Phosphate control on the Th/U variations in ordinary chondrites: Improving solar system abundances

Details Phosphate control on the Th/U variations in ordinary chondrites: Improving solar system abundances Phosphate control on the Th/U variations in ordinary chondrites: Improving solar system abundances

Jan 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001m%26ps...36...63g&link_type=abstract

Meteoritics & Planetary Science, vol. 36, no. 1, p. 63-74 (2001).

Computer Science

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Scientific paper

Isotope dilution thorium and uranium analyses by inductively-coupled plasma mass spectrometry of 12 samples of Harleton (L6) show a much larger scatter than was previously observed in equilibrated ordinary chondrites. Th/U linearly correlates with 1/U in Harleton and in the total equilibrated ordinary chondrite data set as well. Such a correlation suggests a two component mixture and this trend can be quantitatively modeled as reflecting variations in the mixing ratio between two phosphate phases: chlorapatite and merrillite. The major effect is due to apatite variations, which strongly control the whole rock U concentrations. Phosphorous variations will tend to destroy the Th/U vs. 1/U correlation, and measured P concentrations on exactly the same samples as U and Th show a factor of 3 range. It appears that the P variations are compensated by inverse variations in U (a dilution effect) to preserve the Th/U vs. 1/U correlation. Because variations in whole rock Th/U are consequences of phosphate sampling, a weighted average of high accuracy Th/U measurements in equilibrated ordinary chondrites should converge to a significantly improved average solar system Th/U. Our best estimate of this ratio is 3.53 with ?mean = 0.10.

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