Melt-peridotite interactions in shallow mantle at the East Pacific Rise; evidence from ODP Site 895 (Hess Deep)

Author(s): Edwards, Stephen J.; Malpas, John
Author Affiliation(s): Primary:
Manchester Metropolitan University, Department of Environmental and Geographical Sciences, Manchester, United Kingdom
Memorial University of Newfoundland, Canada
Volume Title: Mineralogical Magazine
Source: Mineralogical Magazine, 60( 1(398)), p.191-206. Publisher: Mineralogical Society, London, United Kingdom. ISSN: 0026-461X CODEN: MNLMBB
Note: In English. 40 refs.; illus., incl. 1 table, geol. sketch maps
Summary: 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.
Year of Publication: 1996
Research Program: ODP Ocean Drilling Program
Key Words: 05 Petrology, Igneous and Metamorphic; Basalts; East Pacific; East Pacific Rise; Equatorial Pacific; Hess Deep; Igneous rocks; Leg 147; Mantle; Melts; Mid-ocean ridge basalts; North Pacific; Northeast Pacific; ODP Site 895; Ocean Drilling Program; Pacific Ocean; Peridotites; Plutonic rocks; Ultramafics; Volcanic rocks
Coordinates: N021000 N022000 W1012000 W1014000
Record ID: 1996046958
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland