Glacial alteration as a source of amorphous silica on amazonian mars

Introduction: New results show that cold-climate silica mobility is more efficient than previously reported, and is the dominant weathering process in glaciated mafic bedrock. Based on field work at glaciated volcanic sites, we hypothesize that this is due to both high rates of silica dissolution from mafic bedrock and reprecipitation of silica in the form of opaline silica coatings and other poorly crystalline silicate alteration phases in proglacial sediments. Widespread evidence for past and present-day glaciation on Mars [1-4] means that this cycle must be investigated in order to better interpret alteration signatures on the surface of Mars, especially those in the north polar deposits [5-7]. Amorphous silica in this settings could be interpreted as mineralogical evidence for alteration by meltwater sourced from ice or snow. Field study: To investigate meltwater-driven silica cycling on mafic volcanic bedrock, water and rock samples were collected during June 2015 and July 2016 from glaciated volcanic bedrock in the Cascade Volcanic Arc: Mount Adams (46o9’N, 121o27’W), Mount Hood (45o21’N, 121o42’W), Middle Sister (44o9’N, 121o46’W) and North Sister (44o10’N, 121o47’W). Dominant bedrock compositions for each site are detailed in Table 1. Evidence for subglacial precipitation of poorly crystalline silica was found at North and Middle Sisters in the form of striated rock coatings on recently deglaciated lava flows (Fig 1) [8] and in a silica-enriched poorly crystalline component of glacial flour from proglacial till deposits [9]. Results: Figure 1 shows a representative thermal infrared spectrum of a rock coating from the North Sister proglacial plain. It exhibits similar absorptions to opaline silica and an Al-Si gel, indicating that it is composed primarily of poorly crystalline silica. Figure 2 shows the mean glacial outwash stream silica concentrations. The mafic bedrock sites have greater concentrations of silica in glacial outwash waters compared to the felsic sites. Within each mafic field site, the highest silica concentrations were measured in moraine-sourced springs. Silica cycling in terrestrial glacial systems The high rate of bedrock comminution in subglacial environments results in high rates of both chemical and physical weathering, due to the increased reactive mineral surface area formed through glacial grinding. In most bedrock types, carbonate weathering is enhanced and silica fluxes are depressed in glacial outwash compared with global average riverine catchment runoff due to low temperatures and short residence times [10]. However, in mafic systems, higher dissolved SiO2 concentrations have been observed [11-13]. Additionally, remote sensing has identified high-silica zones in proglacial outwash plains on mafic bedrock [14]. Water composition. The major difference between glacial alteration of volcanic bedrock and more typical continental terrains is the absence of significant dissolved carbonate in the former. In the absence of carbonate minerals which normally dominate dissolution processes at glacier beds [10], carbonation of feldspar can become the dominant weathering process [11]: