10:45 〜 11:00
[SMP28-07] Hydrothermal Alteration of Lower Crust: Plagioclase Replacement by Epidote in Khantaishir ophiolite, Western Mongolia
キーワード：Hydrothermal alteration, Epidote, Gabbro, Khantaishir ophiolite, western Mongolia
The fluid flow in oceanic crust facilitates element and heat transport, accelerates hydration reactions, alters mechanical and rheological properties, and plays a key role in ore deposit formation and crustal deformation. For example, hydrothermal alteration of mafic volcanic rocks and plagiogranites in the oceanic crust typically results in the formation of epidosite, primarily composed of epidote and quartz. The creation of epidosite is believed to be due to high water/rock ratios and is linked to ore deposits, such as volcanic massive sulfide deposits. Fluid infiltration in volcanic rocks, which are more porous and permeable than gabbro layers, tends to occur more easily, while in gabbro layers, fluid movement primarily happens along fractures. Although altered gabbro bodies are commonly found, the detailed mechanisms of element and fluid transport, as well as the changes in porosity during hydrothermal alteration, remain poorly understood.
To explore these processes, we studied hydrothermally altered gabbro in the Khantaishir ophiolite in western Mongolia. The altered gabbro samples consist of primary minerals: pyroxene and amphibole, and secondary minerals such as epidote, chlorite, albite, and amphibole, with minor quartz. Pyroxene contains amphibole inclusions and is diopsidic in composition. Primary plagioclase has been altered mostly to epidote (composed of polycrystals), with minor chlorite and albite. Some pyroxene has been replaced by chlorite, preserving the exsolution of amphibole. The primary amphibole exhibits zoning from hornblende at the core to actinolite at the rim. Epidote is primarily clinozoisite (Al# = 0.92-0.99), and albite has an Xab value ranging from 0.90 to 0.99. Chlorite has an XMg value between 0.72 and 0.75. The aluminum content in hornblende suggests a formation depth of 1-2.5 kbar, while chlorite thermometry indicates the gabbro interacted with fluids at temperatures between 280–300°C.
Mass transport from plagioclase to epidote suggests a gain in CaO (12 wt.%), Al2O3 (5 wt.%), and water (2 wt.%), assuming volume conservation and a Ca# value of 0.82 for the original plagioclase. The gabbro is cut by various veins, including amphibole, chlorite + albite, albite + epidote, and epidote. In the absence of veins, plagioclase is altered directly to epidote, indicating pervasive alteration before the veins were introduced. Epidote pseudomorphs contain numerous mineral inclusions (<5 µm) or pores, indicating changes in porosity during metamorphism. The alteration of the gabbro body from the Khantaishir ophiolite thus records both the cooling history and the evolution of porosity during hydrothermal alteration. We suggest that epidote formed during this alteration may contribute to mass transfer and the evolution of fluid pathways, influencing porosity changes in the lower oceanic crust.
To explore these processes, we studied hydrothermally altered gabbro in the Khantaishir ophiolite in western Mongolia. The altered gabbro samples consist of primary minerals: pyroxene and amphibole, and secondary minerals such as epidote, chlorite, albite, and amphibole, with minor quartz. Pyroxene contains amphibole inclusions and is diopsidic in composition. Primary plagioclase has been altered mostly to epidote (composed of polycrystals), with minor chlorite and albite. Some pyroxene has been replaced by chlorite, preserving the exsolution of amphibole. The primary amphibole exhibits zoning from hornblende at the core to actinolite at the rim. Epidote is primarily clinozoisite (Al# = 0.92-0.99), and albite has an Xab value ranging from 0.90 to 0.99. Chlorite has an XMg value between 0.72 and 0.75. The aluminum content in hornblende suggests a formation depth of 1-2.5 kbar, while chlorite thermometry indicates the gabbro interacted with fluids at temperatures between 280–300°C.
Mass transport from plagioclase to epidote suggests a gain in CaO (12 wt.%), Al2O3 (5 wt.%), and water (2 wt.%), assuming volume conservation and a Ca# value of 0.82 for the original plagioclase. The gabbro is cut by various veins, including amphibole, chlorite + albite, albite + epidote, and epidote. In the absence of veins, plagioclase is altered directly to epidote, indicating pervasive alteration before the veins were introduced. Epidote pseudomorphs contain numerous mineral inclusions (<5 µm) or pores, indicating changes in porosity during metamorphism. The alteration of the gabbro body from the Khantaishir ophiolite thus records both the cooling history and the evolution of porosity during hydrothermal alteration. We suggest that epidote formed during this alteration may contribute to mass transfer and the evolution of fluid pathways, influencing porosity changes in the lower oceanic crust.