Silica Deposits in the Nili Patera Volcanic Caldera, Syrtis Major, Mars

Syrtis Major is a large Hesperian-aged volcanic edifice of basaltic composition. In the central Nili Patera caldera is a small cone with associated flows thought to be dacitic in composition [1], though also spectrally consistent with a silica-rich highly altered basalt [2]. Visible and near-infrared (VNIR) observations of the caldera are consistent with an olivine-rich basalt while the cone and associated flows show weaker absorption features consistent with but not diagnostic of dacite or altered basalt with no olivine and weaker pyroxene absorptions than surrounding terrains. Additionally, several small (0.004-0.1 km2), higher albedo outcrops on the cone and surrounding flows exhibit a strong vibrational absorption centered at 2.2 µm that is diagnostic of OH coordinated with Al or Si. The position (variable, 2.20-2.24 µm), width (0.2 µm), and shape of the band are most consistent with Si-OH whose absorption is modulated by hydrogen bonding with H2O. In outcrop spectra, absorptions at 1.4 µm (due to OH and H2O) and 1.9 µm (H2O) are variable in strength from weak to absent. A strong spectral slope may play a role in subduing the 1.4 µm feature. Similarly, a 1.39 µm absorption characteristic of laboratory measurements of Si-OH has not been uniquely detected, this may also be masked by the strong spectral slope or may point to a different mineral identification, possibly an alterative metal-OH in a phyllosilicate structure. The spectral characteristics of the outcrops are most consistent with partially dehydrated hydrated/hydroxylated silica, i.e. Si-OH bearing materials, with metal OH as a possible alterative. In addition, the morphology of distinct outcrops consistent with fumarole-related deposit of silica or from alteration of basalt. Both formation mechanisms would have required a hydrothermally active environment with acidic waters heated by a magma body and sourced from either the magma or groundwater in the volcanic substrate. The partial dehydration of the silica may result from exposure to the Martian environment [3], though silica deposits elsewhere on Mars remain hydrated [4,5]. The Nili Patera deposits are significant because they show a hydrothermal environment in original geological context, are evidence of Hesperian or younger active hydrothermal environments, and are correlated with one of the very few possible examples of evolved lavas on Mars. [1]Christensen et al. (2005) Nature. 436. 504-509. [2]Wyatt, M. B. and H. Y. McSween (2002) Nature. 417, 263-266 [4]Milliken et al (2008). Geology, v.36 p847-850 [3]Cloutis et al. (2009) Workshop on Modeling Martian Hydrous Environments. Abstract #4002. [5]Elhmann et al. (2009) J. Geophys. Res. In Press