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Melt-Peridotite Interactions in Shallow Mantle at the East Pacific Rise: Evidence from ODP Site 895 (Hess Deep)

Published online by Cambridge University Press:  05 July 2018

Stephen J. Edwards  and
John Malpas
Stephen J. Edwards
Affiliation:
Department of Environmental and Geographical Sciences, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK
John Malpas
Affiliation:
Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland A1B 3X5, Canada
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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.

Keywords

harzburgitemid-ocean ridge basaltmelt-peridotite interactionmantle refertilizationHess DeepEast Pacific RiseODP Leg 147
Type
The 1995 Hallimond Lecture
Information
Mineralogical Magazine , Volume 60 , Issue 398 , February 1996 , pp. 191 - 206
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1996

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Footnotes

*

Present address: Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong.

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