Isotopic variation in Semail Ophiolite lower crust reveals crustal-level melt aggregation

The scale and magnitude of compositional heterogeneity in the mantle has important implications for the understanding of the evolution of Earth. Heterogeneity of the upper mantle is often evaluated based on mid-ocean ridge basalt compositions, despite their homogenisation prior to eruption. In this study we present Nd and Sr isotope data obtained by micro-drilling single plagioclase and clinopyroxene crystals in gabbroic cumulates of the Semail Ophiolite (Oman) and show that mantle source variability is better preserved in the lower crust than in the extrusive suite. Analysis of sub-nanogram quantities of Nd in plagioclase revealed a range in Nd-143/Nd-144(i) in the Wadi Abyad crustal section that is three times greater than recorded in the extrusive suite. The isotopic variability is preserved in plagioclase, whereas clinopyroxene is isotopically homogeneous. These data imply that the mantle is heterogeneous on the scale of melt extraction, and that a significant proportion of homogenisation of erupted melts occurs in the oceanic crust, not the mantle.