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

Abstract

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.