Evidence of Melt percolation and Mantle Wedge Deformation in the Marum Ophiolite (Papua New Guinea)
Abstract:The Marum ophiolite in Papua New Guinea described an ultra-depleted mantle made by dunite and harzburgite showing compositions of supra-subduction zone peridotite. This depleted mantle has been fertilised by diffuse crystallisation of a low proportion of clinopyroxene in the dunite and formation of ol-websterite and ol-clinopyroxenite at cm scale cross-cutting the foliation and the primary pyroxenite layering. This percolating melt shows silica-rich magnesian affinities (boninite-like) related to supra-subduction zone in a fore-arc environment. The peridotite has also been percolated by a melt with more tholeiite affinities precipitating plagioclase-rich wehrlite and thin gabbroic veins.
Crystallographic preferred orientations of minerals show that clinopyroxene and orthopyroxene in the harzburgite and orthopyroxenite veins crystallized under the same deformation constraints as the depleted dunite. However, for low melt proportion, such as the clinopyroxenes in the dunite, their crystallization can be governed by epitaxial growth. Epitaxial growth is also possible in the ol-websterite where an area with low melt fraction (5% of clinopyroxenes), clinopyroxene and orthopyroxene <001> axes are parallel to olivine <100> axes, while in an area with higher melt fraction (28% of clinopyroxenes) clinopyroxenes record their own orientation with <001> axes at nearly 90˚ to the olivine <100> axes suggesting a reorientation of the constraints. The same relationship has been observed within the plagioclase-rich wehrlite suggesting the same deformation conditions during percolation of boninitic affinities melt and a more tholeiitic affinities melt.
The major inferred slip system in olivine is (001) E-type slip with the possible activation of both (001) E-type and (010) A-type slip systems, which are activated at high-temperature low-stress conditions. Olivine E-type slip is predicted to operate within the mantle wedge in a back-arc position however the Marum peridotite has a fore-arc geodynamic position and chemical composition similar to mantle from SSZ. As a consequence, the Marum peridotite is considered as mantle in front of the arc located near the E-type to B-type transition.