I-XE STUDIES OF THE ACAPULCO METEORITE - ABSOLUTE I-XE AGES OF INDIVIDUAL PHOSPHATE GRAINS AND THE BJURBOLE STANDARD

We have measured Kr and Xe by laser volatilization of seventeen indivi dual neutron-irradiated phosphate grains (nine apatite and eight merri llite) separated from the (reclassified) A-chondrite Acapulco. Radioge nic Xe-129 from now-extinct I-129 is only observed in the apatites, wh ich all formed simultaneously (8.1 +/- 1.2 Ma) after Xe closure in the Bjurbole (L4) standard. This relative closure time, when coupled with the Pb-Pb age of Acapulco phosphates (4.557 +/- 0.002 Ga) provides an absolute I-Xe age for these apatites, for Bjurbole and for all sample s previously referenced to the Bjurbole standard, subject to the assum ptions implicit in I-Xe dating. We have also measured Kr and Xe by ste pwise heating of irradiated and unirradiated whole-rock samples of Aca pulco and of unirradiated phosphate concentrates. Iodine-derived xenon in the irradiated whole-rock sample does not yield an I-Xe isochron, consistent with the presence of multiple iodine host phases previously observed. Xenon from the spontaneous fission of now-extinct Pu-244 is observed in the single phosphate grains, and the inferred initial (Pu -244/U-238)0 ratios reflect fractionation of the actinides favoring pl utonium in the merrillites and uranium in the apatites.