Geochimica et Cosmochimica Acta最新文献

Serpentinite dehydration in subduction zones plays a pivotal role in geochemical cycling on Earth. A number of geochemical studies on arc magmas have elucidated the contributions of serpentinite-derived fluids to mantle sources. However, due to complex geological overprints during subduction zone processes, discerning serpentinite signatures in exposed metamorphic rocks within fossil subduction zones remains challenging. In this study we address these difficulties through in-situ investigations of tourmaline, the geochemistry of which reflects the host environment as well as potential fluid-induced processes. The presence of zonations in tourmaline makes it an excellent recorder of consecutive geological events. Integrated major and trace elements along with in-situ boron isotopes of tourmaline from the high-pressure Sopron area (Hungary) in the Eastern Alps were used to unravel fluid action sourced from serpentinite. Despite the presence of color zoning, tourmaline in the orthogneiss (Tur-G) has low X_Mg [Mg/(Mg + Fe)] of ca. 0.3–0.6 and δ¹¹B values of around −11 ‰, along with variable trace element compositions. Petrological observations and geochemical analyses suggest that the inner domains of Tur-G are of igneous origin, while the outer rims are likely affected by subsequent metamorphic events. Tourmaline in metasomatized kyanite-quartzite (Tur-K) veins exhibits distinct geochemical zoning, and preserves metamorphic cores and fluid-induced rims. The inner domains of Tur-K display low X_Mg (<0.6), relatively high trace element concentrations and δ¹¹B values of less than −10 ‰, whereas the overgrowths exhibit extremely high X_Mg values (>0.99), low trace element concentrations and high δ¹¹B values reaching up to +21 ‰, clearly indicating the incorporation of serpentinite-derived Mg-¹¹B-rich fluids. Through comparison with other metamorphic and metasomatic tourmalines in (ultra)high-pressure rocks globally, we establish that tourmaline with high X_Mg > 0.85 and δ¹¹B values >0 ‰ may serve as an effective proxy for detecting serpentinite-derived fluids in subduction zones.

The global carbon and iron cycles are intimately linked as redox-sensitive iron oxides readily bind organic carbon in a variety of environmental settings, including marine and lacustrine sediments. While these iron-organic carbon complexes sequester vast quantities of organic carbon, the composition of the organic matter within them remains unknown for lacustrine environments. Here we present C K_1s and Fe L_3,2 edge Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of surface sediments and authigenic iron complexes from adjacent basins of a pristine boreal lake located in Québec, Canada, with contrasting oxygen exposure regimes. We demonstrate differences in organic carbon speciation in sediments from both basins, as well as co-localization of organic carbon and iron on a sub-micron scale in 100 nm thick samples. Differences in redox cycling across these two basins allow for a direct comparison of the effect of oscillating redox conditions on the composition of organic carbon sequestered by iron. Our results suggest that reactive organic molecules, which may be polysaccharides, were found preferentially associated with iron in the perennially oxic sediments compared to more phenol rich organics in the seasonally anoxic sediments, highlighting the importance of iron oxides in the protection and preservation of labile organic compounds. Traces of aliphatic carbon were observed in sediments from the anoxic basin, alongside carboxyl and aromatic functionalities. This carboxyl-rich aliphatic material could possibly interact with the sediment mineral matrix either through a ligand exchange mechanism between the mineral phases and the carboxyl functionalities, or via non-specific hydrophobic interactions involving the aliphatic moieties. Finally, our work also shows that OC:Fe ratios should be used with caution when inferring a binding mechanism between OC and iron oxides.

Aqueous alteration in planetesimals is one of the earliest geological processes in the solar system. The timing of aqueous alteration sheds light on the timescale of material evolution through water–rock interaction in small bodies. The ⁵³Mn-⁵³Cr decay system, where a short-lived radionuclide ⁵³Mn decays to ⁵³Cr with a half-life of 3.7 Myr, is a powerful tool for dating carbonates in primitive meteorites that formed during aqueous alteration. In CI chondrites and samples returned from asteroid Ryugu, a major carbonate mineral is dolomite (CaMg(CO₃)₂) and could be dated precisely because of their relatively high Mn abundances. However, the lack of a proper dolomite standard for secondary ion mass spectrometry (SIMS) hinders us from obtaining accurate Mn/Cr ratios of carbonates, resulting in erroneous formation ages. In this work, we synthesized Mn-, Cr-, and Fe-bearing crystalline dolomite as standard materials and evaluated the relative sensitivity factor (RSF) of Mn/Cr for SIMS analysis, namely, the ratio of Mn/Cr obtained using SIMS to true Mn/Cr. We found that the RSF values of the dolomite standards range from 0.8 to 0.9, slightly higher than that of calcite (CaCO₃) (∼0.7), and increase with their Fe contents. We used the newly evaluated RSF values to date dolomite in the Ivuna CI chondrite and obtained an initial ⁵³Mn/⁵⁵Mn ratio of (3.95 ± 0.49) × 10⁻⁶ (95 % confidence interval) and the corresponding absolute age of 4564.0 + 0.6/−0.7 Ma. Our new initial ⁵³Mn/⁵⁵Mn ratio is 26 ± 19 % higher than that obtained by a previous study for the same dolomite grain using a calcite standard. This difference is consistent with the difference between the RSF values of dolomite and calcite. Based on these results, we updated the initial ⁵³Mn/⁵⁵Mn ratio previously reported for dolomite in the Ryugu sample A0058 to be (3.21 ± 0.66) × 10⁻⁶, which corresponds to an absolute age of 4562.8 + 1.0/−1.2 Ma. This age seems to be the best estimate for the formation age of dolomite in Ryugu currently available.

By taking advantage of recent analytical advances, we herein develop the ²²⁶Ra/²³⁰Th isotope systematics as a novel tool for quantifying nitrate and dissolved silicate fluxes across the sediment–water interface of the deep-ocean floor. Sediment cores were retrieved from the seabed between 4927 m and 5951 m in the North Pacific Ocean. Downcore profiles of ²³⁰Th and both dissolved and total ²²⁶Ra were measured using a high-sensitivity inductively coupled plasma mass spectrometer. At all study sites, a marked deficit of total ²²⁶Ra with respect to ²³⁰Th was observed between 0 and 20 cm, indicating active migration of soluble ²²⁶Ra from the sediment into the overlying seawater. By constructing the mass balance of ²²⁶Ra in the sediment column, the flux of dissolved ²²⁶Ra across the sediment–water interface was estimated to range from 461 to 1320 dpm m⁻² yr⁻¹. When coupled to a diffusive transport model as developed by early investigators, these flux values of ²²⁶Ra enabled us to calculate the flux of any dissolved constituent of interest by measuring their bottom water concentrations and pore water “saturation” concentrations. Based on the ²²⁶Ra/²³⁰Th disequilibrium approach, the derived fluxes vary between 4.1 and 10.5 mmol m⁻² yr⁻¹ for nitrate and between 11 and 49 mmol m⁻² yr⁻¹ for dissolved silicate. A compilation of nitrate and silicate fluxes from the seabed in the deep Pacific Ocean shows that these values are consistent with historical flux measurements based on the conventional core incubation method in the same study region. In addition, both nitrate and silicate fluxes exhibit a clear depth-dependent trend. Overall, our results suggest that sedimentary diagenetic alterations at the North Pacific Ocean floor below ∼ 5000 m are efficient so that only < 2 % of the particulate organic carbon and < 12 % of the biogenic opal raining to the seafloor are ultimately preserved in the sediment.