 59 carbonatites and associated silicate rocks from different localities and ages ranging from 3000 Ma to present-day, as well as C and O isotopic analysis for 38 carbonatites and Sr isotopic analysis for 41 carbonatites and silicate rocks. In addition, we also report Mg isotope data for 17 Phanerozoic carbonate rocks, with the aim of identifying the isotopic signature of carbonate-rich material potentially recycled to the carbonatite mantle source regions. Collectively, the data reveal a range of stable Mg isotope compositions from δ25/24Mg DTS-2 = −1.20 ± 0.01‰ to +0.08 ± 0.01‰. We observe positive residual deviations after kinetic mass fractionation correction of the Mg isotope data for our carbonatites; an isotope signal that is also present in Phanerozoic carbonates. This observation establishes that a component of the Mg present in these samples experienced mass-dependent equilibrium isotopic fractionation processes, which are significantly larger at low temperatures. Given that the Mg results do not covary with C and O isotopic signals, the magnitude of the fractionation following the equilibrium law observed in carbonatites provides strong evidence for recycled material from the Earth's surface in the mantle source of Ca- and Mg-rich carbonatites. Furthermore, associated silicate rocks present mantle-like Mg isotopic compositions in contrast with the genetically linked carbonatites - based on the Sr isotopes - for some complexes, which is best explained if the carbonatites and associated silicate rocks represent distinct generation of partial melts of a mantle source containing recycled carbonate. [ABSTRACT FROM AUTHOR]

Travertine samples deposited in Earth's surface environments can be used as an effective archive for paleo-climatic reconstruction. As a common element in carbonates, magnesium (Mg) and its isotopic composition in travertine could provide useful information for evaluating paleo-environment changes. In this study, we investigate the Mg isotope systematics in both endogenic travertines (mainly calcite) and spring/stream waters at Baishuitai, Yunnan, SW China. Our results show a systematic increase in δ26Mg value from −1.37 to −1.26‰ for water samples downstream, but varied δ26Mg values between −4.12 and −3.95‰ (average −4.02‰) for solid carbonates, thus a corresponding fractionation Δ26Mg calcite-water between −2.76 to −2.59‰ (mean value of −2.69‰). Therefore, the solid carbonates preferentially incorporate light Mg isotopes during travertine formation. More interestingly, the Mg distribution coefficient (K Mg/Ca) between travertine and water exhibits two variation trends with the calcite deposition rate (R p) along the canal, which can be explained by the change of calcite formation mechanism from direct nucleation to precipitation via amorphous calcium carbonate (ACC) intermediate. In the upper-stream, the direct nucleation of calcite results in the rapid incorporation of Mg ions into crystal lattice, while a relatively slow precipitation of calcite downstream would incorporate Mg via ACC formation pathway in a quasi-equilibrium pattern. This is consistent with the grain size distribution and crystal morphology observed under SEM. Our results show the important control of water Mg/Ca ratios on the calcite precipitation during travertine formation, and imply the potential and complexity of using Mg isotopes of travertine deposits to reconstruct paleo-environments. • Mg isotope fractionation was investigated on naturally-occurring travertine samples from Baishuitai, China. • The Mg/Ca ratios of spring/stream water have important controls on the Mg-Ca exchange and Mg isotope fractionation. • Our results can be explained by changing precipitation mechanism from direct nucleation to the precipitation via ACC. • This study suggests both temperature and Mg/Ca ratio in solution may have significant effects on Mg isotope fractionation. • This study reveals the potential and complexity of using δ26Mg values of travertine deposits as a paleo-proxy. [ABSTRACT FROM AUTHOR]

