Testing oceanic crust–mantle decoupling by Sr–Nd–Hf–Os isotopes of Neo-Tethyan ophiolites

Abstract Oceanic crust and the mantle are thought to have identical Sr–Nd–Hf–Os isotopic compositions. However, there is increasing evidence of isotopic differences between the two units, suggesting the existence of oceanic crust–mantle decoupling. Here we present updated whole-rock and mineral Sr–Nd–Hf–Os isotope data for the Kizildag (southeast Turkey) and Xigaze (south Tibet, China) Neo-Tethyan ophiolites to test if oceanic crust–mantle decoupling characterizes these ophiolites. Mantle peridotites from the Kizildag ophiolite have large variations in clinopyroxene and orthopyroxene Hf isotopic compositions (eHf (t) = −27.7 to +20.6) and unradiogenic Os isotope ratios (187Os/188Os = 0.11853–0.12720). These mantle peridotites have experienced ancient (up to ~1.6 Ga) melt depletion events, as evidenced by their old Re depletion ages (TRD = 1632–357 Ma), although Hf isotopes have been locally reset by recent melt re-fertilization, which likely occurred in a forearc basin setting. Mantle peridotites from the Xigaze ophiolite mainly have DMM (depleted mid-ocean ridge basalt mantle)-like to ultra-depleted clinopyroxene Hf isotopic compositions (eHf (t) = +9.1 to +29.8 for 17 out of 19 analyses), which were extensively overprinted by silicate melt percolation. However, the Os isotopic compositions of the Xigaze mantle peridotites are similar to those of the Kizildag ophiolite, again suggestive of ancient melt extraction long before the Early Cretaceous formation of this oceanic crust. These ancient and ultra-depleted mantle domains are not uncommon in modern oceanic lithosphere, forearc basins and ophiolites, and are less dense than the fertile mantle, and also are too refractory to contribute to the recent generation of oceanic crust. Sampling bias is unavoidable when comparing the oceanic crust and its underlying lithospheric mantle. As such, the oceanic crust alone cannot be used to estimate the isotopic composition of its entire mantle source.