An ~4.35 Ga Ar-Ar Age for GRA 8 and the Complex Chronology of Its Parent Body

Introduction: GRA06128 and GRA06129 (hereafter GRA 8 and GRA 9) are partial melts of a parent body of approximately chondritic composition [1-3]. We [4,5] reported a conventional Sm-Nd isochron age of 4.559±0.096 Ga and a Sm-Nd model age of 4.549±0.036 for combined data for the two rocks. Plagioclase plus whole rock and leachate (~phosphate) samples gave a secondary Sm-Nd age of 3.4±0.4 Ga [5]. An Ar-Ar age of 4.460±0.028 Ga [3,6] was interpreted by [3] as dating metamorphism in GRA 9. We report Ar-Ar ages in the range ~4344-4366 Ma for GRA 8, establishing similar but different Ar-Ar ages for the two rocks, consistent with their different Sr-istopic systematics [5], and discuss these ages in the context of the complex sequence of events that affected these samples (cf. [3]). Ar-Ar Stepped Ar-Release Ages for GRA 8: We did a stepped-temperature Ar extraction (49 steps) of a plagioclase separate of GRA 8 (12.7 mg) of low magnetic susceptibility (Fig. 1). Small age variations occurred among “phases” with different K/Ca at low, intermediate, and high extraction temperatures, and the summed age is 4354 Ma. Partitioned according to the fraction of Ar released, the calculated ages varied from 4326±18 to 4344±14 to 4362±18 Ma (1 ) for 4-14%, 14-47%, and 52-96% Ar released, resp. A single temperature step for 47-52% of the Ar release appeared to mark a transition in the gas release mechanism as also seen in an Arrhenius plot constructed from the data (Fig. 2).