RE-OS ISOTOPIC CONSTRAINTS ON THE ORIGIN OF VOLCANIC-ROCKS, GORGONA ISLAND, COLOMBIA - OS ISOTOPIC EVIDENCE FOR ANCIENT HETEROGENEITIES IN THE MANTLE

The Re-Os isotopic systematics of komatiites and spatially associated basalts from Gorgona Island, Colombia, indicate that they were produced at 155 +/- 43 Ma. Subsequent episodes of volcanism produced basalts at 88.1 +/- 3.8 Ma and picritic and basaltic lavas at ca. 58 Ma. The age for the ultramafic rocks is important because it coincides with the late-Jurassic, early-Cretaceous disassembly of Pangea, when the North- and South-American plates began to pull apart. Deep-seated mantle upwelling possibly precipitated the break-up of these continental plates and caused a tear in the subducting slab west of Gorgona, providing a rare, late-Phanerozoic conduit for the komatiitic melts. Mantle sources for the komatiites were heterogeneous with respect to Os and Pb isotopic compositions, but had homogeneous Nd isotopic compositions (epsilon-Nd = + 9 +/- 1). Initial Os-187/Os-186 normalized to carbonaceous chondrites at 155 Ma (gamma-Os) ranged from 0 to +22, and model-initial mu values ranged from 8.17 to 8.39. The excess radiogenic Os, compared with an assumed bulk-mantle evolution similar to carbonaceous chondrites, was likely produced in portions of the mantle with long-term elevated Re concentrations. The Os, Pb and Nd isotopic compositions, together with major-element constraints, suggest that the sources of the komatiites were enriched more than 1 Ga ago by low (< 20%) and variable amounts of a basalt or komatiite component. This component was added as either subducted oceanic crust or melt derived from greater depths in the mantle. These results suggest that the Re-Os isotope system may be a highly sensitive indicator of the presence of ancient subducted oceanic crust in mantle-source regions.