In situ compositions of Martian volcanics: Implications for the mantle

The primary objective of this analysis is to determine the mineralogic composition of relatively pristine terrains exposed on the surface of Mars. This analysis is conducted using imaging spectrometer data acquired in 1989 by the Imaging Spectrometer for Mars (ISM) instrument that was onboard the Phobos-LI spacecraft. The ISM instrument acquired some of the highest spectral (64 channels between 0.77 and 3.14 mu m) and spatial resolution (approximate to 22 km/pixel) observations to date of Mars, focused on the equatorial region. Relatively unaltered surfaces were identified in these data on the basis of their spectral properties (low albedo, weak spectral slope, absence of ferric absorptions), and were found in volcanic terrains of Syrtis Major, Valles Marineris, Ophir Planum, and Sinus Meridiani/Oxia Palus. Spectra representative of the diversity among terrains were extracted for detailed analysis using the Modified Gaussian Model to determine the primary mafic mineralogy. It was determined that the mafic mineralogy of the regions investigated is dominated by low-calcium and high-calcium pyroxene (LCP, HCP), and that there was homogeneity within a given region but some heterogeneity between regions in the relative abundance of LCP to HCP. The two-pyroxene mineralogy is consistent with the mineralogies of the basaltic SNC meteorites believed to have originated on Mars. Two-pyroxene volcanic mineralogies are indicative of large degrees of partial melting (e.g., komatiites) or a mantle source depleted in aluminum. Integration of these findings with previous studies of Mars and the SNC meteorites indicates that (1) the SNC mineralogies are representative of large volcanic regions of Mars with surface ages as old as 2-3 Ga, (2) the martian mantle was depleted in aluminum at least as long ago as the oldest terrain analyzed (Ophir Planum), and (3) there has been little evolution in the composition of mantle source regions as the 180 Ma SNC mineralogies are comparable to those of the oldest terrains analyzed.