HR: 1330h
AN: V42C-0373    [PDF]
TI: Precise Pt-Re-Os Isotope and PGE Systematics of 2.7 Ga Pyke Hill Komatiites, Canada
AU: * Puchtel, I S
EM: ipuchtel@geosci.uchicago.edu
AF: Dept. of Geophysical Sciences, The University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637 United States
AU: Brandon, A D
AF: NASA Johnson Space Center, Mail Code SR, Houston, TX 77058 United States
AU: Humayun, M
AF: Dept. of Geophysical Sciences, The University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637 United States
AB: The Os isotopic composition of the modern upper mantle is relatively well constrained, but only limited data on the Re-Os systematics of the Archean mantle are available. Pt-Os isotope studies have so far not been performed on Archean mantle-derived rocks, mostly due to analytical limitations and insufficient availability of appropriate geological record. In this study, precise Re-Pt-Os isotopic and PGE abundance data for 2.7 Ga Pyke Hill komatiites have been obtained. These are LILE-depleted mantle melts with 28% MgO characterized by CI chondrite-normalized Pd/Ir = 4.0, Os/Ir = 1.1, and Os = 2.3 ppb. Calculations establish a non-CI-chondritic PGE pattern for the mantle source of the lavas. This pattern is similar to that of an average depleted spinel lherzolite and of enstatite chondrites. The precise Re-Pt-Os isotopic data for bulk rock cumulate komatiite samples from a single lava flow define isochrons on the Re-Os and Pt-Os diagrams with ages of 2725+/-33 and 2843+/-640 Ma and initial 187Os/188Os = 0.10902+/-16 and 186Os/188Os = 0.1198318+/-18, respectively. These correspond to initial gamma(Os) = +0.62+/-0.15 and epsilon(Os) = +0.57+/-0.15, respectively, relative to the chondritic references currently in use. These precise initial values interpreted to represent the composition of the Abitibi komatiite mantle source are higher than those in carbonaceous chondrites, but closely match recently obtained precise Os isotopic compositions of enstatite and ordinary chondrites recalculated at the time of the lava emplacement. These data provide strong evidence that vast regions in the Archean mantle evolved with long-term HSE relative abundances similar to those in enstatite and ordinary chondrites.
DE: 1010 Chemical evolution
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting