16:15 〜 16:30
[SMP26-09] Multiple hydration events and fluids characteristics in amphibolite in the Main Shear Zone, Sør Rondane Mountains, East Antarctica
キーワード：Hydration, Amphibolite, Brittle-viscous transition zones
Fluid flow in the crust induces hydration reactions and contributes to earthquake triggering. Geophysical findings revealed that source regions of slow earthquakes locate brittle-viscous transition zones and associated with intense fluid flux (Behr and Burgmann, 2021). More geological observation of fluid activity is required to provide constraints on the physico-chemical processes related to seismic events. This study provides a primary analysis of multiple hydration processes in amphibolite samples and reveal temperature conditions of fluid infiltration related to brittle-viscous shear deformation. A series of amphibolite samples were collected from the southern side of the Main Shear Zone (MSZ) exposed along the Ketelersbreen, Sør Rondane Mountains, East Antarctica (S 72.103º, E 23.199º) during the 61st Japan Antarctic Research Expedition in 2019-2020.
The amphibolite host rock is associated with epidote alteration (Fig. 1 b, c) and muscovite-calcite-scapolite (Ms-Cal-Scp) alteration (Fig. 1 d, e). Epidote alteration layers occur along high-angle amphibole veins and/or along foliation (Fig. 1 b, c).
Amphibolite host rock mainly contains hornblende, plagioclase, quartz, K-feldspar, titanite and relict clinopyroxene. The relict clinopyroxene (XMg = 0.75) is rare and found as an inclusion in the quartz grains. Epidote alteration layer consist of epidote, calcite, plagioclase, actinolite, quartz, K-feldspar, titanite and apatite. The amphibole veins contain actinolite, hornblende, calcite, epidote, quartz, and apatite. Amphibole grains in the amphibole veins have a hornblende core and an actinolite rim. Ms-Cal-Scp-bearing veins consist of muscovite, calcite, scapolite, hornblende, plagioclase, quartz, zoisite, titanite, chlorite, zeolite and apatite. Foliations around the Ms-Cal-Scp-bearing vein are plastically deformed (Fig. 1e).
The range of XAb in plagioclase in the amphibolite host rock is 0.60-0.64, while that in the Ms-Cal-Scp-bearing veins is 0.54-0.62. The Si contents in amphiboles in the amphibolite host rock are 6.7-6.9 atoms per formula unit (a.p.f.u., O=23), and Na contents are 0.1-0.2 a.p.f.u. For the epidote alteration zone, Si contents in amphiboles varies 7.7-7.9 a.p.f.u, and Na content is 0.1 a.p.f.u. Amphibole veins show wider ranges for Si, 6.8-8.0 a.p.f.u, Na are 0.1-0.2 a.p.f.u. Low temperature actinolite is detected in the epidote alteration zone. Hornblende plagioclase thermometer (Holland and Blundy, 1994) was used to estimate temperature conditions of host rock metamorphism. Temperature conditions estimated for the amphibolite host rock are 595±65 °C in the range of 0-1.5 GPa. Presence of low temperature actinolite and high XAb in plagioclase suggesting lower temperature (<500℃) in the epidote alteration zone.
We analysed Cl content in amphibole in the Ms-Cal-Scp veins and the host rock. The highest Cl concentration was detected in the vein center (0.62 wt%), and the lowest was in the host rock (0.01 wt%). By Raman spectroscopy, we found secondary aqueous fluid inclusions (Fig.1 f) in quartz, calcite, epidote and actinolite. Based on the morphology of secondary fluid inclusion chains, we suggest a several stages of fluid infiltration.
Different types of amphiboles in the veins and alteration layers suggest several fluid activities, including the breakdown of clinopyroxene into actinolite during initial hydration, followed by the infiltration of Cl and CO2-bearing fluids to form calcite and Ms-Cal-Scp veins accompanying brittle-viscous shear deformation, and the formation of high-angle amphibole veins and epidote alteration due to CO2-bearing fluids infiltrating. These activities occurred during the brittle-viscous transitions and cooling of the southern part of the MSZ, providing insights into physico-chemical aspects of fluid-induced rock fracturing during brittle-viscous transitions in the crust.
The amphibolite host rock is associated with epidote alteration (Fig. 1 b, c) and muscovite-calcite-scapolite (Ms-Cal-Scp) alteration (Fig. 1 d, e). Epidote alteration layers occur along high-angle amphibole veins and/or along foliation (Fig. 1 b, c).
Amphibolite host rock mainly contains hornblende, plagioclase, quartz, K-feldspar, titanite and relict clinopyroxene. The relict clinopyroxene (XMg = 0.75) is rare and found as an inclusion in the quartz grains. Epidote alteration layer consist of epidote, calcite, plagioclase, actinolite, quartz, K-feldspar, titanite and apatite. The amphibole veins contain actinolite, hornblende, calcite, epidote, quartz, and apatite. Amphibole grains in the amphibole veins have a hornblende core and an actinolite rim. Ms-Cal-Scp-bearing veins consist of muscovite, calcite, scapolite, hornblende, plagioclase, quartz, zoisite, titanite, chlorite, zeolite and apatite. Foliations around the Ms-Cal-Scp-bearing vein are plastically deformed (Fig. 1e).
The range of XAb in plagioclase in the amphibolite host rock is 0.60-0.64, while that in the Ms-Cal-Scp-bearing veins is 0.54-0.62. The Si contents in amphiboles in the amphibolite host rock are 6.7-6.9 atoms per formula unit (a.p.f.u., O=23), and Na contents are 0.1-0.2 a.p.f.u. For the epidote alteration zone, Si contents in amphiboles varies 7.7-7.9 a.p.f.u, and Na content is 0.1 a.p.f.u. Amphibole veins show wider ranges for Si, 6.8-8.0 a.p.f.u, Na are 0.1-0.2 a.p.f.u. Low temperature actinolite is detected in the epidote alteration zone. Hornblende plagioclase thermometer (Holland and Blundy, 1994) was used to estimate temperature conditions of host rock metamorphism. Temperature conditions estimated for the amphibolite host rock are 595±65 °C in the range of 0-1.5 GPa. Presence of low temperature actinolite and high XAb in plagioclase suggesting lower temperature (<500℃) in the epidote alteration zone.
We analysed Cl content in amphibole in the Ms-Cal-Scp veins and the host rock. The highest Cl concentration was detected in the vein center (0.62 wt%), and the lowest was in the host rock (0.01 wt%). By Raman spectroscopy, we found secondary aqueous fluid inclusions (Fig.1 f) in quartz, calcite, epidote and actinolite. Based on the morphology of secondary fluid inclusion chains, we suggest a several stages of fluid infiltration.
Different types of amphiboles in the veins and alteration layers suggest several fluid activities, including the breakdown of clinopyroxene into actinolite during initial hydration, followed by the infiltration of Cl and CO2-bearing fluids to form calcite and Ms-Cal-Scp veins accompanying brittle-viscous shear deformation, and the formation of high-angle amphibole veins and epidote alteration due to CO2-bearing fluids infiltrating. These activities occurred during the brittle-viscous transitions and cooling of the southern part of the MSZ, providing insights into physico-chemical aspects of fluid-induced rock fracturing during brittle-viscous transitions in the crust.