INITIAL STRONTIUM FOR A CHONDRITE AND DETERMINATION OF A METAMORPHISM OR FORMATION INTERVAL

A precise Rb-Sr internal isochron was determined for Guareña, 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 Guareña 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 Guareña having been metamorphosed 74 million years after its formation. This approach may yield a time" evolution index for the classification of meteorites. © 1969."