Oxide gabbro: from detachment/transform faults to subduction.

Along slow and ultra-slow spreading detachment and transform faults, a large abundance of oxide gabbro is documented and thought to represent the injection of a differentiated Fe-Ti-(V)-(P) saturated nelsonitic melt in gabbroic mushes. The oxide-rich gabbro carapace observed on detachment faults is interpreted as a deformation-assisted melt migration playing a critical role in rock weakening, strain localization, exhumation and evolution of core complexes. Fe-Ti-rich metagabbros are also often found in exhumed high-pressure low-temperature oceanic metamorphic units which allow to understand how they transform during subduction. The aim of this study is to constrain the composition and deformation of oxide gabbros from transform faults and to compare it to (1) oxide gabbros and processes at detachment faults and (2) subducted and exhumed oxide gabbros to characterize their variability and infer their chemical and rheological impacts on the subduction system.

This study focuses on samples collected in the Atlantic Ocean with the submersible Nautile along the northern and southern Vema transform valley walls during the Vemanaute campaign. The nelsonitic melt led to the crystallization of large amount (~10-60%) of V-rich ilmenite and titanomagnetite, F-rich amphibole, olivine and variable amount of apatite, which pervasively intrude the primary gabbro. Thermometric estimates on amphiboles suggest a crystallization at around 800-900°C. Some samples are mylonitic and textures suggest that deformation was coeval with melt infiltration and crystallization. Fe-Ti oxides do not show any internal deformation suggesting rock hardening following melt-rock reaction. Vema oxide gabbros are nonetheless impacted by subsequent hydrothermal alteration with the pseudomorphic replacement of pyroxene into Cl-rich amphibole (~500-700°C) and late alteration phases (e.g., clay).  

Oxide gabbros therefore play a similar role in weakening the oceanic lithosphere on both transform and detachment faults. However, apatite-rich oxide gabbros (with apatite content up to 50% of the melt products) found at the Vema transform fault are not described in detachment faults. Furthermore, melt-rock interactions produce amphibole and olivine in transform faults whereas pyroxene is formed at detachment faults. This indicates important differences on the composition of the nelsonitic melt, with potential differences on the melt source or degree of differentiation. In the case of transform faults, the presence of hydrated phases indicates a hydrated source.

In both cases, the nelsonitic melt intrusion induces a localized drastic change in the rheology and bulk composition of gabbros which will likely hold significant chemical and rheological implications during subduction, particularly along the subduction interface. This phenomenon can be further explored in exhumed ophiolite, such as in Syros, Greece, interpreted as a preserved coherent fragment of a discontinuous, slow-spreading oceanic domain. There, Fe-Ti-rich gabbros consist of blocks distributed in a serpentinite matrix. They play a major role as a calcium source and as a lithological discontinuity localizing fluid pathways which allow to pervasively metasomatize the serpentinite matrix (half of it) into a tremolite-chlorite-talc schist. Such diffuse transformation of a serpentinite unit is bond to impact the chemical and rheological behavior of the subduction interface.