Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon

Water plays a critical role in the evolution of planetary bodies, and determination of the amount and sources of lunar water has profound implications for our understanding of the history of the Earth-Moon system. During the Apollo programme, the lunar samples were found to be devoid of indigenous water. The severe depletion of lunar volatiles, including water, has long been seen as strong support for the giant-impact origin of the Moon. Recent studies have found water in lunar volcanic glasses and in lunar apatite, but the sources of lunar water have not been determined. Here we report ion microprobe measurements of water and hydrogen isotopes in the hydrous mineral apatite, found in crystalline lunar mare basalts and highlands rocks collected during the Apollo missions. We find significant water in apatite from both mare and highlands rocks, indicating a role for water during all phases of the Moon's magmatic history. Variations of hydrogen isotope ratios in apatite suggest the lunar mantle, solar wind protons, and comets as possible sources for water in lunar rocks and imply a significant delivery of cometary water to the Earth-Moon system shortly after the Moon-forming impact

The redox state of arc mantle using Zn/Fe systematics

International audienceMany arc lavas are more oxidized than mid-ocean-ridge basalts and subduction introduces oxidized components into the mantle(1-4). As a consequence, the sub-arc mantle wedge is widely believed to be oxidized(3,5). The Fe oxidation state of sub-arc mantle is, however, difficult to determine directly, and debate persists as to whether this oxidation is intrinsic to the mantle source(6,7). Here we show that Zn/Fe-T (where Fe-T = Fe2+ + Fe3+) is redox-sensitive and retains a memory of the valence state of Fe in primary arc basalts and their mantle sources. During melting of mantle peridotite, Fe2+ and Zn behave similarly, but because Fe3+ is more incompatible than Fe2+, melts generated in oxidized environments have low Zn/Fe-T. Primitive arc magmas have identical Zn/Fe-T to mid-ocean-ridge basalts, suggesting that primary mantle melts in arcs and ridges have similar Fe oxidation states. The constancy of Zn/Fe-T during early differentiation involving olivine requires that Fe3+/Fe-T remains low in the magma. Only after progressive fractionation does Fe3+/Fe-T increase and stabilize magnetite as a fractionating phase. These results suggest that subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge. Thus, the higher oxidation states of arc lavas must be in part a consequence of shallow-level differentiation processes, though such processes remain poorly understood

DYNAMIC MELTING OF THE ICELAND PLUME

Icelandic high-magnesia basalts show striking correlations between major element abundances and incompatible element and radiogenic isotope ratios. The most MgO-rich lavas have the most depleted incompatible element ratios and among the least radiogenic lead isotopes recorded in Atlantic mid-ocean-ridge basalts, highlighting a decoupling of the major and trace element characteristics expected of plume melts. This paradox can be explained by the process that mixes melts segregated from different depths of the melting column. The resulting model provides insight into the processes governing melt compositions at spreading ridges.</p