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There is a renewed interest in K isotope geochemistry driven by the advances in analytical precision and it is emerging as a useful tracer in a variety of disciplines ranging from Earth sciences to biology. However, high-quality K isotopic data for reference materials are still limited but highly needed. Here, we report high-precision stable K isotopic compositions (δ41K) for 23 commercially available international reference materials, including igneous, sedimentary, and metamorphic rocks, as well as an in-house seawater standard. Potassium in digested samples was separated by cation-exchange chromatography with Bio-Rad AG50W-X8 (200–400 mesh) resin in 0.5 N HNO3 media. Potassium isotopes were measured on a Nu Plasma II high-resolution MC-ICPMS. The reproducibility of K isotopic analysis, based on over one year of measurements of pure K solutions and rock standards, was ≤0.06‰ (95% confidence interval). Synthetic solutions made by mixing single element standards to represent various rock matrices confirmed the accuracy of our methods. The 23 reference materials have δ41K values ranging from −0.562‰ to −0.253‰ and the seawater standard has a much higher δ41K value of 0.143‰. The comprehensive dataset presented here provides a reference for quality control and inter-laboratory comparison of high-precision K isotopic analyses for future studies.

The Mg isotope compositions of weathering profiles developed on granite and granodiorite bedrock have been analyzed to investigate Mg isotope behavior during the weathering of felsic rocks in a continental environment. δ26Mg generally exhibits a small increase from the bedrock values, which range from −0.17 ± 0.06‰ to +0.07 ± 0.06‰, until the upper two meters in which a decrease is observed, producing near surface values between −0.14 ± 0.06‰ and +0.09 ± 0.06‰. The small observed increase in isotopic composition below 2 m is likely due to Mg loss to the hydrosphere during biotite weathering and in situ illite production, while the decrease at depths

This study presents a method for high-precision stable potassium (K) isotope analysis using High-Resolution Multi-Collector Inductively Coupled Plasma Mass Spectrometry (HR-MC-ICPMS). Dry and cold plasma is used to suppress the formation of argon hydrides that are direct isobaric interferences on K isotopes. The residual 40Ar1H+ beam is resolved from 41K+ by using pseudo-high resolution mode, providing interference-free shoulders of 39K+ and 41K+ for isotopic measurements. Precision of better than 0.06‰ (95% confidence interval) has been routinely achieved based on multiple measurements of pure K standard solutions and rock standards over a six-month period. Mismatches in K concentration (up to 20%) and acid molarity (up to 67%) between samples and bracketing standards, as well as the presence of substantial amount of matrix element Rb, do not significantly affect the analytical precision and accuracy. Our study documents that high-precision K isotope analysis can be routinely achieved for a wide variety of natural samples by HR-MC-ICPMS, which makes it possible for using K isotopes to trace major geological processes.

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

The element, Mg and Sr isotope ratios of a weathering profile from Hubei, China have been measured in order to document the behavior of Mg isotopes during dolostone weathering. The profile is developed in the joint system, and the weathering intensity increases from the least weathered dolostone towards the joint plane. According to the element and isotope ratios, the weathering profile can be divided into the weakly and intensely weathered zones. In the weakly weathered zone, Mg/Al (atomic) ratios decrease from 4.99 to 1.75 with increasing weathering intensity, while Ca/Al ratios slightly decrease from 13.70 to 9.02. These suggest massive loss of Mg and simultaneous conservation of Ca. δ26Mg values in the weakly weathered zone slightly decrease from − 1.90‰ to − 2.22‰, lighter than the corresponding protolith (− 1.90‰). These element and isotope variations are likely caused by the cooccurrence of dolomite dissolution and calcite re-precipitation, suggesting that the fluids co-existing with the dolostone are saturated with respect to calcite while not saturated with respect to dolomite. By contrast, both Mg/Al and Ca/Al ratios of intensely weathered samples display a decreasing trend towards the joint plane, ranging from 1.62 to 0.16 and 10.24 to 2.05, respectively. The significant loss of Mg and Ca indicates the considerable dissolution of both dolomite and calcite. δ26Mg and 87Sr/86Sr values increase from − 2.22‰ to − 0.41‰ and from 0.71128 to 0.71368, respectively. These observations demonstrate that the isotope compositions of the intensely weathered residues are dominantly controlled by the increasing silicate fractions after carbonate dissolution, given that silicates have considerably high 87Sr/86Sr and δ26Mg values relative to the carbonates. Overall, our study reveals large Mg isotope fractionation during dolostone weathering. The direction and extent of Mg isotope fractionation are mainly governed by the dissolution and re-precipitation of carbonate minerals, which are further controlled by the fluid's saturation with respect to carbonate minerals (calcite and dolomite). Therefore, cautions should be taken when using whole rock Mg isotope data of dolostones to trace paleoclimate change and dolostone genesis. Primary dolomite could be partially reserved in weakly weathered dolostones and step-leaching experiments may help to reveal its bearing Mg isotope signals.

