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Manganese‐chromium systematics in sulfides of unequilibrated enstatite chondrites
Author(s) -
WADHWA Meenakshi,
ZINNER Ernst K.,
CROZAZ Ghislaine
Publication year - 1997
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.1945-5100.1997.tb01266.x
Subject(s) - meteorite , enstatite , chondrite , parent body , geology , sulfide , manganese , systematics , geochemistry , achondrite , mineralogy , analytical chemistry (journal) , chemistry , astrobiology , physics , taxonomy (biology) , botany , organic chemistry , chromatography , biology
— We measured with a secondary ion mass spectrometer Mn/Cr ratios and Cr isotopes in individual grains of Mn‐bearing sulfides ( i.e. , sphalerites, ZnS; alabandites, MnS; and niningerites, MgS) in nine unequilibrated enstatite chondrites (UECs). The goals were to determine whether live 53 Mn (half‐life ∼3.7 Ma) was incorporated in these objects at the time of their isotopic closure and to establish whether Mn‐Cr systematics in sulfides in UECs can be used as a high‐resolution chronometer to constrain formation time differences between these meteorites. Sulfide grains analysed in four of these UECs, MAC 88136 (EL3), MAC 88184 (EL3), MAC 88180 (EL3), and Indarch (EH4), have clear 53 Cr excesses. These 53 Cr excesses can be very large (δ 53 Cr/ 52 Cr ranges up to ∼18,400%, the largest 53 Cr excess measured so far) and, in some grains, are well correlated with the Mn/Cr ratios. Thus, they were most likely produced by the in situ decay of 53 Mn in the meteorite samples. In the remaining five meteorites, no detectable excesses of 53 Cr were found, and only upper limits on the initial 53 Mn/ 55 Mn ratios could be established. The four meteorites with 53 Cr excesses show variations in the inferred 53 Mn/ 55 Mn ratios in various sulfide grains of the same meteorite. The Mn‐Cr systematics in these sulfides were disturbed (during and/or after the decay of 53 Mn) by varying degrees of reequilibration. Provided 53 Mn was homogeneously distributed in the region of the early solar system where these objects formed, the data suggest that the time of the last isotopic equilibration of sulfides in EL chondrites occurred at least 3 Ma after a similar episode in EH chondrites.