Experimental constraints on the serpentinization rate of fore‐arc peridotites: Implications for the upwelling condition of the slab‐derived fluid

Abstract

To constrain the water circulation in subduction zones, the hydration rates of peridotites were investigated experimentally in fore‐arc mantle conditions. Experiments were conducted at 400–580°C and 1.3 and 1.8 GPa, where antigorite is expected to form as a stable serpentine phase. Crushed powders of olivine ± orthopyroxene and orthopyroxene + clinopyroxene were reacted with 15 wt % distilled water for 4–19 days. The synthesized serpentine varieties were lizardite and aluminous lizardite (Al‐lizardite) in all experimental conditions except those of 1.8 GPa and 580°C in the olivine + orthopyroxene system, in which antigorite was formed. In the olivine + orthopyroxene system, the reactions were interface‐controlled except for the reaction at 400°C, which was transport‐controlled. The corresponding reaction rates were 7.0 × 10−12 to 1.5 × 10−11 m s−1 at 500–580°C and 7.5 × 10−16 m2 s−1 at 400°C for the interface and transport‐controlled reactions, respectively. Based on a simple reaction‐transport model including these hydration rates, we infer that penetration of the slab‐derived fluid all the way through a water‐unsaturated fore‐arc mantle is allowed only when focused flow occurs with a spacing larger than 77–229 km in hot subduction zones such as Nankai and Cascadia. However, the necessary spacing is only 2.3–4.6 m in intermediate‐temperature subduction zones such as Kyushu and Costa Rica. These calculations imply that fluid leakage in hot subduction zones may occur after the fore‐arc mantle is totally hydrated, whereas in intermediate‐temperature subduction zones, leakage through a water‐unsaturated fore‐arc mantle may be facilitated.

Anomalous Sound Velocities of Antigorite at High Pressure and Implications for Detecting Serpentinization at Mantle Wedges

“…High V P /V S ratios observed in mantle wedges within subduction zone environments were interpreted on the basis of serpentine elastic properties (Christensen, 1996). Antigorite is an abundant hydrous mineral in subduction zones and contributes to the transportation of water into the mantle, influences slab-mantle coupling, the distribution of slab earthquakes, and triggers partial melting of lithospheric upper mantle (Bostock et al, 2002;Nakatani & Nakamura, 2016;Rüpke et al, 2004;Ulmer & Trommsdorff, 1995).…”

Preferential Orthopyroxene Serpentinization and Implications for Seismic Velocity Interpretation in the Fore‐Arc Mantle

“…The starting material was a powdered mixture of olivine and Opx handpicked separately from a gently crushed San Carlos lherzolite xenolith. The chemical compositions of olivine and Opx have been reported previously (Nakatani & Nakamura, ). The olivine and Opx have X Mg (=Mg/(Mg + Fe) in molar ratio) values of 0.91, and Opx is characterized by a high Al 2 O 3 content (4.07 wt%).…”

“…It is difficult to classify this serpentine variety unambiguously based on the Raman spectrum because the OH‐stretching band, which is decisive for discriminating serpentine varieties, is broad possibly owing to the low crystallinity of the synthesized serpentine. A serpentine variety with similar mineralogical characters was synthesized by Nakatani and Nakamura () at 400–580 °C and 1.3 GPa and was regarded as lizardite. They suggested that high aluminum activity in the system stabilized lizardite even in the high P ‐ T conditions, where antigorite is stable in the less Al 2 O 3 ‐rich system (Caruso & Chernosky, ).…”

“…The general formula of antigorite in the MgO‐SiO 2 ‐H 2 O system can be expressed as Mg 3m–3 Si 2m O 5m (OH) 4m–6 (Mellini et al, ); we assumed a fixed polysomatic formula ( m ) of 17 for antigorite that was expected to form in our experiments through stoichiometric serpentinization, in which the effective olivine/Opx molar ratio is 0.7. When the ratio becomes smaller than 0.7, formation of talc along with serpentine has been observed (Nakatani & Nakamura, ; Figure ). In some experiments, we also used an olivine‐rich powder with an olivine to Opx weight ratio of 7:3 to check the effect of mineral proportion on the reaction stoichiometry.…”

“…Recently, we revealed experimentally that Opx preferentially reacts with water to form talc along with serpentine even in the system where only antigorite is expected to form (olivine:Opx = 1:1 in weight) under the pressure ( P )‐temperature ( T ) conditions corresponding to those of the fore‐arc mantle wedge (400–580 °C and 1.3–1.8 GPa, Nakatani & Nakamura, ; Figure ). Isotropic talc aggregate was suggested to have a V p / V s value as low as ~1.6 (Mainprice et al, ), implying that the observed seismic anomaly in the fore‐arc mantle may be consistently interpreted if talc formation is considered.…”