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

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