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Experimental determination of U and Th partitioning between clinopyroxene and natural and synthetic basaltic liquid

La Tourrette, T. Z. and Burnett, D. S. (1992) Experimental determination of U and Th partitioning between clinopyroxene and natural and synthetic basaltic liquid. Earth and Planetary Science Letters, 110 (1-4). pp. 227-244. ISSN 0012-821X. doi:10.1016/0012-821X(92)90050-6.

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Clinopyroxene-silicate liquid partition coefficients for U and Th have been determined by particle track radiography from 1 atm crystallization experiments at controlled fO_2. Two natural basaltic and one synthetic composition were studied at fO_2 values from the Ni—NiO oxygen buffer to 1 log unit more oxidizing than Fe—FeO (IW+ 1). Over the range of fO_2 values and compositions studied, D_U^(cpx/liq) = 0.0034–0.015,D_(Th)^(cpx/liq) = 0.008–0.036, and D_(Th)/D_U= 3.4–1.1. With decreasing fO_2, D_(Th)/D_U can decrease by up to a factor of 3 for a given composition, primarily from an increase in D_U^(cpx/liq), which we interpret as resulting from an increase in the proportion of tetravalent U in the system with decreasing fO_2. This demonstrates that crystal-liquid U—Th fractionation is fO_2 dependent and that U in terrestrial magmas is not entirely tetravalent. D_(Th)^(cpx/liq) appears to decrease in the two basalts at the lowest fO_2, possibly as a result of changes in composition with fO_2. Our data show the sense of U—Th fractionation by clinopyroxene-liquid partitioning is consistent with previous experimental determinations, in that D_(Th)^(cpx)/D_U^(cpx)> 1 in all cases. This indicates that, during partial melting, the liquid will have a Th/U ratio less than the clinopyroxene in the source. The observed ^(238)—U^(230)Th disequilibrium in MORB requires that the partial melt should have a Th/U ratio greater than the bulk source, and, therefore, cannot result from clinopyroxene-liquid partitioning. Further, the magnitudes of the measured partition coefficients are too small to generate significant U—Th fractionation in either direction. Assuming that clinopyroxene contains the bulk of the U and Th in MORB source, our results indicate that ^(238)U—^(230)Th disequilibrium in MORB may not be caused by partial melting at all.

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Burnett, D. S.0000-0001-9521-8675
Additional Information:© 1992 Elsevier Science Publishers B.V. Received 8 October 1991, Accepted 28 February 1992, Available online 23 October 2002. We thank H. Nagasawa and D. Stakes for providing the basalt samples. Neutron irradiations were carried out by P. Rogers at the TRIGA reactor facility. C. Irvine and P. Carpenter assisted with the electron microprobe analyses. Discussions with M. Baker, J. Beckett, D. Bell, I. Carmichael, M. Johnson, G. Mattioli, G. Rossman and E. Stolper were very helpful. Critical reviews by C. Lesher and B. Mysen resulted in considerable improvements in the manuscript. This research was supported by a NASA Graduate Student Researchers Program Fellowship.
Group:UNSPECIFIED, Division of Geological and Planetary Sciences
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NASA Graduate Student Research FellowshipUNSPECIFIED
Issue or Number:1-4
Record Number:CaltechAUTHORS:20150427-085539852
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Official Citation:T.Z. LaTourrette, D.S. Burnett, Experimental determination of U and Th partitioning between clinopyroxene and natural and synthetic basaltic liquid, Earth and Planetary Science Letters, Volume 110, Issues 1–4, May 1992, Pages 227-244, ISSN 0012-821X, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:56992
Deposited By: Ruth Sustaita
Deposited On:27 Apr 2015 16:49
Last Modified:10 Nov 2021 21:06

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