Experimental investigation into lunar melt density and compressibility : the role of titanium
This study focuses on determining the density and compressibility of four lunar picritic glasses as a function of TiO2 content from 0-11 GPa and 1748-2473 K (1475-2200°C). These glasses are hypothesized to have quenched rapidly as glass beads during lunar fire fountain eruptions. The lunar glass beads have distinctive colors that correspond to TiO2 content. The glasses of interest for this study are the Apollo 15 green glass Type C (A15C) which has a TiO2 content of 0.26 wt%, the Apollo 14 yellow glass (A14Y) which has a TiO2 content of 4.58 wt%, the Apollo 17 orange glass 74220-type (A17O) which has a TiO2 content of 9.12 wt%, and the Apollo 14 black glass (A14B) which has the highest TiO2 content with 16.40 wt%. These glasses are believed to represent primary, unfractionated melts making them excellent candidates for experimental studies into lunar basalt density and eruptability during partial melting of the lunar mantle. We performed sink-float experiments on these lunar glass compositions using a piston-cylinder apparatus (P < 2 GPa) and Walker-style multi-anvil device (P > 2 GPa) in order to bracket the density of these melts. We report new sink-float data for A15C, A14Y, and A17O. We find that with increasing pressure, the melts with less TiO2 are more compressible than high TiO2 melts. This causes the melt with the most TiO2 (A14B) to be the least dense at higher pressures, a complete reversal of what is seen at lower pressures. This change in density and compressibility is attributed to the change from [IV]Ti4+ to [VI]Ti4+ in the melt structure for melts with high TiO2 contents. We have identified density crossovers between these melts and their equilibrium olivines and pyroxenes, and show that these glasses, with the exception of A17O, should be able to rise to the surface as a result of buoyancy forces alone. For the eruption of A17O, we must call upon the rising diapir model of Hess (1991) to explain its eruptability.
Lunar Glasses, Titanium, Sink/Float Technique, Density Crossovers
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