[S-CG41] Hard-Rock Drilling Science: From Continental to Deep Sea Drilling, and Oman Project

[SCG41-02] Oxidation controlled symplectitic formation in olivine gabbros from Oman ophiolite, Oman Drilling Project, Hole GT2A

*Sayantani Chatterjee¹, Debaditya Bandyopadhyay^2,3, Eiichi Takazawa^1,4, Katsuyoshi Michibayashi⁵ (1.Department of Geology, Faculty of Science, Niigata University, Niigata 950–2181, Japan, 2.Department of Geology, University of North Bengal, Darjeeling 734013, India, 3.Department of Geology, University of Calcutta, Kolkata 700019, India, 4.Japan Agency for Marine–earth Science and Technology, Yokosuka 237–0061, Japan, 5.Graduate School of Environmental Studies, Nagoya University, Nagoya 464–8602, Japan)

Keywords:Olivine gabbro, Orthopyroxene-magnetite symplectite, Lower crust, Oman ophiolite

Symplectites are vermicular intergrowth of two or more mineral phases that inferred to be formed via melt-rock reaction, solid-state transformation, oxidation, or during different stages of magmatic crystallization. Here we report orthopyroxene-magnetite symplectitic intergrowth partly surrounding olivine within lower crustal olivine gabbros from Oman ophiolite recovered during Oman drilling project at Hole GT2A. The symplectites are found to be only associated with olivine and typically characterized by orthopyroxene groundmass hosting magnetite lamella. The width of the homogeneous grain of orthopyroxene ranges up to 300μm whereas the width of vermicular intergrowth of magnetite within orthopyroxene ranges between 20-30μm. The intergrowth of fine-grained magnetite rods is sometimes typified by the perpendicular arrangement with respect to the contact of adjacent olivine. Further, fine-grained lamella of magnetite within orthopyroxene generally grades away into irregular coarser-grained variety with increasing distance from olivine. From closer petrographic observation and previously inferred depth of formation of the sample (i.e., ~1kbar) we conclude this texture does not involve reaction (or significant material transfer) between two minerals (e.g., olivine and plagioclase). We thus infer such vermicular intergrowth is formed via olivine oxidation involving the reaction

Olivine + O₂ → Orthopyroxene + Magnetite

In order to test this hypothesis, we have adapted the equilibrium phase equilibria approach and computed several Temperature – Composition diagrams in a fixed pressure (1 kbar) to observe the oxidation effects in different olivine compositions. Our experiments predict the coexistence of olivine with Fo_75-76 and Fo₇₁ with the orthopyroxene (En₇₉ and En₇₆) respectively; which is remarkably similar to the obtained mineral chemistry. Our result suggests the symplectite growth may have occurred between 600-1000°C via subsolidus olivine oxidation and/or reaction with oxidizing melt.

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