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Mars: Identification, Mapping and Modal Mineralogy of Low-Calcium Pyroxene-rich Deposits

PYROXENE-RICH DEPOSITS. F. Poulet, Y. Langevin, M. Le Roux, N. Mangold, J.-P. Bibring, and B. Gondet, IAS, CNRS/Université Paris-Sud, Orsay, 91405, France (francois.poulet@ias.u-psud.fr), LPGN-CNRS, Univ. Nantes, Nantes, 44322, France. Introduction: The surfaces of the low albedo regions of Mars are dominated by basalt in the southern highlands, and weathered basalt in the northern lowlands [1,2]. They have been further characterized into four primary subcomponents by TES [3], while global GRS data suggest that Mars is dominated by basaltic crust [4]. OMEGA observations has identified different mafic, rock-forming minerals, thus revealing a diverse mineralogy from place to place [5,6,2]. Overall, the modal mineralogy of the most low albedo regions has been constrained: plagioclase (40-60% in Vol.) and high calcium pyroxene (20-40%, HCP) are the dominant minerals, low calcium pyroxene (10-15%, LCP) and minor amounts of olivine are also present [6]. Of special interest is the detection of strong signatures of LCP found in very localized areas [5,6]. These LCP outcrops in the ancient cratered terrains have been interpreted to be possible remnants of the early crustal formation. We present a systematic analysis of such outcrops from the OMEGA dataset. Identification: The analysis focuses on NIR reflectance measurements of OMEGA (0.95 to 2.6 μm). Pyroxenes are recognized by the presence of two distinct absorptions centered near 1 and 2 μm, where the band centers shift toward longer wavelengths with increasing calcium content. LCPs have shortwavelength band centers (0.9 and 1.8 μm), whereas HCPs typically have long-wavelength band centers (1.05 and 2.3 μm). A comparison of the spectral characteristics of LCP (red spectra) and HCP (yellow spectrum) is shown on Figure 1A. A spectral criterion was developed for detecting an unique LCP signature in the OMEGA data, namely the shape of the bump at 1.35 μm by calculating the following spectral parameter: R(1.35)/(R(1.07)*R(1.64))^0.5. The threshold depends on the OMEGA cube but typically values larger than 1.05 provide a confident detection. Mapping: Map of the LCP-rich outcrops is given in Fig. 2A. Because the sizes of the spots are small (a few km2 at most), they are hardly visible at a planetary scale. Most of the deposits occur in low albedo, southern, and Noachian terrains, but the distribution is not uniform. Four regions exhibit a larger number of occurences: a region located north of Argyre along the Ladon-Uzboi-Holden corridor and surrounding plains, the walls of Valles Marineris, a portion of the Iapygia quadrangle, and, a Noachian region northeast of the volcanic shield Syrtis Major. They are generally observed in central peaks, crater rims, and isolated massifs. However, detailled mapping with high spatial imagery of each occurrence is still needed to better understand the geological context of the deposits. There is no apparent spatial correlation with olivine as mapped by OMEGA [2].