Dissolution of olivine during natural weathering

摘要

Naturally weathered olivine occurring as phenocrysts in Hawai'ian volcanic rocks from several volcanic centers and reg-olith/outcrop settings, and as tectonized olivines from several metadunite bodies in the southern Appalachian Blue Ridge, areall similarly corroded by natural weathering. Conical (funnel-shaped) etch pits occur as individual pits, base-to-base pairs ofcone-shaped pits, or en echelon arrays. Etch-pit shapes and orientations in the smallest etch-pit arrays visible in conventionalscanning electron microscopy resemble even smaller features previously reported from transmission electron microscopeinvestigations of olivine weathering. 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, and therefore areformed by weathering and not inherited from pre-weathering aqueous alteration (e.g., serpentinization, iddingsitization) ofthese parent rocks. Many etch pits are devoid of weathering products. Natural weathering of olivine is surface-reaction-lim-ited. Similarity of corrosion forms from naturally weathered olivine from multiple igneous and metamorphic parent-rockbodies suggests that olivine weathers in the same manner regardless of its specific crystallization/recrystallization history,eruption/weathering/exposure ages of the olivine's host rock, and the local regolith history.