Geophysical assessment of migration and storage conditions of fluids in subduction zones

By enhancing mass transfer and energy release, the cycle of volatiles and melt is a major component of subduction. Investigating this fluid cycle is therefore critical to understand the past and current activity of subduction zones. Fluids can significantly affect rock electrical conductivity and elastic parameters that are measured using electromagnetic and seismic methods, respectively. This letter emphasizes how these geophysical methods complement each other to provide information about the storage of fluids in subduction systems. By compiling electromagnetic and seismic results from various subduction zones, a possible correlation between electrical conductivity and seismic wave attenuation anomalies in the mantle wedge is observed, consistent with fluid accumulation. A possible relationship between geophysical properties and the slab age is also suggested, whereas no significant trend is observed between electrical conductivity or seismic wave attenuation and estimates of water flux in the mantle wedge. These field-based relationships require further constrains, emphasizing the need for new measurements in the laboratory. Findings Introduction The dynamics and time-evolution of subduction are driven by mechanical and chemical processes that influence buoyancy forces, slab motion, contrasting thermal fields, phase equilibria, and volatile transport. By enhancing mass transfer and energy release, the cycle of fluids in subduction zones is a critical component of slab recycling and continental building processes. A better understanding of the role of melt and volatiles in subduction zones is therefore key to improving our knowledge of the geodynamic processes at work. It can also help us better assess volcanic and earthquake hazards in these contexts. The cycle of fluids is expected to differ significantly between subduction zones. For instance, varying temperatures cause dehydration reactions to occur at shallower depths in the slabs of warm subduction zones (e.g., Southwest Japan, Cascades) compared to slabs of cold subduction zones (e.g., Tonga, Java) (Peacock and Wang 1999). Fluid migration was found to be faster than subduction velocity in warm subduction systems (e.g., approximately 7 cm/year versus approximately 4 cm/year, respectively, in Correspondence: [email protected] School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA © 2014 Pommier; licensee Springer. This is an O Attribution License (http://creativecommons.or in any medium, provided the original work is p Southwest Japan, Kawano et al. 2011), suggesting a continuous hydration of the mantle wedge due to upward fluid migration along the subduction interface. In colder environments, comparable fluid and subduction velocities (e.g., approximately 10 cm/year in Northeast Japan, Kawano et al. 2011) imply that a non-negligible amount of water reaches the lower mantle and triggers melting, as evidenced by geochemical signatures of island arc magmas (e.g., Stolper and Newman 1994; Wallace 2005). Significant water contents in the mantle are suggested by modeling studies. For instance, van Keken et al. (2011) estimated that the global H2O flux to the deep mantle corresponds roughly to one ocean mass over the Earth's history. Fluids influence electrical conductivity and seismic velocity in different ways (see Unsworth and Rondenay 2013). These physical properties are measured using electromagnetic and seismic methods, respectively, offering a unique way to map in situ fluid distributions in real time. When interpreted together with petrological results, geophysical data can be used to constrain fluid chemistry, temperature, fraction, and connectivity. Though some important findings have been obtained to relate electrical and seismic data to fluid distribution, thermal structure, and mineralogy (e.g., Kazatchenko et al. 2004; Hacker and Abers 2004; ten Grotenhuis et al. 2005), further work is required to understand the possible relationships between pen Access article distributed under the terms of the Creative Commons g/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction roperly credited. Pommier Earth, Planets and Space 2014, 66:38 Page 2 of 11 http://www.earth-planets-space.com/content/66/1/38 geophysical parameters sensitive to fluids and subduction dynamics. This letter addresses how electromagnetic and seismic methods complement each other to help define the storage conditions of fluid processes in subduction. It aims to stimulate laboratory investigations that use a joint elec trical-seismic approach and combine geophysical data with subduction settings. Geophysical structure of subduction zones and fluid detection Electrical conductivity structure of subduction zones Because it is sensitive to temperature, composition, and interconnectivity changes, the electrical conductivity of geomaterials provides information about their chemistry and structure (see Pommier 2013). Most electrical images of the subduction zones present two anomalies unnecessarily connected along or above the slab: a backarc conductor and a near-trench conductor (Table 1). These conductive areas are usually interpreted as zones of fluid accumulation, in agreement with petrological modeling (e.g., Schmidt and Poli 1998). The upward migration of fluids from the slab may explain backarc and forearc anomalies. In some subduction zones, the forearc conductor extends Table 1 Location and average electrical conductivity (EC) of m subduction zones Subduction zone Backarc conductor Depth (km) EC (S/m) Distance from trench (km) Depth ( 1. Chile-Bolivia (19.5°S-21°S) 20 0.04.