Rodingite is produced by Ca-rich fluid metasomatism of mafic igneous rocks in oceanic lithosphere. Despite its importance for the composition of subduction zone magmas, its formation mechanism still remains obscure. To address this issue, we investigated the major-trace element and Mg–O–Sr–Nd isotope compositions of rodingites in serpentinites from the Xigaze ophiolite, southern Tibet. Coexisting gabbros and rodingites show consistently depleted Sr Nd isotope compositions, suggesting their formation in a mid-ocean ridge setting. The rodingites exhibit variable δ26Mg values from −0.61‰ to −0.14‰, with an average of −0.37‰ ± 0.28‰ (n = 19; 2 SD), mostly lower than those of −0.26‰ to −0.13‰ for the protolith gabbros. Compared to the relatively homogeneous δ18O values of 6.1‰–6.7‰ for the gabbros, the rodingites show highly variable δ18O values from 3.8‰ to 9.4‰. The rodingites can be subdivided into two groups based on mineral assemblages and formation conditions. Group I rodingite is prehnite-dominated and hydrogarnet-free, formed at temperatures of 200–300 °C, and has higher δ18O values of 8.1‰ ± 1.4‰ (n = 15) and δ26Mg values of −0.32‰ ± 0.19‰ (n = 12). Group II rodingite consists mainly of hydrogarnet and clinopyroxene, formed at temperatures of 300–400 °C, and has lower δ18O values of 5.4‰ ± 2.5‰ (n = 7) and δ26Mg values of −0.44‰ ± 0.35‰ (n = 7). Modeling of the fluid–rock interactions between the protolith gabbro and fluid was undertaken using the Mg O isotope compositions of the serpentinizing fluid inferred from the associated serpentinites. Based on the modeling results, we propose that the metasomatism of gabbros by Ca-rich, low-δ26Mg fluids produced during serpentinization would result in the Mg O isotope compositions of the rodingites. Such rodingites with low δ26Mg values would generate the geochemical heterogeneity in the mantle wedge overlying the subducting oceanic lithosphere and eventually give rise to Ca-rich, low-δ26Mg basaltic magmas. The results indicate that low δ26Mg values of basaltic rocks cannot be unambiguously ascribed to carbonated mantle sources. • The Xigaze rodingites show low δ26Mg and variable δ18O values. • The rodingites were formed through metasomatism of gabbros by serpentinizing fluids under seafloor. • The subduction of rodingites in subduction zones can greatly affect mantle Mg isotopic composition. [ABSTRACT FROM AUTHOR]

The serpentinized mantle wedge is critical for the geochemical cycling of water, volatiles, and fluid-mobile elements in the subduction zone. It is also a major reservoir of magnesium (Mg) in subduction zones, but its Mg isotopic composition is still not well constrained. To investigate Mg isotopic fractionation during mantle wedge serpentinization, and better understand Mg isotopic composition of the mantle wedge, we studied Mg isotopes of mantle wedge serpentinites/serpentinized peridotites exhumed by Mariana forearc serpentinite mud volcanoes. Whole-rock δ26Mg values of these samples vary from −0.29 to −0.03‰. Some serpentinite/serpentinized peridotite samples have significantly elevated δ26Mg values up to −0.03‰, which are caused by seafloor weathering after their exhumation by the mud volcanoes. In contrast, the unweathered serpentinized peridotites have homogeneous δ26Mg values of −0.29 to −0.27‰ (mean δ26Mg = −0.28 ± 0.01‰, 2SD, n = 3), which represent the primary Mg isotopic composition of the Mariana forearc mantle wedge peridotite. However, the unweathered mature serpentinites (i.e., completely serpentinized) have slightly heavier Mg isotopic compositions (δ26Mg = −0.29 to −0.21‰, mean δ26Mg = −0.24 ± 0.05‰, 2SD, n = 16) than the serpentinized peridotites, indicating that Mg isotopes are fractionated during the late-stage mantle wedge serpentinization. It is probably due to the leach of isotopically light Mg by the infiltrating slab fluids after the complete consumption of olivine. By compiling Mg isotopic data of mantle wedge and oceanic mantle peridotites, we find that mantle wedge peridotites have Mg isotopic compositions (δ26Mg = −0.27 ± 0.04‰) that are identical to subcontinental lithospheric mantle peridotites (−0.25 ± 0.04‰), which are more uniform and lighter than those of oceanic mantle peridotites (−0.21 ± 0.12‰). The Mg isotope difference between them is most probably caused by mantle heterogeneity, not by different degrees of early partial melting as indicated by the incremental batch melting modeling. • Mg isotopes of mantle wedge serpentinite/serpentinized peridotite are studied. • Mg isotopes are fractionated during late-stage mantle wedge serpentinization. • Mantle wedge peridotite has more homogeneous Mg isotopes than oceanic mantle peridotite. • Their Mg isotope differences are not caused by partial melting but by mantle heterogeneity. [ABSTRACT FROM AUTHOR]

