A distinct type of MORB formed by two-stage melting of a hybrid mantle during Gondwana breakup

Abstract

The nature of magmatism associated with the breakup of the Gondwana supercontinent remains controversial. Here we report compositions of volcanic glasses from Jurassic (∼155 Ma) seafloor adjacent to the Investigator Ridge, providing new insights on magma generation in the embryonic Indian Ocean. These samples have exceptionally primitive compositions with the highest MgO (∼10.6 wt%) content found thus far in mid-ocean-ridge basalt (MORB) glasses globally. They also have FeO-rich (∼10 wt%) compositions, strongly fractionated HREE patterns (Dy/Yb ∼1.24 versus 1.4–1.5 in the prevailing mantle), highly depleted contents of moderately incompatible elements (Zr, MREE) but elevated contents of highly incompatible elements and enriched Sr–Nd–Hf–Pb isotopic characteristics. A long-lived and hot mantle plume may not be required to explain the composition of these basalts associated with more typical but also Fe-rich MORB in the Argo Basin. Instead, we propose that these magmas can originate at normal or only moderately elevated temperatures from less magnesian mantle consisting of undepleted high-Mg# peridotite and residual, previously melted under thick continental lithosphere low-Mg# eclogite, likely MORB-like recycled oceanic crust. Re-melting of such hybrid mantle occurred during continental breakup, possibly due to induced active upwelling at continental edges and involved interaction with trace element and isotopically enriched subcontinental lithosphere. Together with basalts from the Red Sea deeps, the Jurassic rocks from the Indian Ocean represent a distinct type of MORB formed by multi-stage melting of lithologically heterogeneous mantle during continental breakup.