Melt - Peridotite interactions in shallow mantle at the East Pacific Rise: Evidence from ODP Site 895 (Hess Deep)

Abstract

Ocean Drilling Program (ODP) Leg 147 recently drilled at Site 895 in Hess Deep (eastern Equatorial Pacific), where a structurally dissected section of the East Pacific Rise (EPR) is preserved, and intersected the mantle-crust transition zone of a fast-spreading centre for the first time. The core from Hole 895D (latitude 2°16.635′N, longitude 101°26.777′W) revealed that harzburgite is predominant over dunite in the top section of the Hole, but the reverse relationship is found lower in the section where dunite is closely associated with gabbroic rocks (gabbro and troctolite). Texture, mineralogy and mineral chemistry suggest a two-stage evolution for harzburgite preserved at the transition zone. Harzburgite with a porphyroclastic texture was produced by partial melting of peridotite to, or beyond the clinopyroxene-out phase boundary before or during asthenospheric (>1000°C) flow, which suggests a higher degree of mantle melting than normally expected below mid-ocean ridges. Subsequently, basaltic melt(s) interacted with this refractory harzburgite (olivine + orthopyroxene + spinel), which resulted in dissolution of orthopyroxene, re-equilibration and formation of olivine and spinel, and formation of clinopyroxene ± plagioclase, this is manifested as a progressive conversion of harzburgite to gabbroic rock through an intermediate dunite. At low melt/peridotite ratios, harzburgite was refertilised as the plagioclase component of the melt completely reacted with the peridotite matrix to produce clinopyroxene-spinel intergrowths and Al enrichment in ferromagnesian minerals. At high ratios, orthopyroxene completely dissolved incongruently, plagioclase appeared, and spinel was partially to completely resorbed; this produced olivine-bearing and olivine-free gabbroic rocks. Residual minerals in peridotites adjacent to gabbroic zones were enriched in Fe and Ti and depleted in Al.