5:15 PM - 7:15 PM
[SCG54-P18] Post-antigorite reaction and hydration processes of peridotite under mantle transition zone conditions
Keywords:post-antigorite reaction, hydration of NAMs , Transformation kinetics
Partially serpentinized peridotite is transported into the Earth's interior by subducting plates (slabs), where high-pressure hydrous phases (DHMS: Dense Hydrous Magnesium Silicate) are formed. Upon dehydration, water is released into the mantle transition zone (depths of 410-660 km), where high-pressure polymorphs of olivine, wadsleyite, and ringwoodite (Rw) –nominally anhydrous minerals (NAMs) that can contain 2-3 wt.% H2O–may capture it. This water significantly influences dynamics by promoting non-equilibrium phase transitions and plastic deformation of slabs. However, most previous experiments have been conducted in closed systems, by using metallic capsules, which may differ from the conditions of partially hydrated slab peridotite. As a result, the temperatures and processes of DHMS dehydration within the slab, as well as the interactions of the dehydrated fluid with surrounding peridotite, remain unclear. To investigate this, we conducted dehydration reaction experiments of antigorite in peridotite capsules at mantle transition zone pressures.
Experiments were conducted at 17-20 GPa, and 700-870°C for 0-240 minutes using the D-111 type high-pressure apparatus at Kyushu University. To simulate the dehydration of partially serpentinized peridotite, natural antigorite from Kawarakoba, Nagasaki Prefecture, was placed inside capsules made of Horoman peridotite (Lherzolite) or San Carlos olivine single crystals. The phases, microstructures, and chemical compositions of the recovered samples were examined using FE-SEM, EDS, EBSD, and XRD.
Under our experimental conditions, the antigorite dehydration reaction generally produces Phase D and Superhydrous phase B (ShB) releasing 4.3 wt.% H2O. In the highest temperature run at 870°C, Phase D and ShB further dehydrated into Phase D and Rw in the vicinity of the surrounding peridotite. The ShB dehydration in the peridotite reaction system occurred at ~250°C lower than that in the closed system (Irifune et al., 1998), which is similar to the conditions in open systems from our previous study. The Mg/Si ratio in the dehydrated antigorite region did not change and remained constant (i.e., Mg/Si ~1.5). Three characteristic reaction textures were identified in the surrounding peridotite capsule: 1) a DHMS reaction rim consisting of Phase D+ShB (Mg/Si ~2.0) at the interface between antigorite and olivine; 2) a relatively coarse-grained, equigranular Rw reaction rim outside of the DHMS rim; and 3) the formation of branching Rw veins and massive Rw in the outer olivine. In the longest run of 240 min at 870°C most of the surrounding olivine and pyroxenes transformed to their high-pressure phases on the mm scale, indicating that the dehydrated fluid significantly enhances kinetics.
We also observed that the Rw reaction rim and veins form first, followed by erosion of the former Rw rim by the DHMS region, resulting in double-layered reaction rims. The Rw veins contain a low-molecular weight region at the center of the vein tip, but this feature is absent at the tail end, with the vein largely growing outward. These textures suggest that the dehydrated water not only contributes to the formation of the Rw rim but also further infiltrates the surrounding NAMs region, forming Rw veins. Although we have not examined the quantitative water content, the total loss observed in EDS analysis suggests that the Rw is likely hydrated. Thus, this process dramatically enhances the overall transformation rate and the hydration of NAMs in the surrounding peridotite capsule. The additional fluid supplied from the antigorite region also contributes to the formation of another water-rich DHMS rim. The processes and kinetics of the antigorite dehydration and hydration of NAMs demonstrated in our study provide new insights into water transportation, metastable phase transformation, and the mechanical properties of partially serpentinized slab peridotite at mantle transition zone depths.
Experiments were conducted at 17-20 GPa, and 700-870°C for 0-240 minutes using the D-111 type high-pressure apparatus at Kyushu University. To simulate the dehydration of partially serpentinized peridotite, natural antigorite from Kawarakoba, Nagasaki Prefecture, was placed inside capsules made of Horoman peridotite (Lherzolite) or San Carlos olivine single crystals. The phases, microstructures, and chemical compositions of the recovered samples were examined using FE-SEM, EDS, EBSD, and XRD.
Under our experimental conditions, the antigorite dehydration reaction generally produces Phase D and Superhydrous phase B (ShB) releasing 4.3 wt.% H2O. In the highest temperature run at 870°C, Phase D and ShB further dehydrated into Phase D and Rw in the vicinity of the surrounding peridotite. The ShB dehydration in the peridotite reaction system occurred at ~250°C lower than that in the closed system (Irifune et al., 1998), which is similar to the conditions in open systems from our previous study. The Mg/Si ratio in the dehydrated antigorite region did not change and remained constant (i.e., Mg/Si ~1.5). Three characteristic reaction textures were identified in the surrounding peridotite capsule: 1) a DHMS reaction rim consisting of Phase D+ShB (Mg/Si ~2.0) at the interface between antigorite and olivine; 2) a relatively coarse-grained, equigranular Rw reaction rim outside of the DHMS rim; and 3) the formation of branching Rw veins and massive Rw in the outer olivine. In the longest run of 240 min at 870°C most of the surrounding olivine and pyroxenes transformed to their high-pressure phases on the mm scale, indicating that the dehydrated fluid significantly enhances kinetics.
We also observed that the Rw reaction rim and veins form first, followed by erosion of the former Rw rim by the DHMS region, resulting in double-layered reaction rims. The Rw veins contain a low-molecular weight region at the center of the vein tip, but this feature is absent at the tail end, with the vein largely growing outward. These textures suggest that the dehydrated water not only contributes to the formation of the Rw rim but also further infiltrates the surrounding NAMs region, forming Rw veins. Although we have not examined the quantitative water content, the total loss observed in EDS analysis suggests that the Rw is likely hydrated. Thus, this process dramatically enhances the overall transformation rate and the hydration of NAMs in the surrounding peridotite capsule. The additional fluid supplied from the antigorite region also contributes to the formation of another water-rich DHMS rim. The processes and kinetics of the antigorite dehydration and hydration of NAMs demonstrated in our study provide new insights into water transportation, metastable phase transformation, and the mechanical properties of partially serpentinized slab peridotite at mantle transition zone depths.