Chemical and germanium isotopic compositions of sediments and sedimentary rocks from ODP Sites 129-801 and 185-1149

A new technique for the precise and accurate determination of Ge stable isotope compositions has been developed and applied to silicate rocks and biogenic opal. The analyses were performed using a continuous flow hydride generation system coupled to a MC-ICP-MS. Samples have been purified through anion- and cation-exchange resins to separate Ge from matrix elements and eliminate potential isobaric interferences. Variations of 74Ge/70Ge ratios are expressed as d74Ge values relative to our internal standard and the long-term external reproducibility of the data is better than 0.2? for sample size as low as 15 ng of Ge. Data are presented for igneous and sedimentary rocks, and the overall variation is 2.4? in d74Ge, representing 12 times the uncertainty of the measurements and demonstrating that the terrestrial isotopic composition of Ge is not unique. Co-variations of 74Ge/70Ge, 73Ge/70Ge and 72Ge/70Ge ratios follow a mass-dependent behaviour and imply natural isotopic fractionation of Ge by physicochemical processes. The range of d74Ge in igneous rocks is only 0.25? without systematic differences among continental crust, oceanic crust or mantle material. On this basis, a Bulk Silicate Earth reservoir with a d74Ge of 1.3+/-0.2? can be defined. In contrast, modern biogenic opal such as marine sponges and authigenic glauconite displayed higher d74Ge values between 2.0? and 3.0?. This suggests that biogenic opal may be significantly enriched in light isotopes with respect to seawater and places a lower bound on the d74Ge of the seawater to +3.0?.This suggests that seawater is isotopically heavy relative to Bulk Silicate Earth and that biogenic opal may be significantly fractionated with respect to seawater. Deep-sea sediments are within the range of the Bulk Silicate Earth while Mesozoic deep-sea cherts (opal and quartz) have d74Ge values ranging from 0.7? to 2.0?. The variable values of the cherts cannot be explained by binary mixing between a biogenic component and a detrital component and are suggestive of enrichment in the light isotope of diagenetic quartz. Further work is now required to determine Ge isotope fractionation by siliceous organisms and to investigate the effect of diagenetic processes during chert lithification.

本研究建立了一种可精准、准确测定锗（Ge）稳定同位素组成的新方法，并将其应用于硅酸盐岩石与生物成因蛋白石（biogenic opal）的分析。实验采用连续流动氢化物发生系统（continuous flow hydride generation system）与多接收杯电感耦合等离子体质谱（MC-ICP-MS）联用完成测试。样品通过阴离子交换树脂与阳离子交换树脂（anion- and cation-exchange resins）进行纯化，以分离锗与基体元素并消除潜在的同量异位素干扰。74Ge/70Ge比值的变化以相对于内部标准的δ74Ge值表示，当样品中锗的质量低至15纳克时，数据的长期外部重现性优于0.2‰。本研究给出了火成岩与沉积岩的锗同位素数据，其δ74Ge总变化范围达2.4‰，为测量不确定度的12倍，表明地球体系中锗的同位素组成并非唯一。74Ge/70Ge、73Ge/70Ge与72Ge/70Ge比值的协同变化遵循质量依赖分馏规律，证明锗可通过物理化学过程发生自然同位素分馏。火成岩的δ74Ge变化范围仅为0.25‰，且大陆地壳、洋壳及地幔物质之间无系统性差异。据此可定义全硅酸盐地球（Bulk Silicate Earth）储库的δ74Ge值为1.3±0.2‰。与之形成对比的是，现代生物成因蛋白石（如海洋海绵与自生海绿石）的δ74Ge值更高，介于2.0‰至3.0‰之间。这表明相较于海水，生物成因蛋白石可显著富集轻同位素，并将海水的δ74Ge值下限限定为+3.0‰。由此可见，相较于全硅酸盐地球，海水的同位素组成更偏重，且生物成因蛋白石与海水之间存在显著的同位素分馏。深海沉积物的δ74Ge值处于全硅酸盐地球的范围内，而中生代深海燧石（蛋白石与石英）的δ74Ge值介于0.7‰至2.0‰之间。燧石的δ74Ge值变化无法通过生物组分与碎屑组分的二元混合来解释，这暗示成岩石英发生了轻同位素富集。未来仍需开展进一步研究，以明确硅质生物介导的锗同位素分馏机制，并探究燧石岩化过程中成岩作用对同位素分馏的影响。