A variety of ultramafic and mafic rocks from a well-mapped sampling of the ~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, peridotites, and chromites from other ophiolites. A correlation between Mg # of primary olivine and Os isotopic compositions of whole rock peridotites suggests that the isotopic variability is due to variable extents of partial melting at ~1.5 Ga. This conclusion is also supported by a linear correlation between 187Re/188Os and 187Os/188Os. 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. The results are comparable to global observations of Os isotopic compositions in mid-ocean ridge basalts versus abyssal peridotites, but the geologic relations here are well-defined.