[S-CG58] 岩石―流体相互作用の新展開：表層から沈み込み帯深部まで

[SCG58-P15] Plagioclase Replacement by Epidote during Hydrothermal Alteration of Gabbro from the Khantaishir ophiolite, Western Mongolia

*OTGONBAYAR DANDAR¹、Atsushi Okamoto¹、Masaoki Uno¹、Noriyoshi Tsuchiya¹ (1.Graduate School of Environmental Studies, Tohoku University)

キーワード：Hydrothermal alteration, Epidote, Gabbro , Khantaishir ophiolite, western Mongolia

Fluid flow in oceanic crust triggers element and heat transport, accelerates hydration reactions, changes mechanical and rheological properties, and has a key role in ore deposit formation and crustal deformation ^e.g.,1. Hydrothermal alteration of mafic volcanic rocks and plagiogranites in oceanic crust produces usually epidosite which is consisted mainly of epidote and quartz ^{e.g., 2}. Formation of epidosite is thought to be result of enormous water/rock ratios and be link to ore deposits (for example volcanic massive sulfide) ^e.g.,2-3. In contrast to fluid infiltration into volcanic rock (porous and high permeability than gabbro layer), fluid infiltration into gabbro layer is mainly proceeded along fractures ^{e.g., 4}. Although intensively altered gabbro bodies are found in many localities, detailed mechanism of (1) element and fluid transport, and (2) porosity change on hydrothermal alteration of gabbro is lacking. To gain more understanding of such processes, we investigate hydrothermally altered gabbro body in the Khantaishir ophiolite, western Mongolia. Altered gabbro samples are composed of primary minerals (pyroxene and amphibole) and secondary minerals (epidote, chlorite, albite, and amphibole with minor amount of quartz). Pyroxene contains amphibole inclusions and has diopsidic composition. Primary plagioclase is not remained and is altered into mostly epidote (consisted of poly crystals) with minor amount of chlorite and albite whereas some of pyroxene are replaced by chlorite remaining exsolution of amphibole. Primary amphibole shows zoning from hornblendic composition at core to actinolitic composition at rim. Composition of epidote is clinozoisite (Al# (= Al/(Al+Fe_total)) = 0.92-0.99). Xab (=Na/(Na+Ca)) of Albite varies from 0.90 to 0.99. XMg (= Mg/(Mg+Fe²⁺)) of chlorite ranges from 0.72 to 0.75. Al in horblende indicates a pressure condition of 1-2.5 kbar, implying a formation depth of original amphibole-bearing gabbro. Chlorite thermometry shows that gabbro body in the Khantaishir ophiolite was interacted with fluid under 280-300 °C. Mass transport from plagioclase to epidote indicates the gain of CaO (12 wt.%), Al₂O₃ (5 wt.%), and water (2 wt.%) if we assume a volume constant and Ca# [=Ca/(Ca+Na)] = 0.82 of original plagioclase. Gabbro body is cut by various types of veins including amphibole, chlorite + albite, albite + epidote, and epidote. Without veins, plagioclase in sample is altered to epidote, implying that gabbro body is altered pervasively and then cut by veins. Epidote pseudomoph contain abundant mineral inclusions (<5 µm) or pores, indicating porosity evolution during metamorphism. Alteration of gabbro body from the Khantaishir ophiolite records the cooling history and porosity evolution during hydrothermal alteration. Therefore, we suggest that epidote formed by hydrothermal alteration of gabbro body in oceanic crust may contribute to mass transfer and porosity evolution (fluid pathways) of lower crust.
References
1. Seyfried 1987, Ann. Rev. Earth Planet. Sci. 15:317-35
2. Weber et al., 2021, JGR Solid Earth. 126 e2020JB021540
3. Schiffman and Smith 1988, Journal of Geophysical Research. 93, 4612-4624
4. Zhang et al., 2021, JGR Solid Earth. 126 e2021JB022349