Intermineral oxygen three-isotope systematics of silicate minerals in equilibrated ordinary chondrites

1David McDougal,1,2Daisuke Nakashima,1,3Travis J. Tenner,1Noriko T. Kita,1John W. Valley,4Takaaki Noguchi
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12932]
1WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin, USA
2Department of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University, Sendai, Miyagi, Japan
3Chemistry Division, Nuclear and Radiochemistry, Los Alamos National Laboratory, Los Alamos, New Mexico, USA
4Faculty of Arts and Science, Kyushu University, Fukuoka, Japan
Published by arrangement with John Wiley & Sons

High-precision oxygen three-isotope ratios were measured for four mineral phases (olivine, low-Ca and high-Ca pyroxene, and plagioclase) in equilibrated ordinary chondrites (EOCs) using a secondary ion mass spectrometer. Eleven EOCs were studied that cover all groups (H, L, LL) and petrologic types (4, 5, 6), including S1–S4 shock stages, as well as unbrecciated and brecciated meteorites. SIMS analyses of multiple minerals were made in close proximity (mostly <100 μm) from several areas in each meteorite thin section, to evaluate isotope exchange among minerals. Oxygen isotope ratios in each mineral become more homogenized as petrologic type increases with the notable exception of brecciated samples. In type 4 chondrites, oxygen isotope ratios of olivine and low-Ca pyroxene are heterogeneous in both δ18O and Δ17O, showing similar systematics to those in type 3 chondrites. In type 5 and 6 chondrites, oxygen isotope ratios of the four mineral phases plot along mass-dependent fractionation lines that are consistent with the bulk average Δ17O of each chondrite group. The δ18O of three minerals, low-Ca and high-Ca pyroxene and plagioclase, are consistent with equilibrium fractionation at temperatures of 700–1000 °C. In most cases the δ18O values of olivine are higher than those expected from pyroxene and plagioclase, suggesting partial retention of premetamorphic values due to slower oxygen isotope diffusion in olivine than pyroxene during thermal metamorphism in ordinary chondrite parent bodies.

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