Abstract The fractionation of carbon and magnesium isotopes is a potentially useful tracer of natural weathering in ultramafic catchments and engineered CO2 storage. To evaluate the use of carbon and magnesium isotopes as tracers of ultramafic weathering and CO2 storage, we assessed the carbon and magnesium isotope fractionation between hydrous Mg-carbonate minerals and fluid during a mineral phase transformation from nesquehonite [MgCO3·3H2O] to dypingite [Mg5(CO3)4(OH)2·∼5-8H2O], two common products of ultramafic rock weathering. Batch reactor experiments containing nesquehonite were conducted at 5°C, 25°C, and 35°C and the evolution of mineralogical composition, fluid composition, and isotopic composition were tracked over time. At 5°C, the solid remained nesquehonite throughout the experiments, and isotopic equilibrium did not appear to be achieved between the solid and the fluid phase for either carbon or magnesium. At 25°C, and 35°C a transformation from nesquehonite to dypingite occurred by dissolution and re-precipitation, which resulted in extensive exchange of Mg and C between solid and fluid. The phase transformation caused the initial C and Mg isotopic composition of the solid phase to be overwritten. The extensive isotopic exchange during the phase transformation suggests C and Mg isotopes likely obtained approximate isotopic equilibrium between dypingite and fluid. For dypingite, the Δ13Cdyp-DIC was 4.74±0.12‰ (VPDB) and 4.47±0.17‰ (VPDB) at 25 and 35°C, respectively. The Δ meas 26 M g dyp - f l u i d between solid and the bulk fluid was -0.76‰, and -0.98±0.08‰ for the 25 and 35°C experiments, respectively. There was no clear impact of temperature on Mg or C isotope fractionation. The calculated Δ calc 26 M g dyp - M g 2 + between dypingite and the Mg2+ aquo species rather than bulk aqueous Mg values were positive. This indicates that if dypingite is formed by the incorporation of the free Mg2+ ion in the solid, the solid is preferentially enriched in the isotope of higher mass (26Mg). This is opposite to anhydrous Mg-bearing carbonate minerals, which tend to be depleted in 26Mg relative to the forming fluid. These data will help improve interpretation of carbon and magnesium isotope compositions measured in natural and engineered ultramafic weathering environments, and may help to trace the fate of anthropogenic CO2 during engineered CO2 storage efforts.

中文翻译：

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水合碳酸镁矿物相变过程中的镁和碳同位素分馏

摘要 碳和镁同位素的分馏是超基性集水区和工程 CO2 储存中自然风化的潜在有用示踪剂。为了评估碳和镁同位素作为超镁铁质风化和 CO2 储存示踪剂的使用，我们评估了在从三水菱镁矿 [MgCO3·3H2O] 到 dypingite [Mg5] 的矿物相转变过程中含水碳酸镁矿物和流体之间的碳和镁同位素分馏(CO3)4(OH)2·∼5-8H2O]，超基性岩石风化的两种常见产物。在 5°C、25°C 和 35°C 下进行含有三水菱镁矿的间歇反应器实验，并跟踪矿物组成、流体组成和同位素组成随时间的演变。在 5°C 下，固体在整个实验过程中保持为三水菱镁矿，对于碳或镁，在固相和液相之间似乎没有实现同位素平衡。在 25°C 和 35°C 时，通过溶解和再沉淀发生从三水菱镁矿到 dypingite 的转变，这导致了固体和流体之间 Mg 和 C 的广泛交换。相变导致固相的初始 C 和 Mg 同位素组成被覆盖。相变过程中广泛的同位素交换表明 C 和 Mg 同位素可能在 dypingite 和流体之间获得了近似的同位素平衡。对于 dypingite，Δ13Cdyp-DIC 在 25°C 和 35°C 下分别为 4.74±0.12‰ (VPDB) 和 4.47±0.17‰ (VPDB)。对于 25°C 和 35°C 实验，Δ meas 26 Mg dyp - 固体和本体流体之间的流体分别为 -0.76‰和 -0.98±0.08‰。温度对 Mg 或 C 同位素分馏没有明显影响。计算出的 Δ calc 26 Mg dyp - Mg 2 + 在 dypingite 和 Mg2+ aquo 物质之间而不是大量含水 Mg 值是正值。这表明，如果 dypingite 是通过将游离 Mg2+ 离子掺入固体中而形成的，则该固体优先富含更高质量的同位素（26Mg）。这与无水含镁碳酸盐矿物相反，后者相对于形成流体往往会耗尽 26Mg。这些数据将有助于改进对在自然和工程超基性风化环境中测量的碳和镁同位素组成的解释，并可能有助于追踪工程二氧化碳封存工作期间人为二氧化碳的归宿。计算出的 Δ calc 26 Mg dyp - Mg 2 + 在 dypingite 和 Mg2+ aquo 物质之间而不是大量含水 Mg 值是正值。这表明，如果 dypingite 是通过将游离 Mg2+ 离子掺入固体中而形成的，则该固体优先富含更高质量的同位素（26Mg）。这与无水含镁碳酸盐矿物相反，后者相对于形成流体往往会耗尽 26Mg。这些数据将有助于改进对在自然和工程超基性风化环境中测量的碳和镁同位素组成的解释，并可能有助于追踪工程二氧化碳封存工作期间人为二氧化碳的归宿。计算出的 Δ calc 26 Mg dyp - Mg 2 + 在 dypingite 和 Mg2+ aquo 物质之间而不是大量含水 Mg 值是正值。这表明，如果 dypingite 是通过将游离 Mg2+ 离子掺入固体中而形成的，则固体优先富含更高质量的同位素（26Mg）。这与无水含镁碳酸盐矿物相反，后者相对于形成流体往往会耗尽 26Mg。这些数据将有助于改进对在自然和工程超基性风化环境中测量的碳和镁同位素组成的解释，并可能有助于追踪工程二氧化碳封存工作期间人为二氧化碳的归宿。这表明，如果 dypingite 是通过将游离 Mg2+ 离子掺入固体中而形成的，则该固体优先富含更高质量的同位素（26Mg）。这与无水含镁碳酸盐矿物相反，后者相对于形成流体往往会耗尽 26Mg。这些数据将有助于改进对在自然和工程超基性风化环境中测量的碳和镁同位素组成的解释，并可能有助于追踪工程二氧化碳封存工作期间人为二氧化碳的归宿。这表明，如果 dypingite 是通过将游离 Mg2+ 离子掺入固体中而形成的，则该固体优先富含更高质量的同位素（26Mg）。这与无水含镁碳酸盐矿物相反，后者相对于形成流体往往会耗尽 26Mg。这些数据将有助于改进对在自然和工程超基性风化环境中测量的碳和镁同位素组成的解释，并可能有助于追踪工程二氧化碳封存工作期间人为二氧化碳的归宿。