Widespread replacement dolomite occurs commonly in the geologic record. These dolomites are generally characterized by different crystal sizes and shapes that have at times been interpreted to represent multistage dolomitization; although the variation of dolomite texture could also have resulted from the recrystallization of precursor dolomite. It is important, but challenging, to differentiate these two scenarios to better understand the nature of dolomite and the associated processes of dolomitization. Our study explores this problem by using δ26Mg, δ13C, and δ18O isotopes to characterize the different crystal morphologies exhibited by Ordovician dolomites from the Tarim Basin, China. Although the dolomites show distinct textures and crystal morphologies, there are no discernible trends in their δ26Mg values (from −1.66 to −2.39‰), suggesting that these dolomites were formed by the same dolomitizing fluid. This interpretation is supported by an overlapping range of δ13C values (from 0.46 to −1.89‰). By contrast, the δ18O data demonstrate a wide range of values, from −3.8‰ to −8.8‰, reflecting the different degree of recrystallization with increasing burial temperatures. We suggest that the Mg and C isotopes remained unchanged during recrystallization because Mg and C were rock-buffered, so the recrystallized dolomite inherited Mg and C from the precursor dolomite. Based on these results, it appears that Mg isotopes, together with conventional O C isotopes, can provide a diagenetically robust geochemical tracer for identifying dolomite recrystallization in the geological record. • δ26Mg-δ13C-δ18O and petrography of Ordovician dolomites from the Tarim Basin. • Replacement dolomites show varied crystal textures but uniform δ26Mg & δ13C values. • δ18O values demonstrate a decreasing trend with crystal size. • δ26Mg-δ13C-δ18O and textures of dolomite are resulted from recrystallization. [ABSTRACT FROM AUTHOR]

Equilibrium inter-mineral Mg isotope fractionation factors (103lnα) are key to using Mg isotopes to investigate high-temperature geochemical processes. First-principles calculations based on density functional theory (DFT) have been proven to be a reliable approach to predict the reduced partition function ratio (103lnβ) and the inter-mineral 103lnα values, and there are large amounts of data calculated based on different methods in the literature. We here review previous DFT calculations on inter-mineral 103lnα of Mg isotopes, complement new calculations for some minerals, and thoroughly discuss and compare results from different methods. Two types of approximations, the local density approximation (LDA) and the generalized gradient approximation (GGA), are adopted to model the exchange-correlation potential in DFT calculations. Static calculations and quasi-harmonic approximation (QHA) are both used to calculate the 103lnα. In theory, the QHA method is more reliable but more computationally expensive than the static method, because the former derives the pressure- and temperature-dependent 103lnβ from its volume- and temperature-dependent form via the equation of states. The structural and vibrational properties of mantle minerals predicted by the LDA calculations agree well with experimental results, while the GGA calculations overestimate the volumes and Mg O bond lengths. There are systematic differences between the calculated 103lnβ values using static LDA and GGA calculations; however, both approaches predict similar inter-mineral 103lnα values at high temperatures because the systematic difference between two 103lnβ values cancels out. Furthermore, the inter-mineral 103lnα values predicted by the static LDA method generally agree with those from the LDA + QHA method, which is used to calculate the pressure effect on 