EVIDENCE FOR A LATE THERMAL EVENT OF UNEQUILIBRATED ENSTATITE CHONDRITES - A RB-SR STUDY OF QINGZHEN AND YAMATO-6901 (EH3) AND KHAIRPUR (EL6)

The Rb-Sr whole rock and internal systematics of two EH3 chondrites, Qingzhen and Yamato 6901, and of one EL6 chondrite, Khairpur, were determined. Sulfides were separated using a stepwise dissolution technique. The mineral species in each fraction were estimated based on the chemical analyses of 12 major elements. Tle internal Rb-Sr systematics of the EH3 chondrites are highly disturbed. Fractions corresponding to sulfide phases show excess Sr-87, while other fractions corresponding to silicate phases produce a linear trend on a Rb-Sr evolution diagram. If these linear relations are interpreted as isochrons, the ages of the silicate phases are 2.12 +/- 0.23 Ga and 2.05 +/- 0.33 Ga with the initial Sr isotopic ratios of 0.7112 +/- 0.0018 and 0.7089 +/- 0.0032, for Qingzhen and Yamato 6901, respectively. The process of the isotopic disturbance probably involved the breakdown of the major K-bearing sulfide (djerfisherite), and a lack of isotopic exchange between sulfide and silicate phases indicates moderate temperatures of reheating. Although a complete Sr isotopic re-homogenization among silicate phases was not attained, we interpret the Rb-Sr results as indicative of a late thermal event about 2 Ga ago on the parent bodies of these EH3 chondrites. These ages agree well with previously published K-Ar ages. An older isochron age of 4.481 +/- 0.036 Ga with a low initial Sr isotopic ratio of 0.69866 +/- 0.00038 was obtained for the data from silicate fractions of Khairpur, indicating early petrological equilibration on the parent body of EL6 chondrites.