Hydrothermal Occurrences in Gusev Crater

Exploration of the Gusev crater landing site by the Spirit rover has revealed for the first time, in situ evidence of hydrothermal activity on Mars. Most compelling are eroded outcrops of opaline silica found adjacent to "Home Plate" [1], an eroded stack of volcaniclastic deposits stratigraphically overlain by a vesicular basalt unit [2]. Recent work [3] demonstrates that the silica outcrops occur in a stratiform unit that possibly surrounds Home Plate. The outcrops are dominated by opal-A with no evidence for diagenesis to other silica phases. No other hydrous or alteration phases have been identified within the outcrops; most notable is a lack of sulfur phases. The outcrops have porous and in some cases, brecciated microtextures. Taken together, these observations support the interpretation that the opaline silica outcrops were produced in a hot spring or perhaps geyser environment. In this context, they are silica sinter deposits precipitated from silica-rich hydrothermal fluids, possibly related to the volcanism that produced the Home Plate volcanic rocks. On Earth, debris aprons in which sinter is brecciated, reworked, and cemented, are common features of hot springs and geysers and are good analogs for the Martian deposits. An alternative hypothesis is that the silica resulted from acid-sulfate leaching of precursor rocks by fumarolic steam condensates. But stratigraphic, textural, and chemical observations tend to diminish this possibility [3]. We are conducting extensive laboratory and field investigations of silica from both hot spring/geyser and fumarole environments to understand the full range of mineralogical, chemical, textural, and morphological variations that accompany its production, in order to shed more light on the Home Plate occurrence. The recent discovery of abundant Mg-Fe carbonate (16-34 wt%) in outcrops named Comanche provides possible evidence for additional hydrothermal activity in Gusev [4]. However, the carbonate is hosted by olivine-rich (~40 wt%) volcaniclastic rocks that show no other phases indicative of significant alteration, such as phyllosilicates. Even the presence of so much olivine in the outcrops attests to minimal alteration. This suggests that the carbonate was not derived from hydrothermal alteration of the local rock. Instead, carbonate-bearing solutions sourced from elsewhere in the region may have precipitated carbonate as cement within the olivine-rich host rock [4]. An alternative hypothesis by [5] suggests that Comanche carbonate resulted from direct precipitation of evaporating brine, perhaps related to the putative ancient lake in Gusev crater, which infiltrated the host rock. In either case, the presence of outcrops of abundant carbonate and opaline silica demonstrates a rich and varied aqueous history in Gusev crater. [1] Squyres, S. W., et al. (2008), Science, 320, 1063-1067. [2] Squyres, S. W., et al. (2007), Science, 316, 738-742. [3] Ruff, S. W., et al. (2011), J. Geophys. Res., 116, E00F23, 10.1029/2010JE003767. [4] Morris, R. V., et al. (2010), Science, 329, 5990, 421-424, 10.1126/science.1189667. [5] Ruff, S. W. (2011), Lunar Planet. Sci., XLII, abstract #2708.