Iddingsitization and recurrent crystallization of olivine in basalts from the Simcoe Mountains, Washington

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Abstract
Olivine phenocrysts from Pliocene to Recent basalts of the Simcoe Mountains commonly are altered, either in part or completely, to iddingsite. Overgrowths of fresh olivine (Fo 67 to Fo 21 ) surround or partially surround many of these iddingsitized phenocrysts. Textural evidence suggests that the alteration of olivine phenocrysts occurred just prior to and during extrusion and was not dependent upon the phenocrysts' composition. Similarity in composition of overgrowths and ferriferous groundmass olivine suggests that the overgrowths formed during the groundmass crystallization.

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