Beneath slow‐spreading ridges, melt bodies are generally considered to represent ephemeral magma reservoirs filled with crystal mushes. Formation of the oceanic crust requires at least partial extraction of melts from these crystal mushes. However, melts collection and extraction are processes yet to be fully constrained. We investigate olivine gabbros from the plutonic section recovered at the IODP Hole U1473A, in the Atlantis Bank Oceanic Core Complex (Southwest Indian Ridge), to unravel (i) the process of melt migration through lower crustal crystal mushes, and (ii) the collection and segregation of melts forming discrete microgabbro intervals. Throughout the Hole, fine‐ to coarse‐grained intervals are widespread in olivine gabbros. Along the contacts, coarse‐grained minerals display resorbed grain boundaries against the fine‐grained minerals, suggesting partial dissolution by the melt crystallizing the fine‐grained material. Coarse‐grained plagioclase and clinopyroxene are zoned, showing progressive chemical evolution from more primitive crystal cores to more evolved crystal rims. Fine‐grained minerals are unzoned and chemically similar to rims of coarse‐grained minerals, indicating a genetic relationship. We attribute significant enrichments in the most incompatible elements of plagioclase and clinopyroxene to a magma evolution process associated with reactive melt migration. As temperature decreased, melts residual from the reactive processes were segregated in magma pockets that ultimately crystallize the fine‐grained intervals (microgabbros). We document, for the first time, that these microgabbros are crystallization products of melts modified by reactive melt migration; the melts were extracted from the crystal mush and accumulated into discrete melt‐rich zones. This process could have promoted partial extraction of those melts that in turn potentially contribute to Mid Ocean Ridge Basalts erupted at the seafloor.

Oceanic core complexes (OCCs) represent tectonic windows into the oceanic lower crust and mantle; they are key structures in understanding the tectono‐magmatic processes shaping the oceanic lithosphere. We present a petrological and geochemical study of gabbros collected at the Mado Megamullion, a recently discovered OCC located in the extinct Shikoku back‐arc basin. Bathymetry of the Mado Megamullion reveals spreading‐parallel corrugations extending 25 km from the breakaway to the termination. Samples from several locations include peridotites, gabbros, dolerite and rare pillow basalts. Gabbros range from granular to varitextured olivine gabbros and oxide gabbros. The emplacement of these gabbroic rocks within the oceanic lithosphere was followed by a multiphase tectono‐metamorphic evolution including: (i) dynamic recrystallization within shear zones, developed under granulite‐ to upper‐amphibolite‐facies conditions and (ii) intrusion of highly evolved melts forming felsic segregations. This tectono‐metamorphic evolution recalls that of the lower crust from other OCCs worldwide, demonstrating that this OCC exposes deep‐seated intrusions progressively exhumed by detachment faulting. Nonetheless, the Mado Megamullion lower crustal gabbros show an unusual crystal line of descent, different from what is reported from mid‐ocean ridge lower crustal rocks. We infer that the water‐bearing character of the primary melts in this back‐arc basin triggered the early precipitation of clinopyroxene, soon followed by amphibole and Fe‐Ti oxides. Such modifications in phase saturation are likely to be directly related to the back‐arc setting of the Mado Megamullion. If so, the phase assemblages of oceanic gabbros may be a diagnostic for the tectonic setting of lower crustal rocks in ophiolites.

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

Seafloor anomalies along mid-ocean ridges with exceptionally thick and compositionally distinct basaltic crust, for example, at Iceland, suggest that the underlying mantle is hotter and chemically different from the adjacent subridge mantle. Here we present hafnium and neodymium isotope ratios of peridotites from the Charlie Gibbs Transform Zone, which is located at the southern end of the Reykjanes Ridge south-west of Iceland. These peridotites are strongly depleted in incompatible elements with extremely high hafnium isotope ratios, suggesting that they had already melted to a large extent before being incorporated into the plume, at least 1 billion years ago, and thereby also became less dense. We argue that seismic velocity anomalies, geodynamic models and geochemical affinities of ridge basalts connect the peridotites from the Charlie Gibbs Transform Zone to the ‘Iceland plume’. The thermochemical buoyancy of the moderately hot Iceland plume, but also that of other plumes worldwide, may therefore be strongly influenced by composition. Variable peridotite depletion along the rising Iceland plume could also cause the transient, density-driven pulses in plume flux, which have formed the V-shaped Reykjanes Ridge south of Iceland. Overall, expansion of a ridge-centred plume along adjacent ridges and melting of heterogeneous plume material explains the topographic swell, the seismic anomaly and the formation of V-shaped ridges, as well as the regional distribution of basalts with Icelandic affinity.