Re-Os Systematics of Variably Altered Ultramafic Rocks, North Carolina.

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 ¹⁸⁷Os/¹⁸⁸Os 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 ¹⁸⁷Os/¹⁸⁸Os that range from 0.1141 to 0.1327. Spinels have ¹⁸⁷Re/¹⁸⁸Os 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 ¹⁸⁷Os/¹⁸⁸Os 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 ¹⁸⁷Os/¹⁸⁸Os as low as 0.1141 in spinel indicates melt depletion in precursor materials a minimum of 1.9 Ga ago.