Genesis of Oceanic Oxide Gabbros and Gabbronorites During Reactive Melt Migration at Transform Walls (Doldrums Megatransform System; 7–8°N Mid-Atlantic Ridge)

Abstract The Doldrums Megatransform System (~7–8°N, Mid-Atlantic Ridge) shows a complex architecture including four intra-transform ridge segments bounded by five active transform faults. Lower crustal rocks are exposed along the and Vernadsky walls that bound northernmost segment. recovered gabbros characterized variably evolved chemical compositions, ranging from olivine to gabbronorites oxide gabbros, lack most primitive gabbroic endmembers (troctolites, dunites). Notably, numerous show up 20 vol. % of coarse-grained orthopyroxene. Although covariations in mineral bulk-rock compositions define trends crystallization common parental melt, elevated light over heavy rare earth elements (LREE/HREE) ratios both compositions. These features not consistent with petrological evolution driven solely fractional crystallization, which cannot produce preferential enrichments highly incompatible documented orthopyroxene-bearing lithologies. We suggest crystallized melts percolating partly assimilating pre-existing gabbro matrix. Saturation orthopyroxene selective LREE relative M-HREE triggered an increase assimilated crystal mass, ranges negligible oxide-gabbros abundant gabbronorites. This melt–rock reaction process has been related lateral melt migration beneath ridge-transform intersections, where injected peripheral parts melting region towards zone may interact mush form oxide-bearing gabbronoritic associations.