SiO2 precipitation in olivine: ATEM investigation of two dunites annealed at 300 MPa in hydrous conditions

Two natural dunites were annealed at pressure P = 300 MPa, temperature T = 1373, 1473 and 1573 K, and fO(2) within the stability field of olivine. The starting materials contained small amounts of hydroxyls in the form of min or phases of hydrated minerals, which released an aqueous phase during the experiments. A detailed analytical transmission electron microscopy (ATEM) investigation of these materials revealed that small quantities of two type s of silica-rich glass formed during heat treatment. The first type of glas s, found at triple junctions as rare partially crystallized glass pockets, results from melting dehydration reactions involving the hydrous phases. Th e second type of glass is found as pure silica precipitates (0.1-0.5 mu m i n size, for a total of a few 0.1 vol%) within the olivine grains of specime ns heated to greater than or equal to 1473 K. From considerations of the ki netics of the precipitation at 1473 K, we interpret this silica precipitati on as resulting from the condensation of olivine metallic vacancies promote d by increasing fluid fugacities during the runs. Our observations, thus, d emonstrate that metastable silica can precipitate in olivine from dunites e xperiencing rapid changes in their thermodynamical environment. (C) 2000 El sevier Science B.V. All rights reserved.