B1 - Origin of fractionations of highly siderophile and siderophile volatile elements in lunar rocks and in the Earth

Research areas: Cosmochemistry, geochemistry

Summary

Understanding the composition of highly siderophile elements (HSE; platinum group elements, Re, and Au) and siderophile volatile elements (SVE, e.g. S, Se, Te, Ag) in materials from the Earth and Moon will help to constrain the processes that have fractionated elements with different volatility and affinity to metal and sulfide, and thus will improve models of accretion and core formation in the terrestrial planets. In particular, the composition of objects that formed the 4.5-3.8 Ga old lunar impact basins yields key information about the late-accretion history of the inner solar system.

Similar HSE abundance ratios in the silicate Earth and in ancient lunar impact rocks can be explained, if, prior to 3.8 Ga, the Earth’s surface accreted similar materials as the Moon did, and these materials were subsequently mixed into Earth’s mantle (“late veneer”). Some of these element ratios match those in chondritic meteorites, whereas others are not chondritic.

The non-chondritic ratios have been interpreted to indicate that the late veneer comprised a mixture of carbonaceous chondrite–like material and a minor proportion of fragments of planetesimal or embryo cores. If correct, this hypothesis predicts specific ratios and abundances of some SVEs (e. g., S, Se, Te) in the lunar breccias and in the silicate parts of terrestrial planets, which will be tested in the proposed study. Alternatively, the late veneer may be characterized by a unique HSE composition that reflects different processing conditions of dust precursors of planets and asteroids in the early solar nebula, a hypothesis that can be tested by isotopic studies (subprojects B3, B5). Also, while the Moon is thought to be rather depleted in volatile elements, not all SVEs of similar volatility are depleted equally.

Early studies have shown enigmatic fractionations that remain poorly understood. Some of these fractionations may be affected by limits of analytical detection on concentrations. However, others may reflect volatilization and metal–sulphide–silicate partitioning processes. In order to unravel these complexities, we propose to perform comprehensive studies of the HSE and SVE (chalcophile elements such as S, Se, Te, Cu, Ag, but also others such as Cd, Tl, In) composition of ancient lunar breccias and pristine highland rocks from the lunar crust.

However, as a prerequisite for interpreting these data correctly, we must understand the origin of the fractionation of these elements in pristine lunar highland rocks and in the lunar interior. Thus, to evaluate these partitioning processes we will be using new data from subproject C1 and literature data. Results of this project will provide new constraints on the chemical composition of the late veneer, identify the processes that have fractionated HSEs and SVEs in the Moon, and reveal the connection between the timing of late accretion, volatiles, and magma ocean evolution (C2, C4). It will also further our understanding of relations between lunar breccia compositions, specific impact deposits and their ages (subprojects A1-A4).