Crust‐mantle decoupling in the Gakkel Ridge induced by strong heterogeneity of the asthenosphere

Abstract

Abyssal peridotites can provide complementary information on the compositional features of the asthenosphere, as the refractory mantle within the asthenosphere contributes little to the genesis of mid-ocean ridge basalts (MORB). Here we present major and trace elements of ~ 70 abyssal peridotites from the Sparsely Magmatic Zone (SMZ) and Eastern Volcanic Zone (EVZ) of the Gakkel Ridge, which are residues of the asthenosphere that have undergone < 15% partial melting. Their clinopyroxenes display LREE-depleted and LREE-flat patterns, the latter of which resulted from refertilization by quasi-instantaneous melts in the melting zone beneath the mid-ocean ridge. Compositions of the Gakkel peridotites are highly variable along the ridge axis, which cannot be attributed to the spatial variation of temperature of the asthenosphere. The estimated degrees of melting of the Gakkel abyssal peridotites are higher than the values inferred by seismic thickness of ocean crust along the SMZ and EVZ. This implies the Gakkel abyssal peridotites inherit ancient melting prior to their entering the Gakkel Ridge, which also causes the crust-mantle decoupling in compositions. Moreover, compositions of the Gakkel peridotites differs significantly from subduction-related peridotites. We suggest the asthenosphere beneath the Gakkel Ridge is highly heterogeneous in compositions, which is the culprit of crust-mantle geochemical decoupling. Enriched MORB erupted in the SMZ region were derived from small amounts of enriched components within the asthenosphere, which cannot be represented by the abyssal peridotites exposed in this region.