103lnα. Two sets of inter-mineral 103lnα values of 26Mg/24Mg calculated by static LDA and LDA + QHA methods are present as a function of temperature and/or pressure in this review. Large Mg isotope fractionation exists between some minerals, and inter-mineral 103lnα values could be significantly affected by pressure due to the difference in the pressure slopes of their 103lnβ. The measured forsterite-magnesite and spinel-magnesite Mg isotope fractionation factors in previous experiments are consistent with the calculated results within uncertainties. Using the calculated data, we examine the degree to which the observed inter-mineral Mg isotope fractionation in natural rocks represents equilibrium and model the Mg isotope fractionation during late-stage basalt differentiation. [ABSTRACT FROM AUTHOR]

A sustainable use of soil resources is urgently required to cope with the increasing demand for agricultural products during climate change. To inspire farmers on new soil cultivation methods like subsoil management requires not only yield measurements but also nutrient use efficiency measurements for which analytical tools are still missing. Stable isotopes of the macronutrient magnesium (Mg) are a potential novel subsoil management evaluation tool in agronomy and soil/plant sciences because its isotope ratios shift considerably during Mg uptake by crops. The feasibility of Mg stable isotopes was first demonstrated conceptually by simulating subsoil management on soils with low, middle, and high inventories of bioavailable Mg and crop plants typically cultivated in Germany. This simulation showed that the magnitude of Mg isotope shifts among crops and the exchangeable fraction of Mg in soil is resolvable from the long-term external precision of Mg isotope analyses only if three conditions are met. First, the crop uptake-related Mg isotope fractionation factor should be at the upper end of hitherto published fractionation factors. Second, a high Mg uptake flux of crop plants (e.g., sugar beets) is matched by a low Mg supply from the exchangeable fraction in soil (e.g., sandy soils). Third, subsoil management causes a considerable deepening of the rooting system (e.g., flipping the topsoil root cluster below 30 cm depth). If these conditions are met, Mg stable isotopes can be used in a qualitative manner to identify the main Mg uptake depth, and in a quantitative manner by calculating the Mg use efficiency, defined here as the ratio of Mg uptake versus Mg supply, solely from Mg isotope ratios. This concept was tested for Alfisols on field trials by conducting deep loosening with and without the incorporation of compost. Magnesium isotope shifts in crops and the exchangeable fraction of Mg in soil were mostly unresolvable from the long-term external precision of Mg isotope analyses, which positively tested the Mg isotope concept for well nurtured soils. However, systematic Mg isotope shifts among bulk crops cultivated on and beside a melioration strip were found and attributed to the uplift of isotopically distinct compost-derived Mg on the melioration strip and root restricting layers beside the melioration strip. Given that the Mg isotope composition of the exchangeable fraction barely varies with depth, field-based crop uptake-related 'apparent' Mg isotope fractionation factors of winter wheat and spring barley could be determined, which differed from one another (Δ26Mg wheat-rem. exch. = 0.63 ± 0.05‰, Δ26Mg barley-rem. exch. = 0.37 ± 0.12‰). Nonetheless, the quantitative approach of Mg isotopes was violated when calcareous fertilizer was applied to the field as differences in the isotope-derived Mg use efficiency could be attributed to the uneven distribution of lime-derived Mg with depth. Using Mg stable isotopes as a new geochemical routine for agronomy and soil/plant sciences requires future work focussing on isotope fractionation factors related to crop uptake and intra-plant translocation of Mg – which may depend on species, environmental conditions, and nutrient status – to allow minimally invasive sampling of the soil-plant system and to reduce sample sets. • A conceptual Mg isotope model was established and positively tested in the field. • Mg stable isotopes are a powerful novel tool for subsoil management evaluation. • Mg use efficiency of crops can solely be quantified from Mg stable isotopes. • Lime application limits Mg use efficiency determination solely from Mg isotopes. • A criteria catalogue ensures a successful application of Mg isotopes in agronomy. [ABSTRACT FROM AUTHOR]

