Oceanic gabbros are the most abundant rocks close to Earth's surface. Here we present new data from a consistent profile through the paleocrust of the Samail Ophiolite (Oman), which is thought to provide the best analog for modern fast-spreading oceanic crust. Incompatible trace elements of co-existing plagioclase and clinopyroxene fractionate from the mineral core to rim and up section from layered to foliated to varitextured gabbros. Layered gabbro parental melts correspond to mid-ocean-ridge basalts (MORB), and plagioclase Ca# shows a pronounced inverse zonation. Likely, they crystallized in situ from hydrous melts, compositionally buffered by replenishment at equilibrium to MORB and near steady-state boundary conditions. Further upsection, the compositional variability increases. Foliated gabbro rim and core compositions indicate increased fractionation and disequilibrium to MORB, triggered by open-system fractional crystallization within a heterogeneous magma plumbing structure, characterized by magma mixing, varying ambient water activities, and boundary conditions. Varitextured gabbros are chemically diverse with parental melts partially more primitive than MORB, suggesting that primitive melts directly reach the axial melt lens (AML). REE-in-plagioclase-clinopyroxene thermometry compared to and supported by anorthite-in-plagioclase thermometry reveals a relationship of (Formula presented.) [°C] = 6.1 ± 0.2 × An + 706 ± 19. Crystallization temperatures of the layered gabbros cover a narrow range of 1216 ± 14°C. Considerable temperature variability of 1077–1231°C is observed further upsection, featuring a thermal minimum within the foliated gabbros. This minimum is assumed to represent a zone where the fractionated descending crystal mushes originating from the AML meet with evolved liquids expelled from deeper crustal levels.