A mineralogical study of glacial flour from Three Sisters, Oregon: An analog for a cold and icy early Mars

Geomorphic and mineralogical data from the martian surface indicate liquid water was abundant on the martian surface and near subsurface ∼3.5 to 4 Gyr ago, but whether early Mars had a warm and wet climate or whether it was cold and icy with punctuated periods of warmth is still unknown. Mineral assemblages of sedimentary rocks on Mars help determine past aqueous conditions and sediment sources. Here, we report on the primary and secondary mineral and amorphous assemblage of glacial flour from Collier Glacier valley on the northern flank of North Sister in Oregon, U.S.A. to identify mineralogical characteristics of mafic sediments altered under cold, wet conditions. Collier glacial flour is dominated by primary igneous minerals (plagioclase is dominant, with lesser amounts of pyroxene and olivine) and comprises 10-40 wt.% X-ray amorphous materials. Crystalline secondary phases (e.g., phyllosilicates, zeolite) are not significant contributors to the authigenic alteration assemblage. High-resolution transmission electron microscopic observations of the <2 μm size fraction of the flour demonstrate that the X-ray amorphous materials are both primary (i.e., volcanic glass) and secondary in nature. The secondary X-ray amorphous materials are enriched in Si, Al, and Fe, and we observe incipient phyllosilicate formation associated with primary and secondary amorphous materials. Our results indicate chemical weathering on a cold and icy early Mars would have produced X-ray amorphous materials, but not crystalline secondary phases. We suggest that the abundant X-ray amorphous materials recognized from orbit and in situ on Mars could have formed under cold and periodically wet conditions similar to those on North Sister today. Furthermore, the lack of volumetrically significant phyllosilicate formation in Collier Glacier flour indicates phyllosilicates on Mars did not form in a cold and wet climate.