Hybrid troctolites from mid-ocean ridges: inherited mantle in the lower crust

Studies on olivine-rich troctolites from oceanic ridges propose that hybridized mantle rocks may locally constitute small portions of the lower oceanic crust. The exact reaction process by which they originate is still debated and their hybrid nature is controversial. We show that textural and chemical inheritances of the pre-existing mantle are preserved in olivine-rich troctolites recently sampled at the Central Indian Ridge. The occurrence of a large orthopyroxene of a probable mantle origin suggests that these rocks formed through the reactive overprint of a mantle peridotite. Combining our data with those of olivine-rich troctolites worldwide, we show that the clinopyroxenes from these rocks follow chemical trends slightly distinct to those of the oceanic gabbros. These chemical trends can be ascribed to crystallization from melts assimilating mantle peridotites, suggesting that a “mantle flavor” can be locally retained in these hybrid rocks. The present distribution of Ol-rich troctolites suggests that melt-mantle reaction processes by which these rocks originate is likely to be more diffuse at slower spreading environments, where extensive melt-rock reactions within a thick thermal boundary layer enhances the conversion of the shallow oceanic mantle into hybrid crustal rocks.