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[SCG57-11] Deformation and metasomatic histories of Pinatubo peridotite estimated from microstructural observation
Keywords:Olivine, Deformation, Metasomatism, Slip system
Pinatubo peridotites are composed of relatively coarse olivine grains with several mm in size, and partly fine orthopyroxene and amphibole aggregate with tens to hundreds µm in size. The fine grained aggregate in P-4 is more than in P-3. The grain boundaries of the coarse grains exhibit irregular shapes. And these grains show the undulose extinction and well-developed subgrain boundaries. The coarse olivine grains contain a lot of fluid inclusions. Additionally, the fine olivine grains exist along the secondary inclusions within some coarse olivine grains.
The LPO patterns of the coarse olivine grains in the both samples imply the dominant activation of [100]{0kl} slip system, which is developed under high temperature, low pressure and dry deformation conditions. On the other hand, the dominant slip systems obtained from the direct characterization of dislocations by TEM are [100](001), [001](010) or [001](100), which activate under moderate to high water content condition. The discrepancy of the obtained slip systems is probably caused by the overprinting due to the changing of deformation conditions. Dislocation microstructures are more easily modified by later deformation events than LPOs.
We conclude from the characterization of microstructures in combined with the result of Kawamoto et al. (2013) that Pinatubo peridotite experienced deformation and metasomatic events in the following manner. Pinatubo peridotites have originally deformed under high temperature, low pressure and dry conditions in the back-arc region. Then, they have moved to the fore-arc region due to the corner flow, and have been affected by the fluid-related metasomatism. Finally, they have undergone the annealing process within the upwelling magma.
Referenece: Kawamoto et al. (2013) PNAS, 110, 9663.