09:15 〜 09:30
[SCG41-02] Oxidation controlled symplectitic formation in olivine gabbros from Oman ophiolite, Oman Drilling Project, Hole GT2A
キーワード：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 Fo75-76 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.
References:
Connolly, J.A.D. (2005) Computation of phase equilibria by linear programming: A tool for geodynamic modeling and its application to subduction zone decarbonation. Earth and Planetary Science Letters, 236, 24–541.
Connolly, J.A.D. (2009) The geodynamic equation of state: What and how, Geochemistry. Geophysics, Geosystems, 10, 1–19.
Gallien, F., Mogessie, A., Hauzenberger, C.A., and Bjerg, E., et al. (2012) On the origin of multi-layer coronas between olivine and plagioclase at the gabbro–granulite transition, Valle Fertil–La Huerta Ranges, San Juan Province, Argentina. Journal of Metamorphic Geology, 30, 281–301.
Goode, A. (1974) Oxidation of natural olivines. Nature, 248, 500–501.
Holland, T.J.B. and Powell, R. (1998) An internally consistent thermodynamic data set for phases of petrological interest. Journal of Metamorphic Geology, 16, 309–343.
Joesten, R. (1986) The role of magmatic reaction, diffusion and annealing in the evolution of coronitic microstructure in troctolitic gabbro from Risor, Norway. Mineralogical Magazine, 50, 441–67.
Kelemen, P.B., Matter, J.M., Teagle, D.A.H., Coggon, J.A., and the Oman Drilling Project Science Team (2020) Site GT2: foliated to layered gabbro transition. Proceedings of the Oman Drilling Project: College Station, TX (International Ocean Discovery Program).
Klemme, S. Ivanic, T.J., Connolly, J.A.D., Harte, B. (2009) Thermodynamic modelling of Cr-bearing garnets with implications for diamond inclusions and peridotite xenoliths. Lithos, 112, 986–991.
Powell, R., & Holland, T. (1999) Relating formulations of the thermodynamics of mineral solid solutions: activity modeling of pyroxenes, amphiboles, and micas. American Mineralogist, 84, 1–14.
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 Fo75-76 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.
References:
Connolly, J.A.D. (2005) Computation of phase equilibria by linear programming: A tool for geodynamic modeling and its application to subduction zone decarbonation. Earth and Planetary Science Letters, 236, 24–541.
Connolly, J.A.D. (2009) The geodynamic equation of state: What and how, Geochemistry. Geophysics, Geosystems, 10, 1–19.
Gallien, F., Mogessie, A., Hauzenberger, C.A., and Bjerg, E., et al. (2012) On the origin of multi-layer coronas between olivine and plagioclase at the gabbro–granulite transition, Valle Fertil–La Huerta Ranges, San Juan Province, Argentina. Journal of Metamorphic Geology, 30, 281–301.
Goode, A. (1974) Oxidation of natural olivines. Nature, 248, 500–501.
Holland, T.J.B. and Powell, R. (1998) An internally consistent thermodynamic data set for phases of petrological interest. Journal of Metamorphic Geology, 16, 309–343.
Joesten, R. (1986) The role of magmatic reaction, diffusion and annealing in the evolution of coronitic microstructure in troctolitic gabbro from Risor, Norway. Mineralogical Magazine, 50, 441–67.
Kelemen, P.B., Matter, J.M., Teagle, D.A.H., Coggon, J.A., and the Oman Drilling Project Science Team (2020) Site GT2: foliated to layered gabbro transition. Proceedings of the Oman Drilling Project: College Station, TX (International Ocean Discovery Program).
Klemme, S. Ivanic, T.J., Connolly, J.A.D., Harte, B. (2009) Thermodynamic modelling of Cr-bearing garnets with implications for diamond inclusions and peridotite xenoliths. Lithos, 112, 986–991.
Powell, R., & Holland, T. (1999) Relating formulations of the thermodynamics of mineral solid solutions: activity modeling of pyroxenes, amphiboles, and micas. American Mineralogist, 84, 1–14.