Recycling of ancient sub-oceanic mantle in the Neo-Tethyan asthenosphere: Evidence from major and trace elements and Hf–Os isotopes of the Kop Mountain ophiolite, NE Turkey

Abstract

The asthenosphere beneath both ancient and modern oceans is highly heterogeneous owing to the recycling of continental, sub-arc, or sub-oceanic mantle domains and subducted oceanic crust and pelagic sediments. The identification and discrimination of those “old” mantle domains are important for understanding of the generation of oceanic crust and the tectonic setting of ophiolites. Here we report geochemical data including Re–Os and Lu–Hf isotopic compositions for mantle peridotites from the Kop Mountain ophiolite in the western Neo-Tethys. These peridotites have whole-rock and mineral compositions comparable with those of abyssal peridotites. Trace element modeling of clinopyroxenes indicates that they were subjected to anhydrous melting, and in this respect the rocks are distinguishable from supra-subduction zone peridotites. Re–Os and Lu–Hf isotopic compositions further indicate that these mantle peridotites underwent ancient (at or before 1.52 Ga) melt extraction and Jurassic melt metasomatism. The Kop Mountain mantle peridotites thus represent ancient sub-oceanic mantle domains recycled into the Neo-Tethyan asthenosphere, undergoing earlier melt depletion before being entrained at the Neo-Tethyan ridge axis. Our results, together with data published for other ophiolites, indicate that the Os isotopic distribution of the Neo-Tethyan ophiolites is identical to that of global abyssal peridotites, suggesting similar melting histories. The Neo-Tethyan asthenosphere was highly heterogeneous, probably with numerous recycled continental, sub-arc, and sub-oceanic mantle inputs. The recycling of ancient mantle in the asthenosphere could be an alternative explanation for the generation of anomalous compositions in both modern and ophiolitic oceanic crusts such as K-rich lavas, enriched mid-ocean ridge basalts, and arc-like basalts. The use of basalt geochemical data to discriminate between tectonic settings (i.e., mid-ocean ridge or supra-subduction zone) of ophiolites should thus be approached with caution.