Heterogeneous Earth's mantle drilled at an embryonic ocean

Sanfilippo, A.*; 安邊 啓明   ; 他27名*

Sanfilippo, A.*; Abe, Noriaki; 27 of others*

Mantle processes control plate tectonics and exert an influence on biogeochemical cycles. However, the proportion of the mantle accessible to sampling is minimal, and a significant portion of our knowledge is derived from xenoliths, orogenic peridotite massifs exposed on land, and abyssal peridotites. Sampling of in-situ sections of the mantle is extremely challenging because they are buried beneath a magmatic crust with its sedimentary cover. In this study, we report the lithological characteristics of two mantle sections from an embryonic, very young, (~3 Ma) oceanic basin drilled by the International Ocean Discovery Program (IODP) in the central Tyrrhenian Sea. In contrast to observations made in drilled mantle sections from Mid Ocean Ridges (MORs) and hyperextended passive margins such as those in Iberia and Newfoundland, our findings revealed an exceptionally heterogeneous and generally fertile mantle. This represents a snapshot of young oceanic basement in a continental rifting environment characterised by general igneous starvation. The lithological heterogeneity ranges from fertile lherzolites to depleted harzburgites and dunites, variably interlayered with pyroxenites. Plagioclase- and clinopyroxene-rich layers, hydrous potassic magmatic veins, and gabbroic intrusions in these peridotites indicate substantial mantle re-fertilization and delayed inception of magmatic crust. The diverse lithological recovery indicates that magma-poor rifts do not necessarily require a chemically depleted mantle, too refractory to melt. Instead, we propose that deep lithospheric processes such as mantle refertilisation and prolonged lithospheric thinning hinder melt focusing and the formation of a steady-state spreading centre.