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

这项研究旨在通过利用La-Ce同位素体系，为控制硅酸盐地球分异的地球主体难熔元素组成和地球化学循环提供新的制约因素。高精度的138La-138Ce同位素数据可能为常规同位素系统尚未给出明确答案的几个基本问题提供新的约束。例如，通过比较各种球粒陨石和地幔岩石中的高精度138La-138Ce和147Sm-143Nd同位素数据，可以解决观测到的块状地球和球粒陨石的第142同位素组成之间存在的内在模糊性。在新一代热电离质谱计(TIMS)上，更精确的Ce同位素测量已经成为可能。这种高精度的Ce同位素数据将使我们能够从一个新的角度研究地球的主体成分及其随后的化学分化。后者是拟议研究的主要目标。此外，La-Ce和Sm-Nd同位素组合体系可能区分关于陆壳和地幔之间的地球化学循环的不同假说，例如循环不同类型的海洋沉积物与下陆壳的侵蚀和俯冲。地幔的耗尽程度也可能受到进一步的限制，这是估计全球物质平衡的一个关键参数，并对地球深处是否存在孤立的储藏库产生重要影响。因此，建议测定具有代表性的数量和种类的原始陨石和地幔岩石的Ce-ND同位素组成，即来自大洋中脊的玄武岩(MORB)、来自次大陆岩石圈地幔的橄榄岩和洋岛(OIB)。MORB和OIB数据集将由现有的高精度锶铅氢同位素和微量元素数据补充。陨石和MORB-OIB中Ce-ND同位素数据的对比将为地球大块耐火元素组成提供新的约束条件。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的过程。因此，这项拟议的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.