Is the Chemistry of the Bedrocks at Meridiani Planum Indicative of a Volcanic

Introduction: The Mars Exploration Rover Opportunity has obtained chemical and mineralogical data for exposed bedrocks at Meridiani Planum on Mars. In using these data to formulate a diagenetic model for the bedrocks, a clear distinction must be made between the primary inputs into the rocks and products of secondary processes. The chemical composition of the Meridiani bedrocks provides the most definitive constraint on the nature and origin of primary inputs. In contrast, the current mineralogical composition of the rocks provides uncertain constraints on primary inputs because it is inherently ambiguous whether the minerals that are currently observed reflect primary inputs or secondary products created during diagenesis. Thus, although it has been observed that the rocks mostly lack olivine and other igneous minerals, and may contain phyllosilicates and silica [1,2], neither of these observations provide constraints on the compostion of primary inputs since they may be products of secondary processes. The MER science team has interpreted the Meridiani bedrocks as siliclastic sediments whose composition have been modified by two or more stages of evaporation of sulfate-bearing groundwater [1,2]. As described by McLennan et al. [2], there are a series of chemical inputs into the rocks: (1) deposition of a siliclastic compnent as “basaltic mud,” (2) cementation of this siliclastic component by sulfate salts from groundwater evaporation, followed by erosion, transport, and re-deposition by eolian and fluvial processes, and (3) cementation of the siliciclastic/salt grains by additional sulfate salts derived from one or more additional generations of groundwater evaporation. Based on our independent assessment of their chemical composition, mineralogy, and geologic setting, we recently proposed that the Meridaini bedrocks were instead formed by volcanic processes [3]. In this scenario, the rocks were initially deposited as a series of volcanic ash flows of basaltic composition. Following deposition, the ash deposits were permeated by SO2and steam-rich volcanic vapors that altered the ash at elevated temperatures. In this process, SO2 and H2O in the vapors combined to form sulfuric acid, which then reacted with the rocks, similar to the acidsulfate alteration observed in volcanic environments on Earth. During alteration, the original igneous minerals are replaced by alteration products including phyllosilicates (e.g., nontronite, saponite), amorphous silica, hematite and sulfate salts. Morphological features such as cross and festoon bedding, interpreted by the MER team to result from eolian and fluvial processes [1,4], are also observed in base surge deposits in volcanic settings [3,5] and appear to be consistent with a volcanic scenario. The scale of the deposits appears to be consistent with large volcanic deposits elsewhere on Mars [3]. Chemical constraints on bedrock origin: A fundamental observation made by Oppurtunity is that the elmental compositions of all Meridiani bedrocks reported to date closely resemble typical martian basalt except for high abundances of S and O (Fig. 1) [1,2,6,7]. While some variation has been observed in the chemistry of the rocks in Endurance Crater, the range of compositions is small (Fig. 1) and predominantly reflects variations in Mg and S, with abundance of these components decreasing with depth in the section. Any model for the origin of the rocks, therefore, must reproduce a chemical composition that is enriched in S and O but otherwise identical to pristine martian basalt.