Biotite in olivine gabbros from Atlantis Bank: Evidence for amphibolite-facies metasomatic alteration of the lower oceanic crust

Gabbroic rocks recovered from deep holes drilled in the ocean floor provide us with valuable information about in-situ alteration processes of the lower oceanic crust. We found that the occurrence of biotite is widespread in gabbroic rocks recently drilled from IODP Hole U1473A at Atlantis Bank, near the Southwest Indian Ridge. Biotite is rare in oceanic gabbros, thus we analyzed textural and compositional details of biotite and associated minerals to better understand the conditions governing their formation. In olivine gabbros from Hole U1473A and from nearby ODP Hole 735B, biotite occurs mainly in coronitic aggregates mantling olivine. It also forms monomineralic veins or occurs in biotite-chlorite-amphibole veins within plagioclase grains that contact the coronitic aggregates. The coronitic aggregates typically have an outer biotite-dominated zone and an inner zone mostly made up of Al-poor calcic amphibole. The biotite and the calcic amphibole zones frequently include Al-rich calcic amphibole (± cummingtonite) and talc, respectively. Plagioclase in direct contact with Al-rich calcic amphibole has 65–90 mol% anorthite. The coronitic aggregates also frequently have an outermost zone composed of submicron-scale biotite-chlorite mixtures, which show intermediate optical and chemical characteristics between biotite and chlorite, and a composite pattern of Raman-shift spectra. Most biotite-rich coronitic aggregates occur in proximity to felsic veins or to biotite or alkali feldspar microveins branched from felsic veins, whereas most biotite-chlorite coronas are connected to biotite-, chlorite- or amphibole-bearing microveins. Chlorite coronas around olivine, though rare in Atlantis Bank gabbros, occur in contact with chlorite-bearing microveins and show no relationship with felsic veins. Based on the plagioclase-amphibole equilibrium, we evaluated temperature of 750–850 °C for the formation of the biotite-rich coronitic aggregates. From the modes of occurrence, compositions of minerals, and thermodynamic modeling, we conclude that the biotite coronas formed at higher temperatures and higher SiO₂ and/or K⁺/H⁺ activities than chlorite coronas typically found in olivine gabbros from other mid-ocean ridge localities. The coronitic biotite-chlorite mixtures formed in response to lower SiO₂ and/or K⁺/H⁺ activities, and possibly lower temperature, than the biotite coronas. Such a difference in physical and chemical conditions for corona formation probably reflects the distance from felsic vein/microvein or is related to the relative timing of reactions. The high-temperature metasomatic alteration of lower oceanic crustal gabbros shown in this study is most likely characteristic of oceanic core complexes from ultraslow-spreading ridges.