Subduction factory 1. Theoretical mineralogy, densities, seismic wave speeds, and H₂O contents

Abstract

[1] We present a new compilation of physical properties of minerals relevant to subduction zones and new phase diagrams for mid-ocean ridge basalt, lherzolite, depleted lherzolite, harzburgite, and serpentinite. We use these data to calculate H 2 O content, density and seismic wave speeds of subduction zone rocks. These calculations provide a new basis for evaluating the subduction factory, including (1) the presence of hydrous phases and the distribution of H 2 O within a subduction zone; (2) the densification of the subducting slab and resultant effects on measured gravity and slab shape; and (3) the variations in seismic wave speeds resulting from thermal and metamorphic processes at depth. In considering specific examples, we find that for ocean basins worldwide the lower oceanic crust is partially hydrated (<1.3 wt % H 2 O), and the uppermost mantle ranges from unhydrated to $20% serpentinized ($2.4 wt % H 2 O). Anhydrous eclogite cannot be distinguished from harzburgite on the basis of wave speeds, but its $6% greater density may render it detectable through gravity measurements. Subducted hydrous crust in cold slabs can persist to several gigapascals at seismic velocities that are several percent slower than the surrounding mantle. Seismic velocities and V P /V S ratios indicate that mantle wedges locally reach 60-80% hydration.

Nitrogen recycling in subducted mantle rocks and implications for the global nitrogen cycle

“…924 Table 2. 947 The calculations assume a convergence rate of 0.05 m/year and a total arc length of 44000 km (Straub and Layne, 2003) Densities for the metamorphosed lithological components are based on data for metamorphosed MORB and harzburgite at 600°C and 25 kbar (Hacker et al, 2003) The N concentrations given for the bulk slab calculations reflect median values.…”

Distribution of hydrous minerals in the subduction system beneath Mexico

“…In central Mexico, significant reduction in S wave velocity by more than 20% (Song et al, 2009) at the shallow-to-flat transition of the top interface of the Cocos plate ( $ 20245 km depth) are difficult to explain simply by velocity difference predicted for plausible subducted oceanic crust lithologies such as lawsonite blueschist and epidote blueschist (Currie et al, 2002), which is only less than 7% (Hacker et al, 2003a). In comparison to previously reported values for central Mexico subduction zone, the Vp/Vs ratios (or Poisson's ratios) shown in Audet et al (2009) and Peacock et al (2011) for Cascadia subduction zone are quite extreme, and both Audet et al (2009) and Peacock et al (2011) argued that such high estimates cannot be due to the presence of hydrous minerals alone but are due to free-water at high porepressure.…”

Forearc decoupling of guided waves in the Chile‐Peru subduction zone

“…They contribute to still open geoscientific questions such as phase transformations of the slab and its composition, the depth of the gabbro-eclogite transformation during the metamorphosis of the subducting oceanic crust, and the location of intermediate depth earthquakes relative to the slab surface [e.g., Helffrich and Abers, 1997;Hacker et al, 2003].…”