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Title: Uranium-lead isotope systematics of Mars inferred from the basaltic shergottite QUE 94201

Abstract

Uranium-lead ratios (commonly represented as {sup 238}U/{sup 204}Pb = {mu}) calculated for the sources of martian basalts preserve a record of petrogenetic processes that operated during early planetary differentiation and formation of martian geochemical reservoirs. To better define the range of {mu} values represented by the source regions of martian basalts, we completed U-Pb elemental and isotopic analyses on whole rock, mineral and leachate fractions from the martian meteorite Queen Alexandra Range 94201 (QUE 94201). The whole rock and silicate mineral fractions have unradiogenic Pb isotopic compositions that define a narrow range ({sup 206}Pb/{sup 204}Pb = 11.16-11.61). In contrast, the Pb isotopic compositions of weak HCl leachates are more variable and radiogenic. The intersection of the QUE 94201 data array with terrestrial Pb in {sup 206}Pb/{sup 204}Pb-{sup 207}Pb/{sup 204}Pb-{sup 208}Pb/{sup 204}Pb compositional space is consistent with varying amounts of terrestrial contamination in these fractions. We calculate that only 1-7% contamination is present in the purified silicate mineral and whole rock fractions, whereas the HCl leachates contain up to 86% terrestrial contamination. Despite the contamination, we are able to use the U-Pb data to determine the initial {sup 206}Pb/{sup 204}Pb of QUE 94201 (11.086 {+-} 0.008) and calculate the {mu} valuemore » of the QUE 94201 mantle source to be 1.823 {+-} 0.008. This is the lowest {mu} value calculated for any martian basalt source, and, when compared to the highest values determined for martian basalt sources, indicates that {mu} values in martian source reservoirs vary by at least 100%. The range of source {mu} values further indicates that the {mu} value of bulk silicate Mars is approximately three. The amount of variation in the {mu} values of the mantle sources ({mu} {approx} 2-4) is greater than can be explained by igneous processes involving silicate phases alone. We suggest the possibility that a small amount of sulfide crystallization may generate large extents of U-Pb fractionation during formation of the mantle sources of martian basalts.« less

Authors:
; ;
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
936965
Report Number(s):
UCRL-JRNL-226993
Journal ID: ISSN 0016-7037; GCACAK; TRN: US0806148
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Journal Article
Resource Relation:
Journal Name: Geochimica et Cosmochimica Acta, vol. 71, N/A, September 12, 2007, pp. 5016-5031; Journal Volume: 71
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; 07 ISOTOPES AND RADIATION SOURCES; BASALT; CONTAMINATION; CRYSTALLIZATION; FRACTIONATION; LEACHATES; METEORITES; SILICATE MINERALS; SILICATES; SULFIDES

Citation Formats

Gaffney, A M, Borg, L E, and Connelly, J N. Uranium-lead isotope systematics of Mars inferred from the basaltic shergottite QUE 94201. United States: N. p., 2006. Web.
Gaffney, A M, Borg, L E, & Connelly, J N. Uranium-lead isotope systematics of Mars inferred from the basaltic shergottite QUE 94201. United States.
Gaffney, A M, Borg, L E, and Connelly, J N. Fri . "Uranium-lead isotope systematics of Mars inferred from the basaltic shergottite QUE 94201". United States. doi:. https://www.osti.gov/servlets/purl/936965.
@article{osti_936965,
title = {Uranium-lead isotope systematics of Mars inferred from the basaltic shergottite QUE 94201},
author = {Gaffney, A M and Borg, L E and Connelly, J N},
abstractNote = {Uranium-lead ratios (commonly represented as {sup 238}U/{sup 204}Pb = {mu}) calculated for the sources of martian basalts preserve a record of petrogenetic processes that operated during early planetary differentiation and formation of martian geochemical reservoirs. To better define the range of {mu} values represented by the source regions of martian basalts, we completed U-Pb elemental and isotopic analyses on whole rock, mineral and leachate fractions from the martian meteorite Queen Alexandra Range 94201 (QUE 94201). The whole rock and silicate mineral fractions have unradiogenic Pb isotopic compositions that define a narrow range ({sup 206}Pb/{sup 204}Pb = 11.16-11.61). In contrast, the Pb isotopic compositions of weak HCl leachates are more variable and radiogenic. The intersection of the QUE 94201 data array with terrestrial Pb in {sup 206}Pb/{sup 204}Pb-{sup 207}Pb/{sup 204}Pb-{sup 208}Pb/{sup 204}Pb compositional space is consistent with varying amounts of terrestrial contamination in these fractions. We calculate that only 1-7% contamination is present in the purified silicate mineral and whole rock fractions, whereas the HCl leachates contain up to 86% terrestrial contamination. Despite the contamination, we are able to use the U-Pb data to determine the initial {sup 206}Pb/{sup 204}Pb of QUE 94201 (11.086 {+-} 0.008) and calculate the {mu} value of the QUE 94201 mantle source to be 1.823 {+-} 0.008. This is the lowest {mu} value calculated for any martian basalt source, and, when compared to the highest values determined for martian basalt sources, indicates that {mu} values in martian source reservoirs vary by at least 100%. The range of source {mu} values further indicates that the {mu} value of bulk silicate Mars is approximately three. The amount of variation in the {mu} values of the mantle sources ({mu} {approx} 2-4) is greater than can be explained by igneous processes involving silicate phases alone. We suggest the possibility that a small amount of sulfide crystallization may generate large extents of U-Pb fractionation during formation of the mantle sources of martian basalts.},
doi = {},
journal = {Geochimica et Cosmochimica Acta, vol. 71, N/A, September 12, 2007, pp. 5016-5031},
number = ,
volume = 71,
place = {United States},
year = {Fri Dec 22 00:00:00 EST 2006},
month = {Fri Dec 22 00:00:00 EST 2006}
}
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