10:45 〜 12:15
[SCG58-P21] 沈み込み帯のCO2流体が引き起こす地殻-マントル境界の滑石生成に関する実験的制約
キーワード：沈み込み帯、スロースリップ、炭素固定
The geophysical observations have revealed that mantle wedge corner may be extensively serpentinized. In contrast to H2O, although a large amount of carbon is subducted as carbonates or carboniferous materials, it is still unclear how much carbon is fixed within the mantle wedge. At the slab-mantle interfaces, it is thought that Si-metasomatism occurs to form talc or amphibole via a supply of Si from crust to mantle. As talc shows a low frictional coefficient, it could affect the rheology of the subduction zone interfaces and the possible relation to generation of slow slip events has been proposed 1. Recently, the carbonate veins associated with talc have been reported in the serpentinite body with mantle wedge origin 2. However, it is unclear how CO2 fluids causes the mantle wedge carbonation and talc formation effectively due to the lack of experiments. In this study, we evaluated the effects of silica and CO2 on talc formation through a high P-T experiment.
We used a Griggs-type piston cylinder-apparatus to conduct the experiments on the metasomatic reactions at the crust-mantle boundaries at the mantle wedge condition (500℃, 1 GPa). We used the core samples (a diameter of 6.2mm), composed of the pelitic schist (chlorite zone, Nagatoro, Sanbagawa belt) sandwiched by harzburgite (Horoman peridotite) and serpentinite (antigorite+chrysotile Kanasaki, near Nagatoro) to simulate the boundaries between the metasediments and anhydrous mantle / serpentinized mantle, respectively. Two types of fluids were used; pure H2O fluid and H2O-CO2 fluid by decomposition of Oxalic Acid Dihydrate (OAD). OAD is decomposed to CO2, H2O, and H2 at ~200℃. We set 4wt% of H2O and XCO2 as 20 mol% for the cases of H2O-CO2 experiments.
In the experiments with H2O fluids, a talc layer with a thickness of ~10 microns was formed at the boundary of harzburgite and pelitic schist, forming tensile cracks from the tip of the reaction zone. At the boundary between serpentinite and pelitic schist, Al-rich serpentine was formed as a thin layer at the inner parts of the serpentinite, and thin talc veins were formed inside serpentinite. Within the pelitic schist at both boundaries, albite was preferentially decomposed to form fine-grained Mg-smectite.
In the experiment with H2O-CO2 fluid introduced in these two boundaries, talc and magnesite were formed both within the harzburgite and serpentinite. In contrast, the pelitic schist shows little alternation on both sides. Within harzburgite, the talc and magnesite were formed with mesh-like fractures, and talc was more produced in the orthopyroxene than olivine. Within the serpentinite, magnesite preferentially occurred at the just contact and a large amount of pores and a network of thin talc-magnesite veins within the interiors.
In all conditions, the modal abundances of alteration minerals indicate that Mg was moved from mantle rocks to the pelitic schists, and Si (and Al) are moved from the pelitic schist to the mantle rocks. Talc is formed in response to Si supply as revealed by experiments with H2O fluids, but much more amounts of talc were formed with H2O-CO2 fluid in both hydrous and anhydrous mantles than the CO2-free experiments about 5-30 times larger. The contrasting fracture patterns that developed during magnesite + talc formation could be due to the difference in the reaction properties; the carbonation of the anhydrous mantle is characterized by hydration and significant volume increase, and that of the hydrous mantle by dehydration and lack of large volume increase or reduction. The mass balance relationship of the increase of talc amount and the decrease of olivine/serpentine show harmony with that of assuming all of the talcs are generated by CO2, not by silica. Our results imply that the infiltration of CO2 fluids potentially provides the significant enhancement of talc formation within the mantle wedge, compared to Si-metasomatism, and may cause a pronounced weakening of the subduction zone interface than pure H2O fluid.
Reference
1. Tarling, M. S., Smith, S. A. F. & Scott, J. M. Fluid overpressure from chemical reactions in serpentinite within the source region of deep episodic tremor. Nat. Geosci. 12, 1034 - 1042 (2019).
2. Okamoto, A. et al. Rupture of wet mantle wedge by self-promoting carbonation. Commun. Earth Environ. 2, 151 (2021).
We used a Griggs-type piston cylinder-apparatus to conduct the experiments on the metasomatic reactions at the crust-mantle boundaries at the mantle wedge condition (500℃, 1 GPa). We used the core samples (a diameter of 6.2mm), composed of the pelitic schist (chlorite zone, Nagatoro, Sanbagawa belt) sandwiched by harzburgite (Horoman peridotite) and serpentinite (antigorite+chrysotile Kanasaki, near Nagatoro) to simulate the boundaries between the metasediments and anhydrous mantle / serpentinized mantle, respectively. Two types of fluids were used; pure H2O fluid and H2O-CO2 fluid by decomposition of Oxalic Acid Dihydrate (OAD). OAD is decomposed to CO2, H2O, and H2 at ~200℃. We set 4wt% of H2O and XCO2 as 20 mol% for the cases of H2O-CO2 experiments.
In the experiments with H2O fluids, a talc layer with a thickness of ~10 microns was formed at the boundary of harzburgite and pelitic schist, forming tensile cracks from the tip of the reaction zone. At the boundary between serpentinite and pelitic schist, Al-rich serpentine was formed as a thin layer at the inner parts of the serpentinite, and thin talc veins were formed inside serpentinite. Within the pelitic schist at both boundaries, albite was preferentially decomposed to form fine-grained Mg-smectite.
In the experiment with H2O-CO2 fluid introduced in these two boundaries, talc and magnesite were formed both within the harzburgite and serpentinite. In contrast, the pelitic schist shows little alternation on both sides. Within harzburgite, the talc and magnesite were formed with mesh-like fractures, and talc was more produced in the orthopyroxene than olivine. Within the serpentinite, magnesite preferentially occurred at the just contact and a large amount of pores and a network of thin talc-magnesite veins within the interiors.
In all conditions, the modal abundances of alteration minerals indicate that Mg was moved from mantle rocks to the pelitic schists, and Si (and Al) are moved from the pelitic schist to the mantle rocks. Talc is formed in response to Si supply as revealed by experiments with H2O fluids, but much more amounts of talc were formed with H2O-CO2 fluid in both hydrous and anhydrous mantles than the CO2-free experiments about 5-30 times larger. The contrasting fracture patterns that developed during magnesite + talc formation could be due to the difference in the reaction properties; the carbonation of the anhydrous mantle is characterized by hydration and significant volume increase, and that of the hydrous mantle by dehydration and lack of large volume increase or reduction. The mass balance relationship of the increase of talc amount and the decrease of olivine/serpentine show harmony with that of assuming all of the talcs are generated by CO2, not by silica. Our results imply that the infiltration of CO2 fluids potentially provides the significant enhancement of talc formation within the mantle wedge, compared to Si-metasomatism, and may cause a pronounced weakening of the subduction zone interface than pure H2O fluid.
Reference
1. Tarling, M. S., Smith, S. A. F. & Scott, J. M. Fluid overpressure from chemical reactions in serpentinite within the source region of deep episodic tremor. Nat. Geosci. 12, 1034 - 1042 (2019).
2. Okamoto, A. et al. Rupture of wet mantle wedge by self-promoting carbonation. Commun. Earth Environ. 2, 151 (2021).