Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmmr33c..01a&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #MR33C-01
Physics
3621 Mantle Processes (1038), 3630 Experimental Mineralogy And Petrology, 3919 Equations Of State, 3924 High-Pressure Behavior
Scientific paper
High pressure experiments using the sink/float method have bracketed the density of carbonated partial melt of peridotite and carbonated Apollo 14 black glass melt at high pressures and temperatures. The experiments were designed to determine the compressibility of CO2 in silicate melts and allow prediction of crystal-liquid density crossovers in CO2-bearing planetary magmas. The silicate melt compositions were synthetic mixtures of reagent oxides with CO2 added in the form of CaCO3. Samples were contained in compression-sealed molybdenum capsules. Sink/float marker spheres implemented were gem quality synthetic forsterite (Fo100) and San Carlos olivine (Fo91). Experimental run times were 30 seconds, thus minimizing sphere-liquid reactions and liquid reaction with capsule and pressure media. CO2 (total) in the quench melt run products was estimated by electron microprobe analyses of carbon and oxygen. All experiments were carried out in a Walker multi-anvil apparatus or a Quick Press piston-cylinder device at the Institute of Meteoritics, University of New Mexico. The densities of peridotite partial melt with 5 wt % CO2 and the same peridotite partial melt with no CO2 were determined at 4.3 GPa and ~1825 C. Using the density difference between the carbonated and non-carbonated melts we calculate a partial molar volume of CO2 in peridotite partial melt of approximately 18 cm3mol-1 at these conditions. This represents a 35% decrease in VCO2 compared to estimates for VCO2 at 1-bar (Liu and Lange, 2003), indicating a high compressibility for CO2 in silicate melt over the range 0-4 GPa. Our value of VCO2 is similar to that of estimated for basalt at 19.5 GPa (Ghosh et al., 2007), suggesting that the compressibility of CO2 in silicate melt decreases significantly with pressure, although more experiments are needed to confirm this possibility. Our experiments on Apollo 14 black glass are the first of their kind to simultaneously determine melt density and CO2 solubility in silicate melt at high pressure. This study is also the first to examine the effect of high titanium concentrations on the properties of carbonated silicate melts. We bracketed the density of this silicate melt + 5 wt % CO2 at 1 GPa and 1315 C. and observed the presence of an exsolved fluid phase in the quenched run products, in accord with CO2 super-saturation. At higher pressure and temperature (4.7 GPa and 1600 C, equivalent to the pressure at the center of the Moon) no fluid exsolution was observed in the run product indicating that 5 wt % CO2 was dissolved in the black glass melt at these conditions. Apollo 14 black glass represents the densest known magma in the solar system. Presence of a low density volatile propellant in its mantle source region is required to account for it as a presumptive lunar fire fountain eruptive. Our initial results are consistent with a dissolved CO2 propellant at high pressure in black glass magma that buoyantly drives it to the lunar surface and degasses efficiently by decompression.
Agee Carl B.
Dreeland L. E.
Duncan Megan S.
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