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Oxygen isotope and 26 Al‐ 26 Mg systematics of aluminum‐rich chondrules from unequilibrated enstatite chondrites
Author(s) -
Guan Yunbin,
Huss Gary R.,
Leshin Laurie A.,
MacPherson Glenn J.,
McKeegan Kevin D.
Publication year - 2006
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/j.1945-5100.2006.tb00191.x
Subject(s) - chondrule , enstatite , chondrite , geology , isotopes of oxygen , geochemistry , olivine , meteorite , mineralogy , astrobiology , physics
— Correlated in situ analyses of the oxygen and magnesium isotopic compositions of aluminum‐rich chondrules from unequilibrated enstatite chondrites were obtained using an ion microprobe. Among eleven aluminum‐rich chondrules and two plagioclase fragments measured for 26 Al‐ 26 Mg systematics, only one aluminum‐rich chondrule contains excess 26 Mg from the in situ decay of 26 Al; the inferred initial ratio ( 26 Al/ 27 Al) o = (6.8 ± 2.4) × 10 −6 is consistent with ratios observed in chondrules from carbonaceous chondrites and unequilibrated ordinary chondrites. The oxygen isotopic compositions of five aluminum‐rich chondrules and one plagioclase fragment define a line of slope ˜0.6 ± 0.1 on a three‐oxygen‐isotope diagram, overlapping the field defined by ferromagnesian chondrules in enstatite chondrites but extending to more 16 O‐rich compositions with a range in δ 18 O of about ˜12‰. Based on their oxygen isotopic compositions, aluminum‐rich chondrules in unequilibrated enstatite chondrites are probably genetically related to ferromagnesian chondrules and are not simple mixtures of materials from ferromagnesian chondrules and calcium‐aluminum‐rich inclusions (CAIs). Relative to their counterparts from unequilibrated ordinary chondrites, aluminum‐rich chondrules from unequilibrated enstatite chondrites show a narrower oxygen isotopic range and much less resolvable excess 26 Mg from the in situ decay of 26 Al, probably resulting from higher degrees of equilibration and isotopic exchange during post‐crystallization metamorphism. However, the presence of 26 Al‐bearing chondrules within the primitive ordinary, carbonaceous, and now enstatite chondrites suggests that 26 Al was at least approximately homogeneously distributed across the chondrite‐forming region.