HR: 1340h
AN: V33E-1496    [Abstracts]
TI: Re-Os systematics of komatiites and komatiitic basalts at Dundonald Beach, Ontario, Canada: Evidence for a complex alteration history and implications of a late-Archean chondritic mantle source
AU: * Gangopadhyay, A
EM: amitava@geol.umd.edu
AF: Isotope Geochemistry Laboratory, Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Sproule, R A
EM: rsproule@nickel.laurentian.ca
AF: Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, Sudbury, ON P3E 6B5 Canada
AU: Walker, R J
EM: rjwalker@geol.umd.edu
AF: Isotope Geochemistry Laboratory, Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Lesher, C
EM: lesher@sympatico.ca
AF: Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, Sudbury, ON P3E 6B5 Canada
AB: Re-Os concentrations and isotopic compositions have been examined in one komatiite unit and one komatiitic basalt unit at Dundonald Beach, which is part of the spatially-extensive 2.7 Ga Kidd-Munro volcanic assemblage in the Abitibi greenstone belt, Ontario, Canada. The komatiitic rocks in this locality record at least three episodes of alteration of Re-Os elemental and isotope systematics. First, an average of 40% and as much as 75% Re was lost due to shallow degassing during eruption and/or hydrothermal leaching during or immediately after the lava emplacement. Second, the Re-Os isotope systematics of the rocks with $^{187}$Re/$^{188}$Os ratios $>$1 were reset at $\sim$2.5 Ga, most likely due to a regional metamorphic event. Finally, there is evidence for relatively recent gain and loss of Re. The variations in Os concentrations in the Dundonald komatiites yield a relative bulk distribution coefficient for Os (D$_{Os}$ $^{solid/liquid}$) of 2-4, consistent with those obtained for stratigraphically-equivalent komatiites in the nearby Alexo area and in Munro Township. This suggests that Os was moderately compatible during crystal-liquid fractionation of the magma parental to the Kidd-Munro komatiitic rocks. Furthermore, whole-rock samples and chromite separates with low $^{187}$Re/$^{188}$Os ratios ($<$1) yield a precise chondritic average initial $^{187}$Os/$^{188}$Os ratio of 0.1083 $\pm$ 0.0006 (\gamma$_{Os}$ = 0.0 $\pm$ 0.6). The chondritic initial Os isotopic composition of the mantle source for the Dundonald rocks is consistent with that determined for komatiites in the Alexo area and in Munro Township. Our Os isotope results for the Dundonald komatiitic rocks, combined with those in the Alexo and Pyke Hill areas suggest that the mantle source region for the Kidd- Munro volcanic assemblage had evolved along a long-term chondritic Os isotopic trajectory until their eruption at $\sim$2.7 Ga. The chondritic initial Os isotopic composition of the Kidd-Munro komatiites is indistinguishable from that of the projected contemporaneous convective upper mantle. The uniform chondritic Os isotopic composition of the $\sim$2.7 Ga mantle source for the Kidd-Munro komatiites contrasts with the typical large-scale Os isotopic heterogeneity in the mantle sources for komatiites from the Gorgona Island, present-day ocean island basalts or arc-related lavas. This suggests a significantly more homogeneous mantle source in the Archean compared to the presentday mantle.
DE: 1025 Composition of the mantle
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1099 General or miscellaneous
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting