Reactive flow as dominant evolution process in the lowermost oceanic crust: evidence from olivine of the Pineto ophiolite (Corsica)

The Jurassic Pineto ophiolite from Cor- sica exposes a ~1-km-thick troctolite–olivine-gabbro sequence, interpreted to represent a lowermost sector of the gabbroic oceanic crust from a (ultra-)slow spread- ing system. To constrain the petrogenesis of the olivine- gabbros, minor and trace element analyses of olivine (for- sterite = 84–82 mol%) were carried out. Olivine from the olivine-gabbros is depleted in incompatible trace elements (Sc, V, Ti, Y, Zr and heavy rare earth elements) with respect to olivines from associated troctolites. Depleted incompat- ible element compositions are also shown by olivine (for- sterite = 86 mol%) from a clinopyroxene-rich troctolite. The incompatible element compositions of olivine argue against a petrogenetic process entirely driven by fractional crystallization. We propose that melts migrating throughan olivine–plagioclase crystal mush chemically evolved by reaction with the existing minerals, changing in com- position as it flowed upward. The melt residual from these interactions led to partial dissolution of preexisting olivine and to crystallization of clinopyroxene, generating olivine- gabbro bodies within a troctolite matrix. Reactive flow was the major evolution process active in the ~1-km crustal transect exposed at the Pineto ophiolite, producing litho- logical variations classically attributed to fractional crystal- lization processes.