The Effect of Co2 on Density of Molten Apollo 14 Black Glass at High Pressure

Introduction: Knowledge of the density, compressibility and other physical properties of magmas at high pressure are required in order to understand the differentiation of the lunar interior. We present here the first experiments to determine the effect of dissolved volatiles on the physical properties of ultramafic mantle melts as represented by the lunar picritic glasses. Lunar picritic glasses are thought to be pristine igneous samples derived directly from the deep lunar interior. As probes of the lunar interior, picrite glasses are unique in that they are quenched near-primary liquids that originated at greater depths in the Moon compared to dominantly crystalline mare basalts. They are found as beads in lunar soils and are likely formed during rapid cooling and quenching of fire fountain eruptions on the Moon, probably during the period of peak lunar magmatism, and their compositions span the range of nearly all lunar basalts [1]. The glass beads have distinctive colors that correspond to TiO2 content. For example, Apollo 14 “black glass” has the highest TiO2 content with 16.4 wt%, Apollo 17 “orange glass” is intermediate with 8.6 wt% TiO2, and Apollo 15 “green glass” is lowest with 0.26 wt% [2]. These glasses all have high FeO contents and their melt densities are among highest found on terrestrial planets. The densest melt of all the samples, and to our knowledge the densest known magma in the solar system, is Apollo 14 black glass with 24.5 wt% FeO and calculated 1-bar liquidus density [3] of ~3.13 gcm. This fact led Delano [4] to predict that high-TiO2 black glass melt would be neutrally buoyant relative to coexisting liquidus olivines and pyroxenes at a depth of approximately 500 km in the lunar mantle. He pointed out that lunar magmas with higher TiO2 than black glass may be absent from the lunar surface because they were too dense to rise from their mantle source regions. Circone and Agee [5] carried out high pressure sink/float density measurements on molten black glass and confirmed Delano’s original idea. They found that molten black glass is the most compressible mantle silicate melt yet studied and that it would be negatively buoyant relative to an olivine-pyroxene source rock at depths >400 km. Thus fire fountain eruptions of black glass magma are an enigma, since this dense melt should sink deeper into lunar mantle from its source at >400 km, and not rise to the surface. On the other hand, buoyant rise could be explained by the presence of a density-lowering propellant such as volatile compounds in the black glass source region. Volatiles that include carbon and sulfur compounds, H2O, Cl, and F occur in extremely low concentration levels in lunar glasses however it is conceivable that these have been efficiently degassed and lost to space during eruption. Indeed, studies by Dixon [6] and others show a strong positive pressure dependence on the solubility of H2O and CO2 in terrestrial magmas. These studies indicate that CO2 is significantly more volatile than H2O, hence recent detection of trace amounts of water in lunar glasses [7] encourages the notion that CO2 or CO could also have been present in the source region, but was completely lost during decompression degassing. Fogel and Rutherford [8] proposed that oxidation of a modest amount of graphite in the lunar basalt source region through a reaction such as FeO + C → Fe + CO could produce the required fire fountain propellant. Our understanding of the solubility limits of CO, CO2, or CO3 in lunar melts is at best fragmentary. We are unaware of any data on the solubility of carbon dioxide in lunar basalt compositions at high pressure, and the terrestrial database contains nothing comparable to high-Ti compositions such as black glass. Therefore we are carrying out a systematic experimental study of the effect of pressure on the solubility of carbon in a synthetic Apollo 14 black glass melt. The experiments also allow the determination of the effect of carbon solubility on the density of the molten black glass material, and work is underway up to pressures that include the entire lunar interior (~4.7 GPa).