MULTIPLE LINES OF EVIDENCE FOR DEGASSING OF WATER FROM OLIVINE-PHYRIC SHERGOTTITE LAR 06319
LAR 06319 is an olivine-phyric shergottite with intermediate trace element abundances. It has been noted to be close to a primary magma in composition [1], appears based on trace elements to have undergone closed-system crystallization, but also shows increasing oxygen fugacity in late-stage melt inclusions, inconsistent with closed-system evolution [2]. If, however, the oxygen fugacity increase was caused by degassing, it could occur while the system remains closed for trace elements.
Therefore, we investigated this sample in detail, looking for evidence of degassing. We report 2 grains of magnetite in this meteorite associated with iron sulfides which suggests formation via loss of sulfur from the magma. We calculated that there was insufficient sulfur in the sample for sulfur degassing to increase the oxygen fugacity by the observed amount, requiring the presence of water in addition to sulfur during degassing. Next, we measured volatile compositions in apatite grains and found that although most were water-rich, select apatites were Cl-rich, implying degassing of water during crystallization. Next, we report an oxygen fugacity estimate based on trace element abundances in pyroxenes which shows that the oxygen fugacity did not begin to increase until after substantial pyroxene crystallization, consistent with late degassing. Finally, we note an olivine grain which preserves a boundary created when pyroxene began to crystallize, with compositions that suggest pyroxene formation began after substantial olivine crystallization, consistent with the presence of >1 wt. % water during crystallization.
We conclude that this sample records evidence of the presence and degassing of magmatic water while on Mars, and suggests that the degassing led to increasing magmatic oxygen fugacity as well.
[1] Basu Sarbadhikari et al., 2010
[2] Peslier et al., 2010