Corrosion Textures Formed by Aqueous Alteration of Olivine in Mars Meteorites and Terrestrial Analogs

Olivine phenocrysts in Hawai'ian volcanic rocks from several volcanic centers and regolith/outcrop settings, and tectonized olivines from several metadunite bodies in the southern Appalachian Blue Ridge, are all similarly corroded by funnel-shaped etch pits occurring as individual pits or en echelon arrays. Etch pits occur in samples with chemical and/or mineralogical evidence of weathering, and/or are associated with, or proximal or directly connected to, fractures or exposed outcrop surface. Etch pits form by weathering, and are not inherited from pre-weathering aqueous alteration (e.g., serpentinization, iddingsitization) of these parent rocks. Many etch pits are devoid of weathering products, implying that olivine weathering can take place by a dissolution-reprecipitation mechanism as well as by long-understood replacement mechanisms. Similarity of corrosion forms from naturally weathered olivine from multiple igneous and metamorphic parent-rock bodies suggests that olivine weathers in the same manner regardless of its specific crystallization/recrystallization history. Olivine-corrosion textures are similar in all sample suites, despite differences in crystallization, deformation, eruption/weathering/exposure ages, and local regolith history. Corrosion features identical in size, shape and distribution to the smallest pits of well-characterized terrestrial examples occur in olivines of several Mars meteorite groups, but all examples observed to date occur in Mars meteorite finds with known terrestrial-weathering histories. Similar aqueous corrosion features have not yet been identified in Mars meteorite falls that avoided exposure to terrestrial weathering. This suggests that the Mars meteorites available for study of aqueous alteration phenomena on Mars have been exposed to even less aqueous alteration on Mars than olivine-bearing rocks with minimal terrestrial weathering histories.