Glacier-driven Chemical Alteration of Volcanic Rock: Implications for an Icy Low-Temperature Geochemical Cycle on the Surface of Mars

Abstract:

Cold-climate silica cycling is the predominant weathering process in glaciated mafic volcanic rock. This geochemical cycle results in precipitation of subglacial opaline silica coatings and other poorly crystalline silicate alteration phases. Widespread evidence for glaciation and glaciofluvial deposits on the highly mafic surface of Mars means that this low-temperature geochemical cycle must be investigated in order to better understand potential alteration processes on terrestrial planets.

Two glaciers of the Three Sisters volcanoes in Cascadia are ideal analogs to small glaciers and periglacial terrains on Mars: these glaciers are wet-based and rest predominantly on mafic bedrock. In order to understand glacier-related chemical alteration processes, meltwater from these glaciers was analyzed and remote sensing and lab mineralogical analyses were used to determine the composition of bedrock and alteration products.

Hydrochemistry of glacial mafic weathering systems is unique compared to glaciers located in other types of bedrock. Here we report major ion concentrations in meltwaters for two summer melt seasons. Total cation concentrations range from 3 to 250 μeq/l and dissolved bicarbonate concentrations range from 2 to 200 μeq/l. Other dissolved anions are negligible compared to bicarbonate. Dissolved silica concentrations range from 2 to 260 μmol/l, comparable to total dissolved cation concentrations. The highest cation and silica concentrations were measured in moraine-sourced springs.

Geochemical modeling indicates that the predominant form of chemical weathering in these systems is carbonation of feldspar and reactive volcanic glass, resulting in poorly crystalline silicates in glacial coatings and sediments. This cycle has wide-ranging implications for atmospheric CO₂ drawdown due to cold-climate volcanic rock weathering and is highly relevant for geochemical cycles throughout the history of Mars. Interestingly, opaline silica signatures are associated with some recent glacial deposits on Mars.This cycle is most likely driven by relatively high water-rock ratios, and/or long residence times. Silica dissolution and the subsequent formation of poorly crystalline silicates are the predominant signatures of cold-climate weathering in mafic terrains.