Origin of thorium/uranium variations in chondritic meteorites.

摘要

Isotope dilution thorium and uranium analyses of the Harleton chondrite show a larger scatter than previously observed in equilibrated ordinary chondrites (EOC). The linear correlation of Th/U with l/U in Harleton (and all EOC data) is produced by variations in the chlorapatite to merrillite mixing ratio. Apatite variations control the U concentrations. Phosphorus variations are compensated by inverse variations in U to preserve the Th/U vs. l/U correlation. Because the Th/U variations reflect phosphate sampling, a weighted Th/U average should converge to an improved solar system Th/U. We obtain Th/U = 3.53 (1σ _mean = 0.10), significantly lower and more precise than previous estimates.; To test whether apatite also produces Th/U variations in CI and CM chondrites, we performed P analyses on the solutions from leaching experiments of Orgueil and Murchison meteorites.; A linear Th/U vs. l/U correlation in CI can be explained by redistribution of hexavalent U by aqueous fluids into carbonates and sulfates.; Unlike CI and EOC, whole rock Th/U variations in CMs are mostly due to Th variations. A Th/U vs. l/U linear correlation suggested by previous data for CMs is not real. We distinguish 4 components responsible for the whole rock Th/U variations: (1) P and actinide-depleted matrix containing small amounts of U-rich carbonate/sulfate phases (similar to CIs); (2) CAIs and (3) chondrules are major reservoirs for actinides, (4) an easily leachable phase of high Th/U, likely carbonates produced by CAI alteration. Phosphates play a minor role as actinide and P carrier phases in CM chondrites.; Using our Th/U and minimum galactic ages from halo globular clusters, we calculate relative supernovae production rates for 232Th/ 238U and 235U/238U for different models of r-process nucleosynthesis. For uniform galactic production, the beginning of the r-process nucleosynthesis must be 13 Gyr. Exponentially decreasing production is also consistent with a 13 Gyr age, but very slow decay times are required (35 Gyr), approaching the uniform production. The 15 Gyr Galaxy requires either a fast initial production growth (infall time constant 0.5 Gyr) followed by very slow decrease (decay time constant >100 Gyr), or the fastest possible decrease (≈8 Gyr) preceded by slow infall (≈7.5 Gyr).