2:15 PM - 2:30 PM
[SCG40-13] CO2 metasomatism vs. silica metasomatism on the formation of talc at slab-mantle interface: experimental insights and its implications to slow slip event
Keywords:subduction zone, slow slip event, talc, CO2
Talc is one of the weakest mantle minerals, and the linkage between the formation of talc layer and slow slip event has been proposed (Lindquist et al., 2023). It is commonly believed that talc is formed by Si-metasomatism of mantle at the slab-mantle interface. In contrast, serpentinite bodies with carbonate + talc veins are commonly observed in the high-pressure metamorphic belts (Okamoto et al., 2021), suggesting the influences of CO2-fluids on the formation of talc. However, relative importance of silica and CO2 in fluids on the formation of talc, and on the influences on the mechanical properties are poorly understood. In this study, we used Griggs-type deformation apparatus to investigate the metasomatic reactions at the interface between crustal rocks (pelite or quartzite) and mantle rock (harzburgite or serpentinite) at 500℃ and 1GPa.
In the experiments under hydrostatic conditions, we prepared the three layers of core samples; pelitic schist (the Sanbagawa belt) or Brazilian quartzite was sandwiched between harzburgite (Horoman peridotite, dry mantle) and serpentinite (Mikabu belt, wet mantle). Two types of fluids were introduced: pure H2O fluid and H2O-CO2 fluid. The latter produced by the decomposition of Oxalic Acid Dihydrate (OAD). We maintained 4 wt% H2O and set the XCO2 = 0.2 for the H2O-CO2 experiments. In all conditions, the alteration more proceeded in the mantle rocks than on the crust side. In the experiment with H2O, talc was formed both in harzburgite and serpentinite at the contact with crustal rocks. In the pelitic schist at the contact with ultramafic rocks, albite was selectively replaced by Mg smectite, whereas in the quartzite, a small amount of talc was formed, indicating that counter diffusion of Si from crust to mantle, and Mg from mantle to crust. In the experiments with H2O-CO2 fluids, talc was formed with magnesite both in harzburgite and serpentinite with intense fracturing. The preliminary mass balance calculations reveal that the amount of talc in the ultramafic rocks can be explained solely by the reaction with CO2-fluid, even if quartz-bearing rocks existed at the contact.
We also conducted shear deformation experiments at the same P-T condition, using the precut assembly composed of harzburgite and quartzite in contact at 45° and H2O-CO2 fluid (XCO2 = 0.25). The samples are deformed for 12 hours at a shear rate of 3.3×10-5 mm/s following the hydrostatic reaction for 3 hours. At the contact of the two samples, a layer of about 20 μm thick of talc and magnesite was formed while some area is composed of quartz and magnesite. The deformation was concentrated only in a few μm thick layer of talc. The coefficient of friction was approximately 0.25, suggesting that only a small amount of talc layer was responsible for the deformation.
These experimental results suggest that talc formation at the slab-mantle interface is greatly enhanced by the infiltration of CO2 fluids, at least, at the mantle wedge corner of the warm subduction zone, resulting in a reduction of the strength of slab-mantle boundary. During the metasomatism at the plate interface, not only silica but also other elements such as Mg and Al move significantly, which contributes to the various metasomatic reactions which results in the contrast of strength such as barriers and asperities. Such heterogeneous metasomatic reactions may explain a wide spectrum of the slow slip events observed at the mantle wedge corner.
In the experiments under hydrostatic conditions, we prepared the three layers of core samples; pelitic schist (the Sanbagawa belt) or Brazilian quartzite was sandwiched between harzburgite (Horoman peridotite, dry mantle) and serpentinite (Mikabu belt, wet mantle). Two types of fluids were introduced: pure H2O fluid and H2O-CO2 fluid. The latter produced by the decomposition of Oxalic Acid Dihydrate (OAD). We maintained 4 wt% H2O and set the XCO2 = 0.2 for the H2O-CO2 experiments. In all conditions, the alteration more proceeded in the mantle rocks than on the crust side. In the experiment with H2O, talc was formed both in harzburgite and serpentinite at the contact with crustal rocks. In the pelitic schist at the contact with ultramafic rocks, albite was selectively replaced by Mg smectite, whereas in the quartzite, a small amount of talc was formed, indicating that counter diffusion of Si from crust to mantle, and Mg from mantle to crust. In the experiments with H2O-CO2 fluids, talc was formed with magnesite both in harzburgite and serpentinite with intense fracturing. The preliminary mass balance calculations reveal that the amount of talc in the ultramafic rocks can be explained solely by the reaction with CO2-fluid, even if quartz-bearing rocks existed at the contact.
We also conducted shear deformation experiments at the same P-T condition, using the precut assembly composed of harzburgite and quartzite in contact at 45° and H2O-CO2 fluid (XCO2 = 0.25). The samples are deformed for 12 hours at a shear rate of 3.3×10-5 mm/s following the hydrostatic reaction for 3 hours. At the contact of the two samples, a layer of about 20 μm thick of talc and magnesite was formed while some area is composed of quartz and magnesite. The deformation was concentrated only in a few μm thick layer of talc. The coefficient of friction was approximately 0.25, suggesting that only a small amount of talc layer was responsible for the deformation.
These experimental results suggest that talc formation at the slab-mantle interface is greatly enhanced by the infiltration of CO2 fluids, at least, at the mantle wedge corner of the warm subduction zone, resulting in a reduction of the strength of slab-mantle boundary. During the metasomatism at the plate interface, not only silica but also other elements such as Mg and Al move significantly, which contributes to the various metasomatic reactions which results in the contrast of strength such as barriers and asperities. Such heterogeneous metasomatic reactions may explain a wide spectrum of the slow slip events observed at the mantle wedge corner.