Copper sulfide in a vesicular basalt reveals aqueous liquid on the Moon

Abstract All previous reports of magmatic water on the Moon are structurally-bound OH in minerals or glasses. Here, we report the discovery of ubiquitous Cu1.4−1.8S in pristine Apollo vesicular basalt 15556. This copper sulfide forms multiple, fine-grained coatings draping over troilite grains on vesicle walls or coats fracture surfaces around troilite inside the rock. They also occur as rose-like clusters of bladed crystals of ~ 1 µm size at the edge of the coatings, or massive aggregates of < 0.5 µm small euhedral grains on top of the coatings. The draping nature of the coatings combined with unique geochemical signatures suggests copper sulfide precipitated from aqueous Cu- and S-bearing liquids. This finding demonstrates that extensive crystallization of lunar magma can generate aqueous liquids, some of which may be still stored in intrusive igneous rocks.