HR: 0800h
AN: V31C-1454    [Abstracts]
TI: Re-Os Systematics of Variably Altered Ultramafic Rocks, North Carolina.
AU: * Centorbi, T L
EM: tlcento@geol.umd.edu
AF: Isotope Geochemisty Laboratory, Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Walker, R J
EM: rjwalker@geol.umd.edu
AF: Isotope Geochemisty Laboratory, Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Minarik, W G
EM: minarik@geol.umd.edu
AF: Isotope Geochemisty Laboratory, Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Minarik, W G
EM: minarik@geol.umd.edu
AF: Earth & Planetary Sciences Department, McGill University, 3450 University Street, Montreal, H3A 2A7 Canada
AB: Dunitic ultramafic bodies, presumed to be pieces of dismembered Ordovician ophiolites, crop out along strike within the Blue Ridge Province of the southern Appalachians. These rocks were metamorphosed under highly different conditions. Pristine olivine dominating some bodies may represent either primary mineralogy, or recrystallization under dry conditions. Highly serpentinized rocks, more common in other ophiolites, reflect either post-formation hydrous alteration of peridotite in the upper mantle or lower crust, or effects resulting from emplacement. All of the bodies contain chromian spinel. The presence of spinel with very low Re/Os and high Os concentration allows comparison of putative initial $^{187}$Os/$^{188}$Os for each body with initial isotopic compositions calculated for bulk rocks and olivine separates for the variably altered bodies. These results ultimately should allow a better understanding of alteration effects on the Os isotopic systematics of other ophiolite assemblages. The concentration of Os in spinel separates range from 1.7 to 340 ppb whereas bulk rock varies from 0.9 to 8.4 ppb. Whole rock and spinel separates have present day initial $^{187}$Os/$^{188}$Os that range from 0.1141 to 0.1327. Spinels have $^{187}$Re/$^{188}$Os of 0.0001 to 0.0832 and bulk rocks range from 0.0398 to 1.9712. Initial ratios calculated for 490 Ma range from 0.1027 to 0.1327. Data for some spinel-whole rock pairs indicate significantly different $^{187}$Os/$^{188}$Os at the time of formation on a very local scale. There is also evidence for late stage additions of Re to some whole rocks. Present day $^{187}$Os/$^{188}$Os as low as 0.1141 in spinel indicates melt depletion in precursor materials a minimum of 1.9 Ga ago.
DE: 3660 Metamorphic petrology
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting