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Chondrite chronology by initial 87 Sr/ 86 Sr in phosphates?
Author(s) -
Podosek Frank A.,
Bran Joyce C.
Publication year - 1991
Publication title -
meteoritics
Language(s) - English
Resource type - Journals
eISSN - 1945-5100
pISSN - 0026-1114
DOI - 10.1111/j.1945-5100.1991.tb01030.x
Subject(s) - chondrite , meteorite , geology , ordinary chondrite , geochemistry , metamorphic rock , achondrite , parent body , metamorphism , mineralogy , astrobiology , physics
— We report Rb‐Sr analyses of phosphates from nine ordinary chondrites, more than doubling the number of meteorites for which such data are available. Ordinary chondrite phosphates characteristically have Rb/Sr ratios sufficiently low to permit accurate identification of initial 87 Sr/ 86 Sr, which is generally (but not in all cases) found to be significantly higher than the more primitive initial 87 Sr/ 86 Sr ratios inferred for carbonaceous chondrite refractory inclusions (ALL), basaltic achondrites (BABI), or bulk ordinary chondrites (in the ALL‐BABI range). Such elevation of initial 87 Sr/ 86 Sr is generally considered to reflect isotopic redistribution during metamorphism, and with a model for Rb/Sr in this environment can lead to an inferred metamorphic timescale. For whole rock Rb/Sr the inferred formation intervals are typically tens of Ma (range nil to > 100 Ma). There is no evident relation between initial Sr elevation and metamorphic grade. There is not a clear difference in initial Sr effects between H and L chondrites; LL chondrites show much less (if any) elevation of initial Sr, but data are available for only two meteorites. For the first time it is possible to make a detailed comparison of initial Sr and I‐Xe chronologies for several meteorites: these two potential metamorphic chronometers conspicuously fail to agree, in terms of both age and sequence of ages. A comparably definitive assessment of the comparison between initial Sr and Pb‐Pb chronologies is not yet possible, but presently available data suggest that these two approaches to chondrite chronology also fail to agree. Without a correlation with metamorphic grade, or detailed agreement with an independent chronometer, it remains unclear whether initial 87 Sr/ 86 Sr in phosphates can be translated into a reliable chronometer for ordinary chondrite metamorphism, at least within simple interpretational frameworks.