Mantle wedge water contents estimated from seismic velocities in partially serpentinized peridotites

Abstract

[1] Water carried into subduction zones with the downgoing plate and subsequently released by dehydration reactions at depth affects the composition of the mantle wedge, triggers partial melting and affects subduction zone seismicity. Partially serpentinized peridotite may be a significant reservoir for water in the subducted plate, the mantle wedge and the overriding plate. Here we develop a model that relates the degree of serpentinization and water content of partially serpentinized peridotites to their seismic P-wave velocities. In partially-serpentinized ultramafic rocks, a 1% decrease in P-wave velocity corresponds to a 2.4% increase in serpentine content, and a 0.3% (0.18 moles/m 3 ) increase in H 2 O content (up to a maximum of 13%). Where there is evidence of serpentinization, mantle serpentine content is typically $15%, corresponding to 4 -5 wt% H 2 O (6 -10 moles/m 3 ).

Subduction of the oceanic Hikurangi Plateau and its impact on the Kermadec arc

“…4a) can be observed further north, where the subduction of the Louisville Seamount Chain affects the central Tonga-Kermadec arc 14 . Tomography results from a trench-perpendicular ocean bottom seismometer array oriented east-west across the Kermadec trench to the Havre Trough at B37°S (refs 30,31) show zones of reduced seismic velocities (that is, Vp r7.8 km s À 1 compared with Z8 km s À 1 for mantle peridotite) beneath the arc front MOHO, consistent with Z10% of serpentenized mantle being present beneath the arc 44 . A similar B10-km thick crust-mantle transition layer with lower seismic velocities (that is, Vp ¼ 7.0-7.7 m s À 1 ) has also been identified beneath the Izu-Bonin-Mariana and Tonga arcs, interpreted to be composed of a mixture of crustal and mantle materials 45,46 .…”

Resolving the fine-scale velocity structure of continental hyperextension at the Deep Galicia Margin using full-waveform inversion

“…The other anomalous feature is coincident with fault F5, and reaches the model's maximum allowed velocity of 8.50 km s -1 . We expect the velocity of unaltered upper mantle peridotite to be ~ 8.00 km s -1 , thus making this observation implausible (e.g., Carlson and Miller 2003). These features appear to be artefacts introduced during the FWI process.…”

“…Relative velocity decreases (from the normal fault to the nearest down-dip velocity minima) of ~0.60 km s -1 , ~0.60 km s -1 , ~0.70 km s -1 and ~1.0 km s -1 are observed for faults F5, F6a, F6 and F7, respectively. Using the study of Carlson and Miller (2003) we can approximate the extent of mantle peridotite serpentinisation, based on the observed Pwave velocities. Down-dip of these faults we calculate the degree of serpentinisation, averaged over the resolution length of the FWI, to change from 0 to 20%, 30 to 40%, 30 to 50% and 30 to 60%, for faults F5, F6a, F6 and F7, respectively.…”

Large‐scale rigid‐body rotation in the mantle wedge and its implications for seismic tomography

“…The subjacent mantle is considered to be anhydrous peridotite, whereas the overlying mantle is either anhydrous or hydrated ( 2 wt% water) peridotite depending on the kinematic model for the propagation of slab fluids discussed below. Although mantle wedge rocks have the capacity to absorb up to 8 wt% water during serpentinization [Connolly, 2005], we adopt 2 wt% water as an upper limit to account for heterogeneous hydration resulting from channelization of slab-derived fluids [Davies, 1999] as consistent with observed mantle wedge seismic velocities [Carlson and Miller, 2003]. The boundary conditions account for incoming and outgoing asthenospheric flow under the overriding plate.…”