Modern dolomite formation is conditional and restricted in specific environmental and geochemical conditions, accordingly cannot be used as the analog of ancient dolostones, which normally have platform-wide distribution. The key to understand ancient dolostone formation is to find a dolomitization process that is independent of physical/geochemical environments. It is well-known that ribbon rock (one type of calcareous rhythmites, sensu stricto referring to the alternating limestone and marlstone layers or the limestone-marl alternations) that are deposited in nearly all marine environments throughout the Earth's history, are exclusively, though normally partially, dolomitized. Thus, understanding ribbon rock dolomitization may provide valuable insight into the ‘dolomite problem’. In this study, we measured Mg isotopic compositions (δ26Mg) of ribbon rock samples collected from the Ediacaran Doushantuo Formation in South China. The marlstone layers contain higher dolomite contents and are characterized by higher δ26Mg (ranging from − 2.68‰ to − 1.84‰) than the limestone layers (varying between − 3.21‰ and − 2.53‰). A numerical model calculation indicates that clay minerals cannot provide enough Mg for dolomitization, arguing against the traditional interpretation that dolomitization is resulted from diagenetic Mg release from clays. Instead, contemporaneous seawater might provide sufficient Mg for dolomitization, because both Mg isotopes and dolomite content can be simulated by the Diffusion-Advection-Reaction model. Contemporaneous seawater dolomitization requires kinetic barrier be overcome at shallow depth of sediments. We propose the ribbon rock dolomitization is attributed to differential diagenesis of the clay-rich and clay-poor sediment layers, in which Ca2 +-Mg2 + exchange elevates porewater Mg/Ca in clay-rich layers.

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

Zinc isotopes have been increasingly utilized as a proxy for (bio)geochemical cycles of nutrient Zn in the ecosystem like soils, but the behavior and mechanism of Zn isotope fractionation during weathering and soil formation remain to be constrained. Here we reported zinc contents and isotope data for saprolites and laterites from two well-characterized mafic-rock weathering profiles, formed under a subtropical climate (South Carolina, USA) and a tropical climate (Hainan Island, China), respectively. Leaching experiments with basalt and andesite as the starting materials were also performed for comparison. The leaching results revealed no resolvable Zn isotope fractionation during dissolution of silicate minerals in the absence of organic ligands in solutions. The Hainan laterites show strong Zn depletion and have lower δ66ZnLyon values (0.10‰ to 0.27‰) relative to the parent rock (0.31 ± 0.04‰), which are coupled with light copper isotope compositions in the same samples. These observations suggest that Zn mobilization was achieved via metal-organic complexation and Fe-colloids transportation. By contrast, saprolites from the South Carolina are extremely Zn-rich (ten-fold higher concentration than the parent rock) and most samples have higher δ66Zn values (0.10‰ to 0.65‰) compared with the parent rock (0.24 ± 0.04‰). Zinc isotope ratios of the South Carolina profile are positively correlated with iron isotope ratios (δ56Fe) and Fe3+/FeTotal, the latter of which is related to secondary Fe (hydr)oxide formation. These relationships link the high Zn contents and δ66Zn values of saprolites from the South Carolina profile to the substantial adsorption of Zn onto or isomorphic substitution into Fe (hydr)oxides that prefers heavier Zn isotopes. Our study highlights that Fe (hydr)oxides exert a strong influence on the re-distribution of nutrient Zn in natural soils and the (bio)geochemical cycling in natural soil systems.

Magnesium isotopic ratios have been used as a natural tracer to study weathering processes and biogeochemical pathways in surficial environments, but few have focused on the mechanisms that control Mg isotope fractionation during shale weathering. In this study we focus on understanding Mg isotope fractionation in the Shale Hills catchment in central Pennsylvania. Mg isotope ratios were measured systematically in weathering products, along geochemical pathways of Mg during shale weathering: from bedrock to soils and soil pore water on a planar hillslope, and to sediments, stream water, and groundwater on a valley floor. Significant variations of Mg isotopic values were observed: δ26Mg values (− 0.6‰ to − 0.1‰) of stream and soil pore waters are about ~ 0.5‰ to 1‰ lighter than the shale bedrock δ(26Mg values of + 0.4‰), consistent with previous observations that lighter Mg isotopes are preferentially released to water during silicate weathering. Dissolution of the carbonate mineral ankerite, depleted in the shallow soils but present in bedrock at greater depths, produced higher Mg2 + concentrations but lower δ26Mg values (− 1.1‰) in groundwater, ~ 1.5‰ lighter than the bedrock. δ26Mg values (+ 0.2‰ to + 0.4‰) of soil samples on the planar hillslope are either similar or up to ~ 0.2‰ lighter than the bedrock. Hence a heavy Mg isotope reservoir – complementary to the lighter Mg isotopes in soil pore water and stream water – is missing from the residual soils on the hillslope. In addition, soil samples show a slight but systematic decreasing trend in δ26Mg values with increasing weathering duration towards the surface. We suggest that the accumulation of light Mg isotopes in surface soils at Shale Hills is due to a combined effect of i) sequestration of isotopically light Mg from soil water during clay dissolution–precipitation reactions; and ii) loss of isotopically heavy particulate Mg in micron-sized particles from the hillslope as suspended sediments. This latter mechanism is somewhat surprising in that most researchers do not consider physical removal or particles to be a likely mechanism of isotopic fractionation. Stream sediments (δ26Mg values of + 0.3‰ to + 0.5‰) accumulated on the valley floor are ~ 0.2‰ heavier than the bedrock, and are thus consistent with that mobile particulates are the heavy Mg isotope reservoir. Our study provides the first field evidence that changes in clay mineralogy lead to accumulation of lighter Mg isotopes in residual bulk soils. This example also demonstrates that transport of isotopically distinct fine particles from clay-rich systems could be a new and important mechanism to drive the Mg isotope compositions of silicate weathering residuals. This mechanism drives fractionation in an opposite direction as might be expected from previous studies, i.e. residual soils are driven to lighter Mg values and sediments become isotopically heavier.