Abstract
Low estimated lunar volatile contents, compared with Earth, are a fundamental observation for Earth–Moon system formation and lunar evolution. Here we present zinc isotope and abundance data for lunar crustal rocks to constrain the abundance of volatiles during the final stages of lunar differentiation. We find that ferroan anorthosites are isotopically heterogeneous, with some samples exhibiting high δ66Zn, along with alkali and magnesian suite samples. Since the plutonic samples were formed in the lunar crust, they were not subjected to degassing into vacuum. Instead, their compositions are consistent with enrichment of the silicate portions of the Moon in the heavier Zn isotopes. Because of the difference in δ66Zn between bulk silicate Earth and lunar basalts and crustal rocks, the volatile loss likely occurred in two stages: during the proto-lunar disk stage, where a fraction of lunar volatiles accreted onto Earth, and from degassing of a differentiating lunar magma ocean, implying the possibility of isolated, volatile-rich regions in the Moon's interior.
Highlights
Low estimated lunar volatile contents, compared with Earth, are a fundamental observation for Earth–Moon system formation and lunar evolution
More recent studies of the volatile content in the Moon have shown that isotopes of moderately volatile elements, such as Cl and Zn, are highly depleted, compared with bulk silicate Earth[4,5,6]
Recent work on lunar samples has focused largely on using Zn isotopes to study mare basalts, regolith and pyroclastic glass beads[6,18,20]. These studies have shown that mare basalts are consistently enriched in the heavier Zn isotopes compared with terrestrial basalts, which has been interpreted as a consequence of a whole-scale evaporation event on the Moon[6], or during magma ocean differentiation[4]
Summary
Low estimated lunar volatile contents, compared with Earth, are a fundamental observation for Earth–Moon system formation and lunar evolution. Studies of OH contents and D/H in apatites from mare basalts, as well as Apollo pyroclastic glass beads, have argued for elevated volatile contents in portions of the lunar interior[10,11,12,13] Reconciling these seemingly contrasting observations remains critical to examining models of lunar formation Recent work on lunar samples has focused largely on using Zn isotopes to study mare basalts, regolith and pyroclastic glass beads[6,18,20] These studies have shown that mare basalts are consistently enriched in the heavier Zn isotopes compared with terrestrial basalts, which has been interpreted as a consequence of a whole-scale evaporation event on the Moon[6], or during magma ocean differentiation[4]. Additional data are reported for three lunar regolith samples and several mare basalt samples to confirm previously observed heavy Zn isotope enrichment in these lithologies[6,18,20]
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