The Mg/Ca ratio of seawater increased from ∼1.5 to its present value of 5.4 over the Neogene, indicating a fundamental imbalance in the Mg–Ca cycle during the late Cenozoic. The imbalance in Mg–Ca cycles since the late Cenozoic, however, is contrasted by an arguable constancy in the Mg isotope compositions of seawater (δ 26 Mg sw) reconstructed from several low-resolution carbonate records. Addressing such Cenozoic Seawater Conundrum requires reliable seawater Mg isotopic records. The Xisha Islands in the South China Sea host large Cenozoic dolostone sequences that record seawater chemistry over the past 23 Myr. This study reports the Mg isotopic and trace elemental compositions of island dolostones in the well-studied Xike-1 drill core from the Xisha Islands. Petrographic, C–O–Sr isotopic, and paleomagnetic data from the drill core samples collectively support a "fluid-buffered", syn-depositional origin for the dolostones. The dolostones from the Xike-1 core have a narrow range of δ 26 Mg (− 2.83 ‰ ± 0.12 ‰), confirming that δ 26 Mg sw have been stable around the modern value (−0.83‰) since the late Cenozoic. This record provides a new opportunity to better understand the Mg–Ca cycle during the late Cenozoic. We ran numerical models of global seawater Mg–Ca contents and δ 26 Mg sw with the most recently published flux and isotope fractionation data. The results show that the contrast between the constancy of δ 26 Mg sw values and the dramatic increase in seawater Mg/Ca ratios cannot be explained by a single mechanism (i.e., a decrease in the rate of phyllosilicate formation on the seafloor alone). Instead, processes involving different Mg fluxes need to be coupled to explain the observed trends in Mg content and isotopic compositions of Cenozoic seawater. The solution to the Cenozoic Seawater Conundrum includes simultaneous decreases in the rates of both clay formation and dolomitization on the seafloor, or an increase in the rate of both continental weathering together with decreased seafloor clay formation. Our study underlines the sensitivity of the Mg cycle to the interactions between climate, continental weathering, and seafloor processes during the late Cenozoic. • Seawater δ 26 Mg sw values can be reconstructed from tropical island dolostones. • Record of Xisha Island dolostones shows a stable δ 26 Mg sw for the past 23 million years. • A sharp contrast between stable δ 26 Mg and rising Mg/Ca of seawater in the late Cenozoic. • Changes in dolomitization intensity canceled the Mg isotope effect of changes in clay formation intensity. [ABSTRACT FROM AUTHOR]

Magnesium isotope compositions (δ26Mg) can provide valuable insights into Mg cycling through surface fluid-rock interactions. Isotopic variations of Mg within the oceanic lithosphere imply incorporation of isotopically different Mg, but the underlying mechanisms remain uncertain. In this study, we measure the Mg isotopic composition of the oceanic crust (n = 32) and mantle peridotite (n = 13) from the Oman ophiolite, which has experienced magmatism at the spreading center and subduction zone. The crustal section records δ26Mg values that range from −0.58 to −0.04 ‰. In the upper crustal section, higher Mg-bearing mineral abundance tends to have lighter δ26Mg values. However, the lower crustal section has no correlation with the δ26Mg values, indicating that the hydrothermal Mg sink formed by seawater was significant within the upper 2 km of the oceanic crust. In the mantle section, the δ26Mg values range from −0.39 ‰ to −0.07 ‰. Seven peridotite samples, with a loss of MgO by up to 5.3%, have slightly higher δ26Mg values than the global mantle, which may be attributed to low-temperature weathering. In contrast, three peridotite samples with higher melting degrees (spinel Cr# = Cr/(Cr + Al) atomic ratio) than the adjacent rocks are considered to have been affected by subduction-related interactions, forming lighter δ26Mg values. The isotopically light Mg source was subducted sediments, but its low concentration of Mg requires a water/rock ratio >> 1. Therefore, we conclude that the contribution of slab-derived Mg and modification of mantle δ26Mg values were localized. [ABSTRACT FROM AUTHOR]

