Partial Melting of the Lower Oceanic Crust: Implications for Tracing the Slab Component in the Source of Mid‐Ocean Ridge Basalts

Abstract

AbstractThe potential role of the lower oceanic crust in the source region of mid‐ocean ridge basalts has been examined by a series of partial melting experiments on three natural pyroxenite xenoliths entrained in alkali basalts well representing the lower oceanic crustal material at 1 GPa, 1100–1300°C. Layered experiments were also performed to simulate the reaction between lower oceanic crust‐derived melts and surrounding dunite and have successfully enabled the melt infiltration. Melt compositions are consistently basaltic and vary significantly as a function of melt fraction, and these lower oceanic crust‐derived melts are characterized by high Eu/Eu*, Nb/Ta, and Ba/Th values, especially for experiments conducted at 1300°C. No reaction zone is observed in layered experiments, and our results demonstrate that the compositional variations caused by the reaction with dunitic mantle in the lower oceanic crust‐derived melts are relatively minor at 1 GPa. Our melting experiments show that melting of the lower oceanic crust can provide trace element characteristics such as positive Sr and Eu anomalies and consistent Nb/Ta and Zr/Hf ratios, but it cannot account for the wide major element compositional variations (e.g., high FeOT and TiO2 contents) in mid‐ocean ridge basalt (MORB). Some component other than the surrounding dunite in the upwelling mantle flux is needed to substantially modify the major element compositions of the lower oceanic crust‐derived melts. It is quite likely that these lower oceanic crust‐derived melts could be extracted from the upwelling mantle while still preserving their trace element and isotopic imprintings.