Activated release of alkalis during the vesiculation of molten basalts under high vacuum: implications for lunar volcanism

Abstract

Knudsen cell-quadrupole mass spectrometry was used to study the high-temperature vaporization of Hawaiian basalts, plagioclase, tektites, and samples from the Allende meteorite. Procedures are described by which mass loss rates and vapor pressures of Na and K were measured quantitatively. Gas-rich glassy basalts were observed to vesiculate under vacuum over the 900–1000°C region and simultaneously evaporate alkalis in nonequilibrium fashion at rates (units of μg/g/hr) of approximately 200–300 Na and 75–250 K. Degassed residues of the same basalts demonstrated equilibrium evaporation rates (over the same temperature range) of 60–120 Na and 30–60 K. The gas-deficient plagioclase and tektite sample showed only equilibrium vaporization with rates of 60 Na, 10 K (plagioclase) and 10 Na, 5K (tektites) at 900–1000°C. The Allende meteorite vaporized at rates of 2400 Na and 200 K at 900–1000°C, possibly by the reaction of Na 2O and K 2O with C or S 2, or by the thermal decomposition of nepheline or sodalite. The nonequilibrium vaporization of alkalis from the gas-rich basalts is attributed to vigorous agitation of the melt during its vesiculation by a gas phase composed principally of SO 2, CO 2, H 2O, CO, and H 2S. The major gases released from the Allende meteorite at 900–1000°C are, in order of decreasing abundance, CO, S 2, CO 2, H 2O, SO 2, and H 2S. It is proposed that nonequilibrium vaporization of alkalis during the vesiculation of lunar lavas was responsible for the production of alkali-rich vapors which subsequently deposited plagioclase crystals in the vugs of lunar rocks. The vesiculative, nonequilibrium vaporization of Na and K during a lunar volcanic eruption should be expected to occur at a high rate upon initial extrusion of the lava into vacuum but then decrease by a factor of approximately three when degassing is nearing completion. Vaporization losses remain inadequate to explain the uniformly low alkali concentrations in lunar basalts.