The Rock Canyon Creek REE–F–Ba deposit is a carbonate-hosted REE deposit located in the Alkaline Province of British Columbia, Canada, which hosts most REE mineralization. It shares tectonic, stratigraphic and structural similarities with nearby Mississippi Valley-type deposits. To constrain the dolomitization and mineralization processes, the δ26Mg, 87Sr/86Sr, and elemental compositions of dolomite, calcite, and fluorite were determined. The δ26Mg values of dolomite from the unmineralized zone (−1.83 ‰ and −1.67 ‰) are consistent with marine evaporative dolomite, but those from the mineralized zone (−0.97 ‰ to −0.17 ‰) are higher and more variable. The linear 87Sr/86Sr–Sr/Mg relationships, both in the dolomite and calcite from the unmineralized zone, suggest dolomitization in an evaporative marine environment. The high δ26Mg values with large variation and the non-stoichiometric Ca/Mg ratios in the dolomite from the mineralized zone suggest that this dolomite interacted with a relatively low-temperature hydrothermal fluid (<200 °C). Considering the Na, Ca, Sr, Ba, and LREE-enrichment and low 87Sr/86Sr signature in the dolomite from the mineralized zone as well as mineral paragenesis in the deposit, the most plausible scenario for the formation of the deposit is that marine-evaporitic dolomite re-equilibrated with basinal fluids along permeable zones, and at a later stage, carbohydrothermal REE-bearing fluids infiltrated into the Rock Canyon Creek brecciated zone and deposited the REE mineralization. Late-stage, post-mineralization calcite veins and iron oxides formed in the upper part of the deposit under supergene conditions, as suggested by combination of textural observations, REE patterns, and differences in isotopic compositions between dolomite and calcite from the mineralized zone. This study demonstrates that δ26Mg, 87Sr/86Sr, and element/Mg ratios in carbonate minerals are useful for characterizing mineralizing fluids in carbonate-hosted deposits, and discriminating mineralization-related dolomite from marine evaporative dolomite and unmineralized hydrothermal dolomite. • Mg and Sr isotopes of carbonates from carbonate-hosted REE–F–Ba deposit were determined. • Mg and Sr isotopes from the unmineralized zone suggested evaporative marine dolomitization. • Hydrothermal dolomite formed by a relatively low-temperature (<~200 °C) hydrothermal fluid. • REE mineralization caused by carbohydrothermal fluid after hydrothermal dolomite formation. • Mineralization-related and unmineralized dolomite can be discriminated by Mg isotopes. [ABSTRACT FROM AUTHOR]

• Syn-sedimentary Microbialites and dolomite cements under an anoxic shallow marine setting from the basal Dengying Formation; • Silicate weathering flux at the terminal Ediacaran is supported by positive Mg isotope excursion; • Enhanced silicate weathering flux increased seawater Mg/Ca ratio and alkalinity and facilitated the subsequent earliest biomineralization of animals. The advent of animal biomineralization at the terminal Ediacaran remarkably changed the marine ecological system. Changed seawater Mg/Ca ratio is considered as an important external trigger for this transition, but the intrinsic relationship between seawater Mg/Ca variation and animal biomineralization is not well constrained due to the lack of reliable archives. To constrain the coeval seawater chemistry and explore its role in the origin of metazoan biomineralization, we systematically conducted petrographical and geochemical investigations on dolostones from the terminal Ediacaran (∼551 Ma) Dengying Formation of the Yangtze Block in South China. The well-preserved sedimentary fabrics, growth zones as well as cathodoluminescent bands in microbialites and fibrous dolomite cements in the Algal Dolomite Member of the Dengying Formation indicate these dolomites were formed during syn-sedimentary