Abstract
Mantle processes control plate tectonics and exert an influence on biogeochemical cycles. However, the proportion of mantle sampled in-situ is minimal, as it is buried beneath igneous crust and sediments. Here we report the lithological characteristics of two mantle sections from an embryonic ocean drilled by the International Ocean Discovery Program (IODP) in the Tyrrhenian Sea. Contrary to the mantle drilled at Mid Ocean Ridges (MORs) and hyperextended passive margins, our findings reveal exceptionally heterogeneous and fertile mantle lithologies, ranging from fertile lherzolites to depleted harzburgites and dunites, interlayered with pyroxenites. Plagioclase- and clinopyroxene-rich layers, hydrous potassic magmatic veins, and mafic intrusions indicate substantial mantle refertilization and delayed inception of magmatic crust. We propose that magma-poor rifts do not require a chemically depleted mantle, too refractory to melt. Deep lithospheric processes such as mantle refertilization and prolonged lithospheric thinning delayed melt focusing and the formation of a steady-state spreading center.