Experimental Degassing of Cl, F, OH, and S Bearing Lunar Magmas

The recent determination of volatile abundances in apatite from lunar samples [1-3] has opened the door for a new paradigm of lunar evolution, since apatite has the potential to retain information on the water, F, and Cl abundances of crystallizing lunar magmas. For all of the samples investigated, apatite formed late and will therefore, reflect volatile abundances in late-stage liquids (provided that they were not perturbed during post-magmatic processes). However, the volatile contents of these late-stage liquids may not reflect simple crystallization-induced volatile enrichment of the parental magma, but instead may have been modified by degassing. Since little is known about degassing in mixed volatile systems for lunar compositions, a set of experiments was launched to evaluate the potential effect of this process on volatile abundances for a synthetic composition of Apollo 14053. A series of hydrous glasses were synthetized at 5 kbar and 1280oC in Gr-lined Pt capsule by adding water to the mixture of oxides and silicates which replicate the composition of Apollo 14053 mare basalt with variable amounts of Cl, F, and S. Low P (<1bar) evacuated silica tube experiments were conducted in Fe capsules to simulate first boiling (decompression-induced boiling) by using these hydrous glasses as starting materials, heating each above the liquidus for 3 hours and quenching by dropping the end of the tube holding the sample into cold water. The samples showed strong vesicularity after each run. All samples were analyzed by electron microprobe, and compared with the initial starting material. Mass balance calculations conducted constrain the effect of degassing on volatile abundances.We will report on the compositional changes noted and the implications of the results on the the use of lunar apatite to evaluate primary magma volatile contents. References: [1] F. M. McCubbin et al. (2010) PNAS 107, 11223. [2] J. W. Boyce et al. (2010) Nature 466, 466. [3] J. P. Greenwood et al. (2010) Proc. Lunar Sci. Conf. 41th, 2439.