-0.10 220 2. Chile-Bolivia (17°S-19°S) 100 0.10-1 300 3. Costa Rica 30 0.03-0.13 150 10 4. Mexico 40 0.03-0.20 330 20 5. Philippine Sea 85 0.50 320 40 6. South Chile 25 0.10-0.30 140 15 7. Central Argentina 200 0.03-1 800 30 8. Cascadia, British Columbia 50 0.02-0.05 120 25 9. Cascadia, Oregon 80 0.03-1 120 25 10. Greece 40 0.0020.006 170 11. Mariana 40 0.01 300 <30? 12. Taiwan 35 0.0590.10 100 10 13. Kyushu, Japan 50 1 300 14. Hokkaido, Japan 40 0.01-0.10 400 40 15. North Honshu, Japan 170 0.15-0.30 260 15 16. Central New Zealand 35 0.01-0.04 200 20 n.c, zone close to trench not covered by the electromagnetic survey. from the slab upward and can relate to the arc volcanoes' plumbing system (e.g., Brasse and Eydam 2008), whereas in other subduction contexts, conductivity images suggest a connection between the volcanic plumbing system and the backarc reservoir (e.g., Evans et al. 2013). The fluid fraction is usually estimated from the bulk electrical conductivity value of the anomaly by using two-phase formalisms (e.g., ten Grotenhuis et al. 2005) and by assuming a conductivity value for the liquid phase (preferentially based on laboratory results and in agreement with petrological constraints (Pommier and Garnero 2014)). It is interesting to note that these possible fluid sinks are not vertically aligned with the arc volcanoes at the surface (e.g., Worzewski et al. 2011), though they may be rela ted to the volcanic plumbing system. In case these conductive reservoirs contribute to the volcanic activity, the shift in their location may be due to mantle flow and buoyancy processes in the mantle wedge, as suggested by some numerical experiments (e.g., Gerya and Yuen 2003). The detection of these reservoirs using electromagnetic measurements highlights the fact that electrical studies can be a powerful tool to investigate volcanic plumbing systems in subduction. ain conductors detected in electromagnetic studies of Forearc or trench-close conductor References km) EC (S/m) Distance from trench (km) n.c. Brasse et al. (2002) n.c. Brasse and Eydam (2008) 0.10-0.20 65 Worzewski et al. (2011) 0.10-0.20 40 Joedicke et al. (2006) 0.50 140 Shimakawa and Honkura (1991) 0.03-1 45 Brasse et al.(2009) 0.02 300 or less Booker et al. (2004) 0.02 50 Soyer and Unsworth (2006) 0.03-1 80 Evans et al. (2013) Galanopoulos et al. (2005) 0.006? 50-150? Matsuno et al. (2012) 0.05-0.12 30 Bertrand et al. (2012) n.c. Ichiki et al. (2000) 0.01-0.10 100 Ichiki et al. (2009) 0.10 210 Toh et al. (2006) 0.10 150 Wannamaker et al. (2009) Pommier Earth, Planets and Space 2014, 66:38 Page 3 of 11 http://www.earth-planets-space.com/content/66/1/38 Although conductive anomalies in subduction zones are almost systematically interpreted as interconnected fluids, other materials may present high electrical conductivity. In Figure 1, the electrical conductivity of fluids and other materials at conditions relevant to subduction is compared in a synthetic conductivity profile based on laboratory results. Petrological properties are from Schmidt and Poli (1998) for the slab and mantle wedge, and the thermal profile is derived from Furukawa (1993) and Poli and Schmidt (2002). Melt fraction estimates are from Grove et al. (2012). The electrical conductivity of these materials is calculated using the results by Kristinsdóttir et al. (2010) (chlorite), Wang et al. (2012) (amphibole), Zhu et al. (1999), Xie et al. (2002) and Guo et al. (2011) (serpentinite), Constable (2006) (olivine), and Ni et al. (2011) and Yoshino et al. (2010) (silicate melt). Because some of these electrical measurements were performed at lower pressure than subduction conditions, the effect of increasing pressure on conductivity was accounted for by applying a correction of −0.15 log unit in electrical conductivity per gigapascal, in agreement with observations from experimental studies (e.g., Tyburczy and Waff 1983). Tempe rature corrections were applied to measurements of electrical conductivities of chlorite. Those were made at temperatures <250°C, while petrology studies suggest that chlorite may be stable in the mantle wedge at significantly higher temperatures (up to 1,000°C) (Schmidt and Poli 1998). The electrical conductivity of chlorite at higher temperatures is predicted by extrapolation assuming a constant Arrhenian dependence to temperature over the temperature range of interest. This synthetic model suggests that the contrast in electrical conductivity between stable hydrous minerals in the slab and the mantle can be less than 1 log unit. This observation is consistent with the results from magnetotelluric studies that can hardly distinguish the slab from the surrounding mantle and, therefore, often resort to seismic studies to locate the slab (Brasse and Eydam 2008; Naif et al. 2013). Figure 1B also predicts 200 000 800 600 400 200 0