Initial strontium for a chondrite and the determination of a metamorphism or formation interval

Abstract

A precise Rb-Sr internal isochron was determined for Guarena, an H6 chondrite, yielding an age of 4.56 ± 0.08 × 10⁹ years. Rb-poor, Sr-rich phosphate phases (whitlockite and apatite) were obtained resulting in a precise measurement of the initial Sr isotopic composition (⁸⁷Sr/⁸⁶Sr)_I = 0.69995 ∓ 0.00015. It is shown that from precise age and initial (⁸⁷Sr/⁸⁶Sr)_I measurements we can obtain information on the differential evolution of Rb-Sr systems involving either simple metamorphism of closed systems or multi-stage processes. The high initial ratio obtained for Guarena with respect to the value for the basaltic achondrites is the first clear demonstration of differential Sr evolution of a chondrite and affords the simple explanation of Guarena having been metamorphosed 74 million years after its formation. This approach may yield a "time" evolution index for the classification of meteorites.

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