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

Goreva, J. S. and Burnett, D. S. (2001) Phosphate control on the thorium/uranium variations in ordinary chondrites: Improving solar system abundances. Meteoritics and Planetary Science, 36 (1). pp. 63-74. ISSN 1086-9379. https://resolver.caltech.edu/CaltechAUTHORS:20141111-104435610

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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 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 σ_(mean) = 0.10.


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http://dx.doi.org/10.1111/j.1945-5100.2001.tb01810.xDOIArticle
http://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2001.tb01810.x/abstractPublisherArticle
Additional Information:© 2001 The Meteoritical Society Received 2000 May 8; accepted in revised form 2000 September 20. The authors are grateful to J. H. Jones and M. Ebihara for critical and constructive reviews of the manuscript. This work was supported by NASA grant NAG5-4319.
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NASANAG5-4319
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Record Number:CaltechAUTHORS:20141111-104435610
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20141111-104435610
Official Citation:GOREVA, J. S. and BURNETT, D. S. (2001), Phosphate control on the thorium/uranium variations in ordinary chondrites: Improving solar system abundances. Meteoritics & Planetary Science, 36: 63–74. doi: 10.1111/j.1945-5100.2001.tb01810.x
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:51566
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:11 Nov 2014 21:10
Last Modified:03 Oct 2019 07:34

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