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

Research areas: Cosmochemistry, geochemistry


The Moon is believed to be more strongly depleted in volatile elements compared to Earth. This has been most commonly attributed to volatile loss during the formation of the Moon and/or extensive degassing during its magma ocean stage. However, the detailed origin of this depletion is not understood, and the abundances of some siderophile volatile elements (SVE) such as S, Se, Te, Zn, Cd, In and Tl are quite variable in highland rocks and in mare basalts. Thus, to trace the composition of material accreted on the Moon prior to 3.8 Ga, in our previous work on lunar impactites and some pristine FAN and Mg-suite samples we applied isotope dilution methods to obtain highly siderophile element (HSE) abundance data and SVE data. In contrast to the HSE, the budget of most SVE in lunar impactites appears to be predominantly controlled by target rock compositions and not by impactors. However, assessing the role of different fractionation processes of the SVE has been a major challenge, because both lunar core formation (e.g., metal-silicate or sulfide-silicate partitioning), degas- sing of lunar magmas and presumably the moon-forming event have affected abundances of these elements in lunar rocks. To address these challenges, here we propose to investigate the coupled behavior of mass-dependent stable isotope variations of Zn, Cu, K and S in the same sample aliquot of carefully selected lunar lithologies for which SVE mass fraction data and other information are available. We choose these elements because in existing data on lunar magmatic rocks, they show different extents of.

Major questions that will be tested are as follows: (1) Do depletions and stable isotope fractionations of Zn, Cu and other SVE mostly reflect local processes such as degassing of lunar magmas or are some of these fractionations intrinsic to the Moon? (2) Since preliminary data on lunar impactites and pristine highland rocks indicate different volatilities of SVE compared to solar gas condensation, parent body metamorphism and degassing of terrestrial magmas, are these volatility differences also seen for the SVE of mare basalts? (3) Is core formation responsible for the depletion and fractionation of some less volatile SVE in mare basalts and in Mg-suite samples? (4) Finally, did impact-induced heating lead to volatile metal loss in lunar impactites.

The rocks that will be studied to test the volatilization behaviour of volatile metals include well-characterized mare basalts of different compositional groups, pristine highland rocks and suitable ancient lunar impactites. The isotopic and concentration data will be compared to complementary experimental volatilization studies in subproject B7. The primary, longer-term objective is to better understand the Moon’s volatile element budget in the context of the volatile accretion history of Earth and Mars.