Experimental simulation of oxygen isotopic exchange in olivine and implication for the formation of metamorphosed carbonaceous chondrites

Abstract

Abstract We have conducted hydration–dehydration experiments on terrestrial olivine to investigate the behavior of oxygen isotopic fractionation to test the hypothesis that multiple cycles of aqueous and thermal processing on a parent asteroid comprise a genetic relationship between CM 2s and metamorphosed carbonaceous chondrites ( MCC s). Two experiments were undertaken. In the first experiment, serpentine was obtained by hydrating terrestrial olivine (Fo 90.9 ) in the laboratory. During this experiment, olivine was reacted with isotopically heavy water (δ 18 O 21.5‰) at T = 300 °C, = 300 bar, for 100 days. The oxygen isotopic composition of the experimental serpentine was enriched in 18 O (by 10 ‰ in δ 18 O) due to exchange of oxygen isotopes between olivine and the 18 O‐rich water. Dehydrated serpentine was then produced during laboratory heating experiment in vacuum, at T = 930 °C, for 1 h. The oxygen isotopic composition of the dehydrated serpentine was enriched in 18 O by a further 7 ‰. The net result of the hydration–dehydration process was an enrichment of 18 O in the final material by approximately 17‰. The new experimental results suggest that the oxygen isotopic compositions of MCC s of the Belgica‐like group, including Dhofar 225 and Dhofar 725, could be derived from those of typical CM 2 chondrites via several cycles of hydration–dehydration caused by aqueous alteration and subsequent thermal metamorphism within their parent asteroids.