Volatile loss history of the Moon from the copper isotopic compositions of mare basalts

• Copper isotope data reported for Apollo mare basalts and lunar basaltic meteorites. • Large range of Cu isotope compositions in mare basalts from +0.55 to +3.94 ‰. • The BSM estimate of +0.57 ± 0.15 ‰ is heavier than the BSE (+0.5 ± 0.10 ‰). • The Moon experienced volatile loss during the Moon-forming impact or lunar magma ocean at- or near-equilibrium conditions. The abundances and isotopic signatures of volatile elements provide critical information for understanding the delivery of water and other essential life-giving compounds to planets. It has been demonstrated that the Moon is depleted in moderately volatile elements (MVE), such as Zn, Cl, S, K and Rb, relative to the Earth. The isotopic compositions of these MVE in lunar rocks suggest loss of volatile elements during the formation of the Moon, as well as their modification during later differentiation and impact processes. Due to its moderately volatile and strongly chalcophile behaviour, copper (Cu) provides a distinct record of planetary accretion and differentiation processes relative to Cl, Rb, Zn or K. Here we present Cu isotopic compositions of Apollo 11, 12, 14 and 15 mare basalts and lunar basaltic meteorites, which range from δ 65 Cu of +0.55±0.01 ‰ to +3.94±0.04 ‰ (per mil deviation of the 65 Cu/ 63 Cu from the NIST SRM 976 standard), independent of mare basalt Ti content. The δ 65 Cu values of the basalts are negatively correlated with their Cu contents, which is interpreted as evidence for volatile loss upon mare basalt emplacement, plausibly related to the presence Cl- and S-bearing ligands in the vapour phase. This relationship can be used to determine the Cu isotopic composition of the lunar mantle to a δ 65 Cu of +0.57 ± 0.15 ‰. The bulk silicate Moon (BSM) is 0.5‰ heavier than the bulk silicate Earth (+0.07 ± 0.10 ‰) or chondritic materials (from -1.45 ± 0.08 ‰ to 0.07 ± 0.06 ‰). Owing to the ineffectiveness of sulfide segregation and lunar core formation in inducing Cu isotopic fractionation, the isotopic difference between the Moon and the Earth is attributed to volatile loss during the Moon-forming event, which must have occurred at- or near-equilibrium.