diagenesis. The distribution patterns of rare earth elements in these dolomite components documented relatively anoxic shallow water conditions. Furthermore, negative correlations between O and Mg isotope compositions in well-preserved dolomite components indicate the variable mixture of freshwater with seawater in the shallow platform environment. The pervasive influence of freshwater on seawater indicates an enhanced riverine input, and it should be dominated by silicate weathering flux due to the heavier Mg isotopic compositions in freshwater endmember. Enhanced silicate weathering flux at the basal Dengying Formation may be related to the abundant erosions and exposures caused by the significant changes in Earth's surface tectonic and climatic conditions at the late Ediacaran. Consequently, the increased silicate weathering flux would deliver more Mg cations and net alkalinity to the oceans than the mixed weathering flux, which probably facilitated the subsequent skeletonization of early animals. [ABSTRACT FROM AUTHOR]

Tracking the final fate of subducting carbon is crucial to understanding global carbon cycles and climate changes in the history of the Earth. Available geochemical tracers such as carbon isotopes are apt to identify recycled organic carbon but usually insufficient to discriminate between primordial carbon in the mantle and carbon derived from recycled carbonate sediments. In the past decade, magnesium and zinc isotope systematics have been proposed as novel proxies for subducting carbon owing to the noticeable isotopic offsets between carbonate sediments and the mantle (i.e., δ26Mg carbonate < δ26Mg mantle ; δ66Zn carbonate > δ66Zn mantle). Nonetheless, isotopic effects induced by subduction-zone processes and crystal-melt differentiation may obscure the information of Mg and Zn isotopic compositions of mantle-derived magmas. In this paper we firstly discuss how these processes modify the Mg and Zn isotopic systematics of mantle-derived magmas. Based on the fact that different carbonate species (calcite, dolomite, and magnesite) possess distinct Mg and Zn contents and their stabilities in subduction zones vary with pressure, we then develop the two isotope systematics as tools to track the final storage depth of subducting carbon. We test this application by collating available Mg and Zn isotopic compositions of ultramafic xenoliths and basaltic lavas sourced from various mantle depths. The lack of light Mg and heavy Zn isotopic anomalies of global arc lavas supports experimental and theoretical prediction that the dissolved carbonate species in the sub-arc mantle−if any−is dominated by calcium-rich carbonate. The findings of pervasive low-δ26Mg and high-δ66Zn ultramafic xenoliths and basaltic lavas sourced from the sub-continental lithospheric mantle (SCLM) suggest that the SCLM is an important storage of subducting carbon via metasomatism by dolomite that can be substantially dissolved by supercritical fluids at depths of >160 km. Intraplate alkali basalts with low δ26Mg and high δ66Zn are commonly restricted to the regions with stagnant slabs at depths of ~410–660 km, suggesting that the mantle transition zone is another global storage of subducting carbon composed mainly of Mg-rich carbonates. Overall, observations on mantle-derived rocks, with Mg and Zn isotopes as the tracers, indicate that a significant flux of Earth's surface carbon has survived the arc regime and been recycled into the deeper mantle. Future studies that explore a quantitative relationship between Mg-Zn isotopic ratios and the flux of subducting carbon will further promote the application of the paired isotopic proxies. • Factors obscuring the interpretation of Mg and Zn isotopic data of mantle-derived rocks or magmas were reviewed. • Developing Mg and Zn isotopes as novel proxies for the storage depths of subducting carbonates in the mantle. • Test this application using available Mg and Zn isotope data of natural samples from various mantle depths. • A significant flux of Earth's surface carbon have survived the arc regime and been recycled into the deeper mantle. [ABSTRACT FROM AUTHOR]

