Constraints on the cooling history of the H-chondrite parent body from phosphate and chondrule Pb-isotopic dates from Estacado

Abstract

To constrain the metamorphic history of the H-chondrite parent body, we dated phosphates and chondrules from four H6 chondritic meteorites using U-Pb systematics. Reconnaissance analyses revealed that only Estacado had a sufficiently high 206Pb/204Pb ratio suitable for our purposes. The Pb- Pb isochron date for Estacado phosphates is measured to be 4492 +/- 15 Ma. The internal residue second leachate isochron for Estacado chondrules yielded the chondrule date of 4546 +/- 18 Ma. An alternative age estimate for Estacado chondrules of 4527.6 +/- 6.3 Ma is obtained from an isochron including two chondrules, two magnetically separated fractions, and four bulk chondrite analyses. This isochron date might represent the age of termination of Pb diffusion from the chondrules to the matrix. From these dates and previously established closure temperatures for Pb diffusion in phosphates and chondrules, we estimate an average cooling rate for Estacado between 5.5 +/- 3.2 Myr/degrees C and 8.3 +/- 5.0 Myr/degrees C. Using previously published results for Ste. Marguerite (H4) and Richardton (H5), our data reveal that the cooling rates of H chondrites decrease markedly with increasing metamorphic grade, in agreement with the predictions of the onion-shell asteroid model. Several issues, however, need to be addressed before confirming this model for the H-chondrite parent body: the discrepancies between peak metamorphic temperatures established by various mineral thermometers need to be resolved, diffusion and other mechanisms of element migration in polycrystalline solids must be better understood, and dating techniques should be further improved.