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Intermineral oxygen three‐isotope systematics of silicate minerals in equilibrated ordinary chondrites
Author(s) -
McDougal David,
Nakashima Daisuke,
Tenner Travis J.,
Kita Noriko T.,
Valley John W.,
Noguchi Takaaki
Publication year - 2017
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.12932
Subject(s) - chondrite , systematics , silicate , geology , isotopes of oxygen , geochemistry , isotope , silicate minerals , meteorite , mineralogy , astrobiology , chemistry , physics , biology , nuclear physics , taxonomy (biology) , botany , organic chemistry
Abstract 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 ( EOC s) using a secondary ion mass spectrometer. Eleven EOC s 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 δ 18 O and Δ 17 O, 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 Δ 17 O of each chondrite group. The δ 18 O 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 δ 18 O 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.