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 acqu ired in 1989 by the Imaging Spectrometer for Mars (ISM) instrument tha t was onboard the Phobos-LI spacecraft. The ISM instrument acquired so me of the highest spectral (64 channels between 0.77 and 3.14 mu m) an d spatial resolution (approximate to 22 km/pixel) observations to date of Mars, focused on the equatorial region. Relatively unaltered surfa ces were identified in these data on the basis of their spectral prope rties (low albedo, weak spectral slope, absence of ferric absorptions) , and were found in volcanic terrains of Syrtis Major, Valles Marineri s, Ophir Planum, and Sinus Meridiani/Oxia Palus. Spectra representativ e of the diversity among terrains were extracted for detailed analysis using the Modified Gaussian Model to determine the primary mafic mine ralogy. It was determined that the mafic mineralogy of the regions inv estigated is dominated by low-calcium and high-calcium pyroxene (LCP, HCP), and that there was homogeneity within a given region but some he terogeneity between regions in the relative abundance of LCP to HCP. T he two-pyroxene mineralogy is consistent with the mineralogies of the basaltic SNC meteorites believed to have originated on Mars. Two-pyrox ene volcanic mineralogies are indicative of large degrees of partial m elting (e.g., komatiites) or a mantle source depleted in aluminum. Int egration of these findings with previous studies of Mars and the SNC m eteorites indicates that (1) the SNC mineralogies are representative o f 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 li ttle evolution in the composition of mantle source regions as the 180 Ma SNC mineralogies are comparable to those of the oldest terrains ana lyzed.