Phosphate control on the thorium/uranium variations in ordinary chondrites: Improving solar system abundances

Isotope dilution thorium and uranium analyses by inductively-coupled plasma mass spectrometry of 12 samples of Harleton (L6) show a much larger scatte r than was previously observed in equilibrated ordinary chondrites. Th/U li nearly correlates with 1/U in Harleton and in the total equilibrated ordina ry chondrite data set as well. Such a correlation suggests a two component mixture and this trend can be quantitatively modeled as reflecting variatio ns in the mixing ratio between two phosphate phases: chlorapatite and merri llite. The major effect is due to apatite variations, which strongly contro l the whole rock U concentrations. Phosphorous variations will tend to dest roy 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 ac curacy Th/U measurements in equilibrated ordinary chondrites should converg e to a significantly improved average solar system Th/U. Our best estimate of this ratio is 3.53 with sigma (mean) = 0.10.