Samples for estimating primary volatiles in Martian magmas and ancient atmospheric pressures on Mars

Inclusions of glass are likely to be present in phenocrysts in volcanic rocks from Mars, because these occur in volcanic rocks from both Earth and Moon. The usefulness of the inclusions depends upon their size and composition. The compositions of tiny inclusions may be modified by diffusion during growth of the enclosing crystal, the modifications increasing with melt viscosity (silica). Slow cooling results in crystallization and possible redistribution of volatiles, the effects increasing with decreasing silica. Primary volatile concentrations are best sought in inclusions larger than about 50 micrometer diameter in olivine or chromite crystals from quickly cooled basaltic scoria. Such crystals may be present in sands, but it would be preferable to extract them from individual rocks which could be dated and compositionally characterized. This would allow eventual understanding of the role of time and place in outgassing and volcanism on Mars. Analyses of volatiles in inclusions of more siliceous glass in non-basaltic rocks will reveal whether deep outgassing occurs and whether surface volatiles are recycled. Most volcanic crystals contain inclusions, but large inclusions can be uncommon. In the case of terrestrial basalts sample masses of several hundred grams are generally sufficient.