Aqueous corrosion of olivine in the Mars meteorite Miller Range (MIL) 03346 during Antarctic weathering: Implications for water on Mars

摘要

Several nakhlites (clinopyroxenite meteorites from Mars) contain olivine phenocrysts with corrosion features identical in size, shape and distribution to the smaller etch-pits of well-characterized weathered terrestrial olivine. Miller Range (MIL) 03346 is an Antarctic nakhlite find, recovered after long exposure to Antarctic conditions. The distribution of discrete olivine etch-pits almost exclusively within a few hundred microns of allocation MIL 03346,171's documentably exposed surface suggests that they formed by terrestrial weathering in Antarctica. The small size of olivine etch-pits in MIL 03346,171 relative to commonly much larger etch-pits in even incipiently weathered terrestrial examples suggests that the duration of its exposure to weathering conditions was short, or the weathering conditions to which it was exposed did not favor olivine corrosion (in the form of etch-pit formation), or both. Time-scales for the formation of etch-pits, estimated from experimentally determined dissolution rates of olivine over a range of pHs, are comparable to the measured terrestrial age of the meteorite and short relative to the time available for possible similar corrosion on Mars. Etch-pits of the observed size on MIL 03346 olivine phenocrysts would be relatively easy to form supraglacially under brief episodic acidic Antarctic conditions, but the terrestrial age of MIL 03346 is long enough that its olivine might have been weathered to the observed state by englacial films of alkaline Antarctic water. The paucity of similar etch-pits in olivine from the interior of MIL 03346 suggests that olivine in this Mars meteorite was exposed to even less aqueous alteration after iddingsitization during its 1.3 billion years on Mars than its exterior was subjected to during its Pleistocene-Holocene exposure to Antarctic weathering conditions. (C) 2016 Elsevier Ltd. All rights reserved.