Chemically heterogeneous asthenosphere beneath the Gakkel Ridge constrained by abyssal peridotites

The refractory mantle contributes little to the genesis of mid-ocean ridge basalts (MORB), thus observations of the component of the asthenospheric mantle based on the MORB alone are incomplete. In this study, we present both major and trace element compositions of ~70 abyssal peridotite samples from the Sparsely Magmatic Zone (SMZ) and Eastern Volcanic Zone (EVZ) of the Gakkel Ridge. Compositional data indicate that they are mantle residues of the asthenosphere after variable degrees of partial melting. Their clinopyroxenes display two different types of REE patterns, i.e., LREE-depleted and LREE-flat. The latter suggests that some Gakkel peridotites have been refertilized by quasi-instantaneous melts that retained in the melting column. The Gakkel peridotites show large geochemical variability along the ridge axis at length-scales which are too short to be thermally driven. Degrees of partial melting modelled by peridotite geochemistry are greater than those inferred seismically by crustal thicknesses in the SMZ and EVZ. This implies that compositional variations in those abyssal peridotites are inherited from prior melting. In addition, the composition of the Gakkel peridotites differs significantly from that of the subduction-related peridotites. Trace element modelling further supports the presence of a geochemically decoupled crust-mantle. We suggest that the strong heterogeneity of theasthenosphere beneath the Gakkel Ridge is the dominant driver of crust-mantle geochemical decoupling. In particular, in the SMZ region, the small amount of enriched mantle domains in the asthenosphere become the source of the enriched MORB, while massive refractory mantle inherited from prior melting hardly contributes to the SMZ basalts. Therefore, compositional signatures of asthenospheric mantle inferred from MORB of amagmatic zones along mid-ocean ridges may considerably overestimate the proportion of enriched mantle.