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

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 CM2s and metamorphosed carbonaceous chondrites (MCCs). 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 (O-18 21.5 parts per thousand) at T=300 degrees C, PH2O=300bar, for 100days. The oxygen isotopic composition of the experimental serpentine was enriched in O-18 (by 10 parts per thousand in O-18) due to exchange of oxygen isotopes between olivine and the O-18-rich water. Dehydrated serpentine was then produced during laboratory heating experiment in vacuum, at T=930 degrees C, for 1h. The oxygen isotopic composition of the dehydrated serpentine was enriched in O-18 by a further 7 parts per thousand. The net result of the hydration-dehydration process was an enrichment of O-18 in the final material by approximately 17 parts per thousand. The new experimental results suggest that the oxygen isotopic compositions of MCCs of the Belgica-like group, including Dhofar 225 and Dhofar 725, could be derived from those of typical CM2 chondrites via several cycles of hydration-dehydration caused by aqueous alteration and subsequent thermal metamorphism within their parent asteroids.