Occurrences of orthopyroxene in primitive layered gabbros from the Hess Deep Rift, East Pacific Rise

Abstract

One of enigmas of the layered gabbros which IODP Exp.345 drilled at the Hess Deep Rift is the fact that Opx occurs as a cumulus phase. In the Multi-textured layered gabbro which constitutes the upper unit of the Hole U1415P, we can observe the following occurrences of Opx: (1) Opx+Cpx+Pl vein parallel to the layered structure of its surrounding troctolite. (2) Undeformed Opx+Pl veinlets in kinked Ol. (3) Opx in the concave of anhedral Ol. (4) Opx in the surroundings of Cr-spl adjacent to the Ol+Pl domain. The occurrences of Opx suggest the reaction between Ol and SiO₂-saturated melt.