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

Plastic Deformation of Plagioclase in Oceanic Gabbro Accreted at a Slow‐Spreading Ridge (Hole U1473A, Atlantis Bank, Southwest Indian Ridge)

“…This effect of the progressive recrystallization can combine with the change in the dominant active slip system at decreasing deformation temperatures to produce the observed fabric strength decrease. Other mechanisms than dislocation creep could also account for this fabric strength decrease, such as the heterogeneous nucleation of amphibole grains (up to ∼14% in ultramylonites) or the activation of diffusion creep thanks to hydrothermal fluid circulation (e.g., Mehl & Hirth, 2008; Nozaka et al., 2019). In addition, the reduced grain size in ultramylonites (median = 21 μm) and the likely decrease in deformation temperature in the presence of hydrothermal fluids, which must induce significant chemical potential gradients, could favor the activity of grain‐size‐sensitive processes such as granular flow (e.g., Mehl & Hirth, 2008; Oliot et al., 2010, 2014; Paterson, 1995; Stünitz & Fitz Gerald, 1993).…”

“…A decrease of 2%–3% in An content is measured from the core to rims in some samples, as well as in partially albitized gabbros where it decreases to An 32 (Nozaka et al., 2019). Clinopyroxene is augite; its Mg# varies from 0.48 to 0.86 with a mean at 0.74 in gabbros and olivine‐gabbros, and 0.70 in oxide‐bearing‐gabbros (Dick, Kvassnes, et al., 2019; Nguyen et al., 2018; Nozaka et al., 2019). The Mg# of olivines varies from 0.46 to 0.80 in all gabbroic rocks (Dick, Kvassnes, et al., 2019; Nguyen et al., 2018; Nozaka et al., 2019).…”

“…The Mg# of olivines varies from 0.46 to 0.80 in all gabbroic rocks (Dick, Kvassnes, et al., 2019; Nguyen et al., 2018; Nozaka et al., 2019). Orthopyroxene is hypersthene, and has a Mg# varying from 0.56 to 0.79, with a mean at 0.67 (Dick, Kvassnes, et al., 2019; Nozaka et al., 2019).…”

“…Clinopyroxene is augite; its Mg# varies from 0.48 to 0.86 with a mean at 0.74 in gabbros and olivine‐gabbros, and 0.70 in oxide‐bearing‐gabbros (Dick, Kvassnes, et al., 2019; Nguyen et al., 2018; Nozaka et al., 2019). The Mg# of olivines varies from 0.46 to 0.80 in all gabbroic rocks (Dick, Kvassnes, et al., 2019; Nguyen et al., 2018; Nozaka et al., 2019). Orthopyroxene is hypersthene, and has a Mg# varying from 0.56 to 0.79, with a mean at 0.67 (Dick, Kvassnes, et al., 2019; Nozaka et al., 2019).…”

“…Plagioclase found in Hole U1473A gabbroic lithologies has a composition ranging between andesine and labradorite (An 36 to An 71 ), with more An rich compositions in gabbros and olivine‐gabbros (An 40 to An 71 ) than in oxide‐bearing‐gabbros (An 36 to An 45 , Dick, Kvassnes, et al., 2019; Nguyen et al., 2018; Nozaka et al., 2019). A decrease of 2%–3% in An content is measured from the core to rims in some samples, as well as in partially albitized gabbros where it decreases to An 32 (Nozaka et al., 2019). Clinopyroxene is augite; its Mg# varies from 0.48 to 0.86 with a mean at 0.74 in gabbros and olivine‐gabbros, and 0.70 in oxide‐bearing‐gabbros (Dick, Kvassnes, et al., 2019; Nguyen et al., 2018; Nozaka et al., 2019).…”