Origin of Th/U variations in chondritic meteorites

Geochemistry

Goreva, Julia S. ; Rossman, George Robert (co-advisor)

University:California Institute of Technology

Department:Geological and Planetary Sciences

关键词: Geochemistry;   

Others  :  https://thesis.library.caltech.edu/8202/1/Goreva_js_2001.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

Isotope dilution thorium and uranium analyses of the Harleton chondrite show alarger scatter than previously observed in equilibrated ordinary chondrites (EOC). Thelinear correlation of Th/U with 1/U in Harleton (and all EOC data) is produced byvariation in the chlorapatite to merrillite mixing ratio. Apatite variations control the Uconcentrations. Phosphorus variations are compensated by inverse variations in U topreserve the Th/U vs. 1/U correlation. Because the Th/U variations reflect phosphateampling, a weighted Th/U average should converge to an improved solar system Th/U.We obtain Th/U=3.53 (1_-mean=0.10), significantly lower and more precise than previousestimates.

To test whether apatite also produces Th/U variation in CI and CM chondrites, weperformed P analyses on the solutions from leaching experiments of Orgueil andMurchison meteorites.

A linear Th/U vs. 1/U correlation in CI can be explained by redistribution ofhexavalent U by aqueous fluids into carbonates and sulfates.

Unlike CI and EOC, whole rock Th/U variations in CMs are mostly due to Thvariations. A Th/U vs. 1/U linear correlation suggested by previous data for CMs is notreal. We distinguish 4 components responsible for the whole rock Th/U variations: (1) Pand 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) aneasily leachable phase of high Th/U. likely carbonate 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, wecalculate relative supernovae production rates for ²³²Th/²³⁸U and ²³⁵U/²³⁸U for differentmodels of r-process nucleosynthesis. For uniform galactic production, the beginning of ther-process nucleosynthesis must be less than 13 Gyr. Exponentially decreasing production is alsoconsistent with a 13 Gyr age, but very slow decay times are required (less than 35 Gyr),approaching the uniform production. The 15 Gyr Galaxy requires either a fast initialproduction growth (infall time constant less than 0.5 Gyr) followed by very low decrease (decaytime constant greater than 100 Gyr), or the fastest possible decrease (≈8 Gyr) preceded by slow in fall(≈7.5 Gyr).