講演情報
[T1-O-9]Hydrothermal Alteration and Porosity Evolution of Lower Crust: Plagioclase Replacement by Epidote in the Khantaishir ophiolite, Western Mongolia
*OTGONBAYAR DANDAR1, ATSUSHI OKAMOTO1, MASAOKI UNO2, NORIYOSHI TSUCHIYA1,3 (1. Graduate School of Environmental Studies, Tohoku University, 2. Department of Earth and Planetary Science, The University of Tokyo, 3. National Institute of Technology, Hachinohe College)
キーワード：
Hydrothermal alteration、Epidote、Gabbro、Khantaishir ophiolite、western Mongolia
Fluid flow in the oceanic crust is key to heat and element transport, hydration reactions, and changes in crustal properties, and it plays a role in ore formation and tectonic activity. Hydrothermal alteration of mafic rocks and plagiogranites often produces epidosite, rich in epidote and quartz, typically linked to high water-to-rock ratios and volcanic massive sulfide deposits. Fluids penetrate more easily in porous volcanic rocks, while in denser gabbros, flow is mostly limited to fractures. Despite the frequent occurrence of altered gabbro, the details of element transport, fluid movement, and porosity evolution during alteration remain unclear.
To study these processes, we examined hydrothermally altered gabbro from the Khantaishir ophiolite in western Mongolia. The samples contain primary pyroxene and amphibole, and secondary epidote, chlorite, albite, and minor quartz. Diopsidic pyroxene includes amphibole, and original plagioclase is mostly replaced by polycrystalline epidote, with some chlorite and albite. Pyroxene is partly replaced by chlorite, preserving exsolved amphibole. Amphibole shows zoning from hornblende to actinolite. Epidote is primarily clinozoisite, albite is sodium-rich <0.90–0.99>, and chlorite has XMg values of 0.72–0.75. Hornblende chemistry indicates formation at 1–2.5 kbar, and chlorite thermometry suggests alteration temperatures of 280–300°C.
Element transfer from plagioclase to epidote reflects gains in CaO <12 wt.%>, Al2O3 <5 wt.%>, and H2O <2 wt.%>, assuming volume conservation and a plagioclase Ca# of 0.82. The gabbro contains various veins <amphibole, chlorite + albite, albite + epidote, and epidote>, but in vein-free areas, plagioclase is directly replaced by epidote, indicating early pervasive alteration. Epidote pseudomorphs with fine inclusions or voids suggest porosity changes during metamorphism. This alteration records both thermal history and porosity evolution, with epidote formation likely promoting mass transfer and fluid pathways in the lower oceanic crust.
To study these processes, we examined hydrothermally altered gabbro from the Khantaishir ophiolite in western Mongolia. The samples contain primary pyroxene and amphibole, and secondary epidote, chlorite, albite, and minor quartz. Diopsidic pyroxene includes amphibole, and original plagioclase is mostly replaced by polycrystalline epidote, with some chlorite and albite. Pyroxene is partly replaced by chlorite, preserving exsolved amphibole. Amphibole shows zoning from hornblende to actinolite. Epidote is primarily clinozoisite, albite is sodium-rich <0.90–0.99>, and chlorite has XMg values of 0.72–0.75. Hornblende chemistry indicates formation at 1–2.5 kbar, and chlorite thermometry suggests alteration temperatures of 280–300°C.
Element transfer from plagioclase to epidote reflects gains in CaO <12 wt.%>, Al2O3 <5 wt.%>, and H2O <2 wt.%>, assuming volume conservation and a plagioclase Ca# of 0.82. The gabbro contains various veins <amphibole, chlorite + albite, albite + epidote, and epidote>, but in vein-free areas, plagioclase is directly replaced by epidote, indicating early pervasive alteration. Epidote pseudomorphs with fine inclusions or voids suggest porosity changes during metamorphism. This alteration records both thermal history and porosity evolution, with epidote formation likely promoting mass transfer and fluid pathways in the lower oceanic crust.