New constraints on the bulk composition and compositional evolution of the Earth from La-Ce isotope measurements

本研究的目的是提供新的限制地球的整体难熔元素组成和地球化学循环，利用镧铈同位素系统的硅酸盐地球分异。高精度的138 La-138 Ce同位素数据可能会对传统同位素系统尚未提供明确答案的几个基本问题提供新的约束。例如，在大块地球和南极陨石的142 Nd同位素组成之间观察到的差异中固有的模糊性，可以通过比较各种南极陨石和地幔衍生岩石中的高精度138 La-138 Ce和147 Sm-143 Nd同位素数据来解决。在新一代热电离质谱仪（TIMS）上，Ce同位素的测量精度提高了约四倍。这种高精度的Ce同位素数据将允许从一个新的角度研究地球的整体组成及其随后的化学差异。后者是拟议研究的主要目标。此外，La-Ce和Sm-Nd同位素组合体系还可能区分不同的陆壳-地幔地球化学循环假说，如不同类型的海洋沉积物再循环与下陆壳的侵蚀和俯冲。对地球地幔的消耗程度也可能有进一步的限制，这是估计全球质量平衡的一个关键参数，对地球深部是否存在孤立的储层有重要影响。因此，建议确定具有代表性数量和种类的原始陨石和幔源岩石的Ce-Nd同位素组成，即，大洋中脊玄武岩（MORB）、陆下岩石圈地幔橄榄岩和洋岛玄武岩（OIB）。MORB和OIB数据集将得到现有高精度Sr-Pb-Hf同位素和微量元素数据的补充。陨石和MORB-OIB的Ce-Nd同位素数据的比较将提供新的限制地球的整体难熔元素组成。MORB、橄榄岩和OIB数据将揭示地幔亏损的程度，以及导致地幔成分不均匀性的过程。因此，这项研究可以从根本上提高我们对塑造地球系统的关键参数和过程的理解。

This study aims to provide novel constraints on Earths bulk refractory element composition and the geochemical cycles that govern silicate Earth differentiation by exploiting the La-Ce isotope system. High-precision 138La-138Ce isotope data may provide novel constraints on several fundamental questions to which the conventional isotope systems have not yet provided definite answers. The inherent ambiguity in the observed difference between the 142Nd isotope composition of bulk Earth and chondritic meteorites, for example, could be resolved by comparing high-precision 138La-138Ce and 147Sm-143Nd isotope data in a variety of chondritic meteorites and mantle-derived rocks. About four times more precise Ce isotope measurements have become possible on a new generation of thermal ionization mass spectrometers (TIMS). Such high-precision Ce isotope data will allow investigating Earths bulk composition and its subsequent chemical differentiation from a novel perspective. The latter is the main objective of the proposed study. In addition, the combined La-Ce and Sm-Nd isotope systems can potentially distinguish between different hypotheses proposed for geochemical cycling between the continental crust and mantle, such as recycling different types of marine sediment versus erosion and subduction of lower continental crust. Further constraints will also be possible on the extent of depletion of the Earths mantle, which is a crucial parameter for estimating global mass balances and has important ramifications on the putative existence of an isolated reservoir in the deep Earth. Hence, it is proposed to determine the Ce-Nd isotope composition of a representative number and variety of pristine meteorites and mantle-derived rocks, i.e., basalts from mid ocean ridges (MORB), peridotites from the subcontinental lithospheric mantle, and ocean islands (OIB). The MORB and OIB data set will be complemented by existing high-precision Sr-Pb-Hf isotope and trace element data. The comparison of Ce-Nd isotope data in meteorites and MORB-OIB will provide novel constraints on Earths bulk refractory element composition. The MORB, peridotite and OIB data will reveal the extent of mantle depletion, and the processes leading to compositional heterogeneity in the Earths mantle. The proposed study could thus fundamentally improve our understanding of key parameters and processes that shape the system earth.