Magmatic volatiles in primitive lunar glasses: I. FTIR and EPMA analyses of Apollo 15 green and yellow glasses and revision of the volatile-assisted fire-fountain theory

Abstract

Apollo 15 green and yellow glasses have been examined by FTIR and EPMA for dissolved C, S, and Cl. Neither the green glass, the volcanically produced yellow glass, nor the impact produced yellow glasses show any signs of FTIR active dissolved C species down to FTIR detectability (κ50–100 ppm). Glass hydroxyl contents are also below FTIR detectability (κ10–50 ppm). Sulfur and Cl contents are close to or below electron probe detection (S = 100–350 ppm, Cl = detection to κ 100 ppm). Thermochemical modeling demonstrates that C solubility is above FTIR detection limits only under relatively oxidizing conditions. If the lunar mantle is at an oxygen fugacity of less than 0.5 log units below IW, C exsolution from picritic melts is not an effective process for driving lunar pyroclastic eruptions. Almost all the current estimates of lunar mantle f O2 point to a lunar mantle more reducing than 0.5 log units below IW. This is consistent with the lack of dissolved FTIR detectable C observed in this study. Calculations of volatile losses from lunar melt spheres during eruption, due to diffusion followed by evaporation, suggest that significant amounts of C and other volatiles are lost from the melt. This is an alternative explanation for the lack of dissolved volatiles in the Apollo 15 glasses. Diffusive volatile loss is an effective means of generating the fumarolic gasses that accompanied mare volcanism and is implied by the volatile coatings on lunar picritic glass spheres. The oxidation of graphite and the fragmentation of the melt are the essential tenets of the volatile assisted fire-fountain theory. This process is still the most plausible explanation for lunar fire-fountaining; however, the conclusions of our study suggest that carbon exsolution from the melt played only a minor role in driving the eruptions. Graphite oxidation alone is capable of producing the desired pyroclastic effect. The oxidation of graphite can proceed via the reduction of Cr 2 O 3 , TiO 2 , and FeO. Each one of the above components alone has the capacity to produce impressive amounts of CO gas from reduction by C. This is a result of the large reduction capacity of elemental C.