Siderophile volatile element inventory in lunar magmatic rocks and mantle sources

Abstract

Elevated water contents in various lunar materials have invigorated the discussion on the volatile content of the lunar interior and on the extent to which the volatile element inventory of lunar magmatic rocks is controlled by volatility and degassing. Abundances of moderately volatile and siderophile elements can reveal insights into lunar processes such as core formation, late accretion and volatile depletion. However, previous assessments relied on incomplete data sets and data of variable quality. Here we report mass fractions of the siderophile volatile elements Cu, Se, Ag, S, Te, Cd, In, and Tl in lunar magmatic rocks, analyzed by state-of-the-art isotope dilution-inductively coupled plasma mass spectrometry. The new data enable us to disentangle distribution processes during the formation of different magmatic rock suites and to constrain mantle source compositions. Mass fractions of Cu, S, and Se in mare basalts and magnesian suite norites clearly correlate with indicators of fractional crystallization. Similar mass fractions and fractional crystallization trends in mafic volcanic and plutonic rocks indicate that the latter elements are less prone to degassing during magma ascent and effusion than proposed previously. The latter processes predominate only for specific elements (e.g., Tl, Cd) and complementary enrichments of these elements also occur in some brecciated highland rocks. A detailed comparison of elements with different affinities to metal or sulfide and gas phase reveals systematic differences between lunar magmatic rock suites. The latter observation suggests a predominant control of the variations of S, Se, Cu, and Ag by mantle source composition instead of late-stage magmatic degassing. New estimates of mantle source compositions of two low-Ti mare basalt suites support the notion of a lunar mantle that is strongly depleted in siderophile volatile elements compared to the terrestrial mantle.