HR: 09:45h
AN: V21F-08    [Abstracts]
TI: 3.2 to 3.5 Ga Re-Os Model Ages for Detrital Chromite from Jack Hills, Western Australia: Implications for Pilbara and Yilgarn Craton Evolution
AU: Valley, J W
EM: valley@geology.wisc.edu
AF: Univ. of Wisconsin, 1215 W. Dayton, Madison, WI 53706
AU: Cavosie, A J
EM: acavosie@uprm.edu
AF: Univ. of Puerto Rico, PO Box 9017, Mayaguez, PR 00681
AU: * Shirey, S
EM: shirey@dtm.ciw.edu
AF: Dept. of Terrestrial Magnetism, 5241 Broad Br. Rd. NW, Washington, DC 20015
AU: Wilde, S A
EM: S.Wilde@curtin.edu.au
AF: Curtin Univ. of Technology, Kent Street, Perth, WA 6102 Australia
AB: Detritus from the ca. 3.0 Ga Jack Hills metasedimentary belt in Western Australia has been studied in detail due to the presence of detrital zircons with ages as old as 4.4 Ga. These Archean coarse-grained clastic sediments are exceptionally mature; along with zircon, chromite is typically the only other `heavy' mineral present. Prior studies have demonstrated that a spectrum of zircon ages is preserved in the sediments; zircons older than 3.8 Ga are from unknown sources, however younger peaks in zircon age histograms correlate to known regional igneous events in the NW Yilgarn craton of 3.7-3.6, 3.5-3.4, and 2.7-2.6 Ga ages. The age of the detrital chromites has been studied relative to the zircon ages to constrain the source of the chromites. Detrital grains of chromite range from 50-500 microns in size, and occur as euhedral octahedra and rounded spherical grains, suggesting both proximal and distal populations. The range in major element chemistry for chromites of both morphologies is as follows (n=64 grains): Cr2O3=44-50 wt.%, FeO = 24-29 wt.%, Al2O3=18-22 wt.%, MnO = 1.0-4.0 wt.%, ZnO = 1.0-4.0 wt.%, and MgO = 1.0-1.6 wt.%. The Jack Hills chromites are uniformly high in ZnO, which is anomalous for chromites in the exposed Yilgarn Craton; a survey of >7500 chromite EMPA analyses from mineral exploration reports shows that values of ZnO > 1 wt.% are rare. To explore the relation between the Jack Hills chromites and zircons we have analyzed detrital chromites from samples collected along two measured sections in the metasediments for their Re and Os isotopic compositions. Each 50 mg aliquot contained hundreds of detrital grains of both morphologies. Analyses of unleached chromite yield measured ¹⁸⁷Re/¹⁸⁸Os of 0.0060 to 0.0517 and ¹⁸⁷Os/¹⁸⁸Os values of 0.1057 to 0.1081. The Re and Os isotopic compositions for three of the four chromite aliquots analyzed so far are remarkably similar given the detrital history of the grains, suggesting a common-age source for the chromites (Tma of 3.2-3.5 Ga). One chromite aliquot has a resolvably lower ¹⁸⁷Os/¹⁸⁸Os (0.1050) and might contain some material from older igneous events. These Re-Os compositions of the Jack Hills chromites are strikingly similar to Talga-Talga chromites from the Pilbara Craton (Bennett et al., 2002). While the ZnO content of the Pilbara chromites is unknown, the similarity in Re-Os chemistry suggests there may be significant overlap in composition and age. Current models for the docking of the Pilbara and Yilgarn cratons indicate that this collision is a Proterozoic event (the Capricorn Orogen). Although Proterozoic age detrital zircons have been reported from the Jack Hills (Cavosie et al., 2004), the chromites in this study are from samples that only yielded Archean age zircons. The Re-Os chemistry of the Jack Hills chromites suggests the two cratons may have had an earlier association.
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 1115 Radioisotope geochronology
DE: 3621 Mantle processes (1038)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005