U-Th-Pb fractionation in Archaean lower continental crust: Implications for terrestrial Pb isotope systematics

Bolhar, Robert, Kamber, Balz S. and Collerson, Kenneth D. (2007) U-Th-Pb fractionation in Archaean lower continental crust: Implications for terrestrial Pb isotope systematics. Earth And Planetary Science Letters, 254 1-2: 127-145. doi:10.1016/j.epsl.2006.11.032

Author Bolhar, Robert
Kamber, Balz S.
Collerson, Kenneth D.
Title U-Th-Pb fractionation in Archaean lower continental crust: Implications for terrestrial Pb isotope systematics
Journal name Earth And Planetary Science Letters   Check publisher's open access policy
ISSN 0012-821X
Publication date 2007-02-15
Sub-type Article (original research)
DOI 10.1016/j.epsl.2006.11.032
Volume 254
Issue 1-2
Start page 127
End page 145
Total pages 19
Place of publication Amsterdam
Publisher Elsevier Science Bv
Collection year 2008
Language eng
Subject C1
260100 Geology
780104 Earth sciences
Abstract Pb isotopic compositions are reported for leached feldspars and whole rock samples of felsic to ultramafic, aillphibolite to granulite facies xenoliths from the Bearpaw Mountains in the northern Wyoming Craton, Montana, USA. Two-point dates obtained for leached feldspar-whole rock pairs provide a record of Pb isotopic homogenisation at 1.6-2.1 Ga, during an inferred Palaeoproterozoic then-no-tectonic event. Model ages for leached feldspars, assuming single-stage melt extraction front depleted mantle, indicate formation of the protoliths from 2.8 to 4.0 Ga. The inferred timing of the Palaeoproterozoic high-grade metamorphism and extended Archaean crust formation is in agreement with available geochronological data for the Wyoming Craton. Following Palaeoproterozoic isotope resetting, the second stage of Ph isotope evolution Suggests prolonged residence in a low U/Pb environment. Isotope resetting was apparently accompanied by a significant decrease in U-238/Ph-204 (mu-values), reflecting substantial loss of the highly incompatible and mobile element U and, by inference, other heat-producing elements. Geothermal considerations suggest that unstratified continental crust, as approximated by a "reconstituted" average xenolith composition from the Wyoming Craton, would have been thermally unstable at lower crustal levels at the time of formation and metamorphism, if abundances of heat-producing elements had been unaffected by the U-Th-Pb fractionation event. In contrast, differentiated continental crust comprising a chemically depleted lower layer and an enriched upper layer would have reached thermal stability at the time of high-grade metamorphism. On this basis, the Wyoming Craton xenoliths data are proposed as an approximation to the lower continental crust. In Pb-207/Pb-204 vs. Pb-206/Pb-204 space the xenoliths plot to the left of the meteorite isochron and above the depleted mantle evolution curve, in contrast to most previous data from lower crustal xenolith suites. Simple mass balance, using an average of the studied xenoliths from the Wyoming Craton as approximation to lower continental crust and published compositions for upper continental crust, yields a Pb isotopic composition of bulk continental crust that plots close to the meteorite isochron. Hence, it is proposed that Archaean lower crust, as exemplified by the Wyoming Craton xenoliths, displays the essential geochemical and isotopic requisites to be a major reservoir to balance the Ph isotope composition of bulk silicate earth. (c) 2006 Elsevier B.V. All rights reserved.
Keyword Geochemistry & Geophysics
heat-producing elernents
lower continental crust
terrestrial Pb isotope paradox
Wyoming craton
Garnet Granulite Xenoliths
Wyoming Province
Core Formation
Mantle Differentiation
Ultramafic Xenoliths
Lithospheric Mantle
Potassium Feldspars
Q-Index Code C1
Q-Index Status Confirmed Code

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Created: Mon, 18 Feb 2008, 16:27:35 EST