Development of oxy-symplectites in the oceanic lower crust at Atlantis Bank Oceanic Core Complex, Southwest Indian Ridge- manifestation of fluctuating oxidation state

Symplectitic intergrowth of orthopyroxene (host) with magnetite and minor ilmenite (lamellae) are recorded from Atlantis Bank, an Oceanic Core Complex on Southwest Indian Ridge. The texture is typically developed in oxide-rich gabbros recovered from the lower crustal section in this region and formed at the expense of olivine. Usually, the intergrowth primarily occurs where the olivine is in contact with magmatic magnetite and/or ilmenite. The maximum length and width of the lamellae in thin section go up to 350 µm and 20 µm respectively. The vermicular lamellae commonly maintain orthogonal relationship to the olivine grain boundary. A significant characteristic feature of this symplectitic intergrowth is the presence of domain structure, where each domain is characterized by the different orientation of vermicular lamellae. Locally, occurrences of amphibole rim abutting the intergrowth are noted. Development of orthopyroxene + Fe-Ti oxide symplectite is linked to the olivine oxidation endorsed by the interstitial Fe-Ti oxides during the progressive evolution of the lower oceanic crust at Atlantis Bank. Consequently, the occurrence of this texture dominantly from the oxide-rich lithologies bears significance. A phase equilibria modelling adopted to better comprehend the relationship between the state of oxidation and symplectite formation at Atlantis Bank indicates that this texture can be developed at relatively lower oxidation state at lower temperature for the observed range in olivine compositions. The formation temperature of the oxy-symplectites estimated from ilmenite-magnetite pairs ranges from ~730° to 450°C over an oxygen fugacity range of -0.86 to +3.83 (FMQ buffer). We suggest that, the development of this intergrowth took place under the influence of late-stage Fe-Ti oxides at subsolidus condition, which fostered the olivine oxidation. Temperature and oxygen fugacity ranges estimated from majority of the discrete ilmenite and magnetite pairs away from the symplectites however suggest relatively higher temperature and lower oxidation state (temperature ranges from 720° to 550°C, and oxygen fugacity ranges from -1.91 to +2.77 (FMQ buffer)). Higher oxygen fugacity values recorded from the symplectites further validate the effect of oxidation.