Crystal-ring microstructures in Oceanographer Fracture Zone basalts

Pillow basalt fragments from the Oceanographer Fracture Zone contain spherical microstructures seen in section as rings of equant microphenocrysts of olivine±plagioclase±augite. They are uniformly ∼ 1 mm in diameter and have the same vitreous or crystalline material on the inside and outside. These structures occur in the same sections as vesicles and segregation vesicles and are clearly distinguishable from them. The occurrence of these structures in pillow basalt fragments suggests a mode of formation resulting from pillow basalt extrusion. Mechanical disruption of the partially solidified inner surface of the pillow skin produces a dispersion of cool globules in the pillow interior. Crystals adhere to these tacky globules during continued pillow extrusion in a manner analogous to the formation of armored mud balls.