Re-Os Isotopic Systematics of the Taitao Ophiolite, Southern Chile

The Os isotopic compositions of some ophiolitic materials may record the Os isotopic composition of the convecting upper mantle through time. Interpretation of Os data for old ophiolites can be compromised by post-formation open-system behavior, e.g. recent Re gain or loss. Such effects are particularly exacerbated in mafic rocks that dominate the upper crustal portions of most ophiolites. The high Re/Os of old mafic rocks also require large corrections in calculating initial ¹⁸⁷Os/¹⁸⁸Os. Study of a very young ophiolite should reduce some of the age inherent issues. The Taitao Ophiolite is located just 50 km south of the present-day Chile Triple Junction and may be less than 10 Ma. We have examined a variety of ultramafic and mafic rocks from a large areal sampling of the ophiolite. The calculated initial ¹⁸⁷Os/¹⁸⁸Os ratios of most mafic rocks are substantially enriched relative to chondritic, suggesting a transfer of radiogenic Os to the rocks or their mantle sources from either subducting crust or seawater. Peridotitic rocks all have Os concentrations consistent with concentrations present in the upper mantle. The calculated initial ¹⁸⁷Os/¹⁸⁸Os ratios of these rocks range from 0.1236 to 0.1260. Such a range of isotopic compositions for a single ophiolite is typical and may reflect melt enrichment processes or fluid transport of radiogenic Os. Linking the Os isotopic compositions of whole rocks with Sr and O will help to constrain the processes involved. Additional analysis of separated chromites will help to define the absolute range in Os isotopic compositions throughout the ophiolite.