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The behavior of actinides, phosphorus, and rare earth elements during chondrite metamorphism

Murrell, M. T. and Burnett, D. S. (1983) The behavior of actinides, phosphorus, and rare earth elements during chondrite metamorphism. Geochimica et Cosmochimica Acta, 47 (11). pp. 1999-2014. ISSN 0016-7037. https://resolver.caltech.edu/CaltechAUTHORS:20150424-143531352

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Abstract

New data on the U, Pu, and P distributions in less metamorphosed H-chondrites (type 3–5), coupled with literature results, permit a provisional picture to be assembled of the chemistry of these elements and for the rare earth elements in ordinary chondrites and the changes brought about by chondritic metamorphism. Preferential associations of phosphates with metals and/or sulndes in all chondrites strongly indicate an “initially” siderophile or conceivably chalcophile character for P in ordinary chondrite precursor materials with phosphate subsequently formed by oxidation. This oxidation occurred prior to or during chondritic metal-silicate fractionation. Uranium is initially concentrated in chondrule glass at ~ 100 ppb levels with phosphates (primarily merrillite) in H-3 chondrites being essentially U-free (<20 ppb). As chondrule glass devitrified during metamorphism, U migrated into phosphates reaching ~ 50 ppb in Nadiabondi (H-5) merrillite and 200–300 ppb in merrillite from equilibrated chondrites but “froze out” before total concentration in phosphates occurred. Relative ^(244)Pu fission track densities in the outer 5 μm of olivine and pyroxene grains in contact with merrillite and with chondrule mesostasis in Bremervörde (H-3) give Pu(mesostasis)/Pu(merrillite) <0.01, implying total concentration of Pu in phosphates. Similarly, no detectable Pu (<0.1 ppb) was found in chondrule mesostasis in Tieschitz and Sharps; whereas, direct measurements of tracks in phosphates in H-3 chondrites are consistent with high (≳10 ppb) Pu concentrations. Thus, a strong Pu-P correlation is indicated for ordinary chondrites. There is variable Pu/U fractionation in all chondritic phosphates reaching an extreme degree in the unequilibrated chondrites; therefore, the Pu/U ratio in phosphates appears relatively useless for relative meteorite chronology. Literature data indicate that the REE are located in chondrules in unequilibrated chondrites, most likely in glass; thus there may also be strong Pu/Nd fractionation within these meteorites. Like U, the REE migrate into phosphates during metamorphism but, unlike U, appear to be quantitatively concentrated in phosphates in equilibrated chondrites. Thus relative ages, based on Pu/Nd, may be possible for equilibrated chondrites, but the same chronological conclusions are probably obtainable from Pu concentrations in phosphates, i.e., on the Pu/P ratio. However, Pu/P chronology is possible only for ordinary chondrites; so there appears to be no universal reference element to cancel the effects of Pu chemical fractionation in all meteorites. Available data are consistent with — but certainly do not prove-that variations in Pu/P represent age differences, but if these age differences do not exist, then it is conceivable that the solar system ^(244)Pu/^(238)U ratio, important for cosmochronology, is still lower than the presently accepted value of 0.007.


Item Type:Article
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http://dx.doi.org/10.1016/0016-7037(83)90216-8DOIArticle
http://www.sciencedirect.com/science/article/pii/0016703783902168PublisherArticle
ORCID:
AuthorORCID
Burnett, D. S.0000-0001-9521-8675
Additional Information:© 1983 Pergamon Press Ltd. Received 26 January 1983, Accepted 11 August 1983, Available online 14 April 2003. We wish to thank J. H. Jones, H. Palme, and F. A. Podosek for their careful and most helpful reviews of this paper and P. Pellas for invigorating discussions. We are indebted to J. R. Arnold, R. S. Clarke, Jr., J. N. Grossman, C. B. Moore, K. Nishiizumi, P. Pellas, G. J. Wasserburg, J. T. Wasson, and D. S. Woolum for providing the meteorite samples used in this study. The cooperation of Grossman and Wasson in sharing the Chainpur sections was of particular importance to us. We thank G. Croxaz and L. M. Ross for their assistance with track replication and S. Riley for his general assistance throughout these experiments. This research was supported by NSF grant EAR 81-21381.
Funders:
Funding AgencyGrant Number
NSFEAR 81-21381
Issue or Number:11
Record Number:CaltechAUTHORS:20150424-143531352
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20150424-143531352
Official Citation:M.T Murrell, D.S Burnett, The behavior of actinides, phosphorus, and rare earth elements during chondrite metamorphism, Geochimica et Cosmochimica Acta, Volume 47, Issue 11, November 1983, Pages 1999-2014, ISSN 0016-7037, http://dx.doi.org/10.1016/0016-7037(83)90216-8. (http://www.sciencedirect.com/science/article/pii/0016703783902168)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:56968
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:24 Apr 2015 22:13
Last Modified:09 Mar 2020 13:18

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