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

本研究旨在利用La-Ce同位素系统对地球体积耐火元素组成和控制硅酸盐地球分异的地球化学旋回提供新的约束条件。高精度138La-138Ce同位素数据可能为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，通过比较各种球粒陨石和地幔源岩石中的高精度138La-138Ce和147Sm-143Nd同位素数据，可以解决块状地球和球粒陨石中观测到的142同位素组成差异固有的模糊性。在新一代热电离质谱仪（TIMS）上，大约四倍的精确Ce同位素测量成为可能。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总体组成及其随后的化学分化。后者是本研究的主要目的。此外，La-Ce和Sm-Nd组合同位素系统可以潜在地区分大陆地壳和地幔之间地球化学循环的不同假设，例如不同类型的海洋沉积物的再循环与下大陆地壳的侵蚀和俯冲。地球地幔的枯竭程度也可能受到进一步的限制，这是估计全球质量平衡的一个关键参数，并对地球深部孤立储层的假定存在产生重要影响。因此，建议对具有代表性的原始陨石和地幔源岩石（即中洋脊玄武岩（MORB）、次大陆岩石圈地幔橄榄岩（OIB）和洋岛（OIB））进行Ce-Nd同位素组成测定。MORB和OIB数据集将由现有的高精度Sr-Pb-Hf同位素和微量元素数据补充。陨石和MORB-OIB中Ce-Nd同位素数据的比较将为地球体积耐火元素组成提供新的约束。MORB、橄榄岩和OIB数据将揭示地幔衰竭的程度，以及导致地球地幔成分非均质性的过程。因此，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.