Mantle Wedge Water Contents Estimated From Ultrasonic Laboratory Measurements of Olivine‐Antigorite Aggregates

• Published in: Geophysical research letters Vol. 49; no. 10
• Main Authors: Wang, Duojun, Wang, Libing, Zhang, Rui, Cai, Nao, Zhang, Jikai, Chen, Peng, Cao, Yang
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.05.2022
• Subjects: Acoustic velocity; Acoustics; Aggregates; Anomalies; Earth mantle; Lava; Minerals; Moisture content; Oceans; Olivine; P-waves; Peridotite; Pressure; Pressure dependence; Reservoirs; Seismic activity; Seismic propagation; Seismic wave propagation; Seismic waves; Serpentine; Serpentinization; Slabs; Spatial distribution; Subduction; Subduction (geology); Subduction zones; Velocity; Water circulation; Water content; Water reservoirs; Water transport; Wave propagation; Pacific Ocean
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2022GL098226

While it is known that serpentinized peridotite acts as a water reservoir in the mantle wedge of a subduction zone, the spatial distribution and quantity of the water stored in these reservoirs, as well as the acoustic velocity of the olivine‐antigorite aggregates, are not well constrained. Here, we report the propagation of seismic waves through synthetic olivine–antigorite aggregates, which are used as proxies for various mantle wedge lithologies, by varying the amount of antigorite at pressures up to 8 GPa. Our results indicate that the acoustic velocity is strongly dependent on the proportion of antigorite in the sample and somewhat dependent on pressure. We empirically explore the relationship between the acoustic velocity and the degree of serpentinization as well as pressure to map the water content in the mantle wedge. Our estimations show that the water content among the subducting slabs around the Pacific Ocean is between 0.5 and 5.0 wt%. Plain Language Summary The presence of low‐velocity zones in the mantle wedge has long been attributed to the hydration of mantle peridotite, which in turn becomes serpentine minerals. However, the direct links between low‐velocity anomalies, the degree of mantle serpentinization, and water circulation are not firmly established due to the lack of direct experimental data from partially serpentinized peridotites. Because these variables have large uncertainties, it is often difficult to determine the water content of the mantle wedge using seismic observations. In this study, we measured the acoustic velocity through synthetic olivine–antigorite aggregates, which are the proxies for mantle wedge lithologies. We explore two important equations between the acoustic velocity and the degree of serpentinization as well as pressure. Combined with geophysical observations, we could map the degree of serpentinization and water contents of the subducting slabs around the Pacific Ocean. Our results provide a clue to study the water transport between the surface and the Earth's interior. Key Points Synthetic olivine‐antigorite aggregates serve as proxies for mantle wedge lithologies We empirically explore the relationship between the acoustic velocity and the degree of serpentinization and pressure The degree of serpentinization and the water content of the mantle wedges of 19 typical subduction zones are determined