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

Abstract

Abstract-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 TWU 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 TWU measurements in equilibrated ordinary chondrites should converge to a significantly improved average solar system TWU. Our best estimate of this ratio is 3.53 with omem = 0.10.

Unraveling the evolution of chondrite parent asteroids by precise U-Pb dating and thermal modeling

“…Chondrules and fractions of chondrule fragments have uranium concentrations of 9 -62 ppb, similar to or slightly higher than the values of ca. 10 ppb measured in whole rock samples of Richardton (Tilton, 1973;Tatsumoto et al, 1973;Abranches et al, 1980) and in other chondrites (Unruh, 1982;Göpel et al, 1994;Goreva and Burnett, 2001). Concentrations of U in the lowdensity fractions are similar.…”

A gamma‐ray spectroscopy survey of Omani meteorites

“…The measured activities of 232 Th eq are relatively uniform for all chondrites and correspond to an average of 50 ± 12 ppb Th (mean L chondrites: 40 ± 7 ppb, Th/U = 3.53; Goreva and Burnett ). Good agreement is seen between the values from the different transitions, before and after 220 Rn in the decay chain.…”

Characterization of Mason Gully (H5): The second recovered fall from the Desert Fireball Network

“…While both merrillite and apatite are present, U concentrations in apatite are as much as 159 higher (Crozaz et al 1989). Furthermore, Goreva and Burnett (2001) have shown that the relatively high P/Cl ratios in OCs, as well as heterogeneously distributed Cl, results in homogeneous distribution of merrillite and very heterogeneous distribution of chlorapatite. The low U value in MG suggests that mg-sized sampling is too small to obtain reliable results for this element.…”