Saline lakes are sensitive to climatic changes, however, it is challenging to reconstruct paleoclimate based on terrestrial evaporite records using conventional elemental and isotopic proxies. Magnesium is a major element in saline lakes, and the Mg isotope composition of brine is responsive to climate-driven processes such as carbonate precipitation and freshwater input. However, little has been explored on the application of Mg isotopes to studies of saline lakes. In this study, the Middle Pleistocene halite deposit from the Xiaoliangshan (XLS) evaporite section in the Qaidam Basin, Northwest China, was selected as a case to evaluate the response of Mg isotopes in the saline lake to environmental events. The Mg isotope data of the halite are complemented by geochemical analyses major elements and Sr-Cl isotopes of the halite component, and C-O isotopes of the associated carbonates. The 87Sr/86Sr ratios of the halite remained homogeneous (0.7111 to 0.7112) throughout the section, suggesting that the material source did not change significantly during the precipitation of the halite beds. By contrast, element ratios (Mg/Na and K/Na) and stable isotope ratios of C, O, Cl, and Mg show remarkable fluctuations along with the sediment profile. Based on the C-O-Cl isotope data, we identified events of freshwater recharging and desalination in an overall dry climatic background that were recorded in the halite. Notably, halite δ26Mg values vary by up to 2‰ in the section (from −1.63‰ to 0.46‰), and the low δ26Mg signature of halite was interpreted to reflect the input of light Mg isotopes into the saline lake in a freshwater recharging event, while the high δ26Mg values were produced by the precipitation of carbonate minerals under arid climatic conditions. Collectively, we suggest that Mg isotopes in terrestrial halite could be a sensitive tracer of basin hydrology. This study for the first time demonstrates the potential of Mg isotopes in terrestrial evaporites to unravel paleoclimatic events. • The first report of δ26Mg values of halite samples from terrestrial saline lakes. • The δ26Mg values of terrestrial halite show stratigraphic variation of >2‰. • The δ26Mg values of brine are controlled by the balance of river input and carbonate precipitation. • δ26Mg values of halite deposits in terrestrial saline lakes are sensitive to basin hydrology. [ABSTRACT FROM AUTHOR]

Carbonate weathering regulates the short-term carbon (C) cycle and global climate due to its fast response to hydrological processes. The carbonate weathering flux needs to be well constrained to better understand the climate change at short time scale. Riverine magnesium (Mg) isotopes are sensitive to primary mineral dissolution and so have great potential to trace carbonate weathering. Global large rivers draining continental crust dominate weathering flux to the oceans, but how riverine Mg isotopes respond to carbonate weathering remains unclear. The Yangtze River drainage basin (YRDB) was selected to test the robustness of riverine Mg isotopes (δ26Mg) in tracing continental carbonate weathering because it spans a wide range in lithology, geomorphology and climate. The riverine δ26Mg values within the YRDB show a decreasing trend from the headwater to the mainstream ranging from −1.36‰ to −0.59‰. The dissolved δ26Mg have strong negative correlations with carbonate weathering rate and intensity within the YRDB, indicating a sensitive response of riverine δ26Mg to the carbonate weathering flux. In a compilation of Mg fluxes and δ26Mg in the world's largest rivers, there is similar dominance of carbonate weathering on riverine Mg fluxes and isotopes. Therefore, we propose that riverine δ26Mg in large rivers are a robust tracer of carbonate weathering intensity. Intensifying carbonate weathering under global warming tends to increase riverine Mg and C fluxes to the oceans and thus the atmospheric CO 2 sink at the millennial time scale. • The first systematic riverine Mg and Sr dataset within the Yangtze River drainage basin was provided. • Carbonate weathering dominates Mg isotopes in global large rivers, with limited fractionation. • Riverine Mg isotopes of global large rivers can trace carbonate weathering intensity at the continental scale. [ABSTRACT FROM AUTHOR]