Résumé
The composition of the intrusive gabbroic lower oceanic crust remains poorly characterized in comparison to the extrusive portion of the oceanic crust, especially for intermediate-fast spreading mid-ocean ridges. This is a consequence of limited exposures of extant lower oceanic crust or ophiolites similar to mid-ocean ridge crust. One of the best analogues for mid-ocean ridge crust is the southern Samail ophiolite that formed during a period of rapid seafloor spreading above a nascent subduction zone. Here, we focus on the geochemical stratigraphy (whole rock and mineral major and trace element compositions) of the 5,200 m-thick, lower crustal, Wadi Khafifah section of the Wadin Tayin massif in the southern Samail (Oman) ophiolite. Gabbros from the lowermost 3,700 m of this section (the “lower gabbros”) show no systematic changes in composition with height above the Mantle Transition Zone. In contrast, gabbros from the uppermost 1,500 m (the “upper gabbros”) display marked increases in incompatible trace element concentration with increasing height. Liquids in equilibrium with the lower gabbros have major and trace element compositions that overlap with those measured in the upper gabbros and sheeted dikes. Upper gabbros preserve mineral cores with primitive major element compositions that overlap with the range of lower gabbros; however, upper gabbro whole rock compositions are significantly more enriched in incompatible trace elements relative to the lower gabbros. Our data reveal that the upper gabbros are a composite of accumulated minerals derived from primitive melts and a large fraction of evolved melts derived from the fractionation of the lower gabbros. We propose a new “Full Sheeted Sills” model for the lower oceanic crust in which primitive magmas from the mantle are emplaced throughout the lower crust and crystallized in situ. After diking events, evolved magmas leave the lower gabbros and replenish the upper gabbros, thereby contributing to the higher incompatible trace element budget in the upper gabbros relative to the lower gabbros. Our reconstructed bulk compositions of the lower plutonic crust and the bulk oceanic crust from the Wadi Khafifah section yield a plausible primary mantle-derived magma composition in equilibrium with depleted mid-ocean ridge basalts mantle.