Direct evidence of volcanic outgassing of Na and K on the Moon from Apollo orange beads

Abstract

We report the first discovery of a pervasive Na-K-sulfate formed from vapor condensed Na-K-S on the surface of Apollo 74220 orange beads. The molar (Na + K)/S values (A) of different grains range between 1.3 and 2.3, indicating that the original condensates comprise Na-K-sulfides with possible excess native S (for A < 2) or excess Na and K (for A > 2). Together with the previous report of the Zn-Na-S-Cl condensates that contain mostly metallic Zn, our observations show that Na(g) and K(g) condense as sulfides before Zn(g) condenses in its native state. Both thermochemical calculations of lunar volcanic gas and direct observations are inconsistent with the common assumption that moderately volatile elements vaporize and condense as chlorides. The textural relationship of Na-K-S condensates underlying Zn-Na-S-Cl condensates suggests Na-K-S was deposited before Zn-Na-S-Cl, which depleted sulfur from volcanic gas and inhibited the formation of ZnS. These forms of condensates require the volcanic gas that carried 74220 orange beads to be highly reducing (made of mostly H2) with a sufficient partial pressure of S2(g) or H2S(g) to enable interactions between Na and K with S.