Siderophile volatile element inventory of lunar mare basalts: Constraints on magmatic processes and mantle sources

Elevated contents of water and moderately volatile elements in some 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 element volatility and degassing. In order to constrain magmatic processes and mantle source compositions, we obtained a comprehensive data set for mass fractions of the moderately to highly volatile elements Cu, Se, Ag, S, Te, Cd, In, and Tl in various low- and high-Ti mare basalts. Mass fractions of Cu, S, Se, and Ag in each suite are mainly controlled by fractional crystallization. In contrast, Te, Cd, In, and Tl display disturbed fractional crystallization trends, most likely due to late magmatic degassing and recondensation of volatile species of these elements. Low-Ti mare basalt suites display constant ratios of specific siderophile volatile elements (e.g. Cu/Ag, Cu/S, S/Se), which we interpret as characteristics of their mantle sources. High-Ti mare basalt suites differ from low-Ti mare basalts by their significantly lower Cu/S, but higher S/Se ratios. Fractional crystallization modeling reveals that these differences are inherited mainly from their source regions in the lunar mantle. Despite the systematically different element ratios, low- and high-Ti mare basalt source compositions are characterized by consistently low mass fractions of siderophile volatile elements. Our new data support the hypothesis of volatile loss prior to formation of the lunar mantle sources and reveal element ratios in the lunar mantle that are significantly different from the terrestrial mantle.