Isotopic systematics of ultramafic and mafic rocks from the Taitao Ophiolite, southern Chile

A variety of ultramafic and mafic rocks from a large, well-mapped sampling of the ca. 6 Ma Taitao Ophiolite, Chile, have been examined. Calculated initial 187Os/188Os ratios of the peridotitic rocks range from 0.1168 to 0.1282. Similar ranges of Os isotopic compositions have been reported for abyssal peridotites and peridotites and chromites from some other ophiolites. A correlation between the Mg # of primary olivine grains and Os isotopic compositions of whole rock peridotites suggests that the isotopic variability is due to variable extents of partial melting at approximately 1.5 Ga. This conclusion is also supported by a linear correlation between 187Re/ 188Os and 187Os/188Os, although how this relation was retained through a second, recent melting event is unclear. The ancient melting event requires that this km-scale block of the mantle remained isolated and unmixed within the convecting upper mantle for a period of ∼1.5 Ga. Calculated initial 187Os/188Os ratios of the mafic rocks are substantially enriched relative to the peridotites, suggesting that these rocks were derived via partial melting of portions of the mantle with higher, long/term Re-Os ratios than the ambient upper mantle. The results are comparable to global observations of Os isotopic compositions in mid-ocean ridge basalts (MORB) versus abyssal peridotites. At Taitao, however, the geologic relations are generally well defined.