Cadmium isotope fractionation during metal-silicate partitioning - Results and implications for Earth's volatile accretion
Cadmium isotope fractionation during metal-silicate partitioning - Results and implications for Earth's volatile accretion
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金属硅酸盐分配过程中的镉同位素分馏 - 结果及其对地球挥发性吸积的影响
DOI：
10.1016/j.chemgeo.2022.121293
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发表时间：
2023
期刊：
影响因子：
3.9
通讯作者：
Pickard H
中科院分区：
文献类型：
--
作者：
Pickard H
在1.5 GPa和1508 ~ 1843 K条件下进行了金属硅酸盐分配实验，以约束岩心形成过程中的Cd分配和同位素分馏。在研究条件下，Cd在金属相和硅酸盐相之间的分配过程中没有明显的稳定同位素分馏，平均为∆114Cdmet-sil= - 0.02±0.09‰（2SD,n= 7）。两个研究镉的硫化物-硅酸盐分配的实验得出的分馏因子分别为- 0.04±0.06‰和- 0.23±0.07‰（2SE），其中后者的结果是在短期内得到的，可能不能代表完全平衡。总之，这些发现表明，在地核分离过程中，Cd同位素分馏要么不存在，要么非常小。为了更好地约束岩心形成过程中Cd金属硅酸盐的分配，将本研究和以往研究的Cd分配数据结合起来进行多元线性回归分析。与先前的工作一致，分析表明Cd金属硅酸盐配分不受温度和压力的显著影响，但受金属相S含量的影响。此外，金属中C和Si的存在降低了Cd的亲铁性。根据对地核组成的估计，数据表明，单阶段地核形成事件的金属-硅酸盐分配系数dcd约为0.4。然而，考虑到地核中轻元素组成的不确定性，目前不能排除dcd值大于1是地核形成的原因。本研究的结果以及硅酸盐土和球粒陨石的组成数据被应用于质量平衡计算，以限制地球主要阶段吸积物质在晚期贴面形成之前的Cd特征。该模型表明，地球吸积的主要阶段涉及的物质的平均Cd同位素组成比已知的碳质和辉化辉石球粒陨石轻。最有可能的是，这要么反映了目前可用的少量精确数据对这些陨石的特征描述不佳，要么反映了陆地挥发物清单的很大一部分是从与碳质和顽辉石球粒陨石没有直接关系的物质中获得的。此外，根据吸积模型，陆地吸积很可能不包括超过地球质量2%的富含挥发物的晚期贴面，这些模型认为挥发物的输送主要发生在主要吸积阶段，与岩心形成同时发生。
Metal-silicate partitioning experiments were carried out at 1.5 GPa and 1508 to 1843 K to constrain Cd partitioning and isotope fractionation during core formation. At the studied conditions, there was no significant stable isotope fractionation during Cd partitioning between metal and silicate phases with a mean ∆114Cdmet-sil= −0.02 ± 0.09‰ (2SD,n= 7). Two experiments that investigated sulphide-silicate partitioning of Cd yielded fractionation factors of −0.04 ± 0.06‰ and − 0.23 ± 0.07‰ (2SE), whereby the latter result was obtained for a short run that may not represent full equilibrium. In summary, the findings suggest that Cd isotope fractionation during segregation of Earth's core was either absent or very minor. The Cd partitioning data of this and previous investigations were combined in multiple linear regression analyses to better constrain Cd metal-silicate partitioning during core formation. In accord with earlier work, the analyses reveal that Cd metal-silicate partitioning is not significantly impacted by temperature and pressure but affected by the S content of the metal phase. In addition, it is shown that the presence of C and Si in the metal reduce the siderophile character of Cd. Based on estimates for the composition of Earth's core, the data suggest a metal-silicate partition coefficient DCdof about 0.4 for a single-stage core formation event. However, given uncertainties about the light element composition of Earth's core, DCdvalues larger than 1 cannot be ruled out at present for core formation. The results of this study and data on the composition of the bulk silicate Earth and chondritic meteorites were applied in mass balance calculations to constrain the Cd signature of Earth's main stage accretion material prior to delivery of the late veneer. The modelling indicates Earth's main stage of accretion involved material with an average Cd isotope composition that was lighter than that of known carbonaceous and enstatite chondrites. Most likely, this reflects either poor characterisation of these meteorites by the few precise data currently available or that a significant fraction of the terrestrial volatile inventory was acquired from material not directly related to carbonaceous and enstatite chondrites. Furthermore, terrestrial accretion most likely did not encompass the addition of a volatile-rich late veneer exceeding 2% of Earth's mass, in accord with accretion models, which invoke that volatile delivery occurred primarily during main stage accretion, alongside core formation.