MAGNESITE DISSOLUTION IN NEAR SATURATED BRINES: UNEARTHING MARS' AQUEOUS ALTERATION HISTORY

Magnesite has been detected in the Nili Fossae region and the martian meteorite ALH84001; however carbonates are not common on the current Martian surface. Sulfate and chloride evaporates on Mars surface imply brines of sulfate and chloride composition weathered rocks in Mars past and may be active at low temperatures observed on Mars today. This study of magnesite dissolution in brines creates a window for understanding the geological presence of the water cycle, microbes and habitability on Mars. A sample of magnesite was obtained from Bramado, Brazil via Excalibur Minerals and was characterized using electron microprobe, XRD, and BET analysis. Magnesite was crushed using a mortar and pestle, sonicated and washed to clean mineral surfaces prior to reaction, and dried. Batch experiments were conducted in triplicate with 1g MgCO₃ L^-1 solution for 52 day in ultra pure water (a_w=1), 3 molal NaCl (a_w=0.75)_, 4.5 molal CaCl₂ (a_w=0.32) and 1.5 molal Na₂SO₄ (a_w=0.91).

Surface area normalized dissolution rates (log mol m^-2 s^-1) in UPW (-4.16) and NaCl brine (-3.96) are similar. However the 2x faster rate of dissolution in Na₂SO₄ brine (-3.50) suggest that the sulfate ion preferentially complexes Mg. In contrast, the slower dissolution rate in CaCl₂ brine (-4.56) indicates that magnesite weathering decreases as water activity decreases.

Carbonates, while not common, are present on Mars, which means either weathering occurs in concentrated chlorine-rich brines or aqueous alteration in some areas of Mars was limited. However, the detection of magnesium sulfates on Mars by OMEGA and Odyssey, suggest magnesite dissolution in sulfate brines may have occurred and formed these deposits. These data suggest that weathering on Mars may have occurred in relatively high water activity solutions, which improves the case for Mars past habitability.