Controls of faulting and reaction kinetics on serpentinization and double Benioff zones

Abstract

[1] The subduction of partially serpentinized oceanic mantle may potentially be the key geologic process leading to the regassing of Earth's mantle and also has important consequences for subduction zone processes such as element cycling, slab deformation, and intermediate-depth seismicity. However, little is known about the quantity of water that is retained in the slab during mantle serpentinization and the pattern of serpentinization that may occur during bending-related faulting; an initial state that is essential for quantifying subsequent dehydration processes. We present a 2-D reactive-flow model simulating hydration processes in the presence of faulting at the trench outer-rise. We find that the temperature dependence of the serpentinization rate in conjunction with outer-rise faulting results in plate age and speed dependent patterns of hydration. Serpentinization also results in a reduction in surface heat flux toward the trench caused by advective downflow of seawater into the reaction region. Observed heat flow reductions are larger than the reduction due to the minimum-water downflow needed for partial serpentinization, predicting that active hydrothermal vents and chemosynthetic communities should also be associated with bend-fault serpentinization. Our model results agree with previous studies that the lower plane of double Benioff zones can be generated due to dehydration of serpentinized mantle at depth. More importantly, the depth-dependent pattern of serpentinization including reaction kinetics predicts a separation between the two Benioff planes consistent with seismic observations.

Seismic imaging of slab metamorphism and genesis of intermediate-depth intraslab earthquakes

“…If this mechanism actually operates in the mantle, it could explain the formation of double seismic zones. Iyer et al (2012) suggested that the double hydrated zone at the depths of the slab mantle, through slab unbending ( Fig. 22; Faccenda et al 2012) and maximum serpentinization at c. 270°C, may together cause the variations in the observed separation of the double seismic zone.…”

“…The possibility of deep hydration of the slab mantle prior to subduction was also demonstrated by a recent 2-D reactive-flow model simulation of hydration processes at the trench-outer rise region (Iyer et al 2012). Iyer et al (2012) developed a reaction-transport model that enables better quantification of the amount and depth at which hydration occurs in an incoming oceanic plate due to normal faulting caused by the bending force. The hydration patterns determined with this model show the formation of a highly serpentinized band in the slab mantle centered at the 270°C isotherm.…”

“…This bending force causes normal faulting in the trenchouter rise, and seawater enters along faults, in which hydrous minerals are then formed (Fig. 2) (e.g., Kirby, 1995;Peacock, 2001;Ranero et al 2003;Faccenda et al 2009;Iyer et al 2012). (3) Seawater enters along transform faults near mid-ocean ridges because of fault movement, and hydrated minerals form along these faults as well (e.g., Bonatti and Honnorez, 1976;Gregg et al 2007).…”

Serpentinization of mantle peridotites along an uplifted lithospheric section, Mid Atlantic Ridge at 11° N

“…Oceanic peridotites acquire H 2 O during serpentinization (up to~13%), together with significant amounts of B and Cl; their subduction may carry part of this boron-enriched water back into the mantle (Barnes and Sharp, 2006;Bonatti et al, 1984;Boschi et al, 2008;Iyer et al, 2012;Kodolányi et al, 2012;Rüpke et al, 2004;Scambelluri et al, 2004a,b;Ulmer and Trommsdorff, 1995;Vils et al, 2009). Low-T ultramafic rock-seawater reactions can release Mg to the ocean (Niu, 2004;Snow and Dick, 1995), deposit carbonates (Bonatti et al, 1980) and uptake organic carbon (Delacour et al, 2008;Früh-Green et al, 2003) that can also be recycled later through subduction (Kerrick and Connolly, 1998).…”

Competition Between Crystallization‐Induced Expansion and Creep Compaction During Gypsum Formation, and Implications for Serpentinization

“…We use equation to determine the reaction extent at any time but modify it to include a linear dependence on porosity to mimic the effect of water supply on the reaction rate (e.g., Iyer et al, ), so that To describe creep in the product gypsum phase we make use of the work of de Meer (), who found that the bulk strain rate due to uniaxial creep of wet gypsum can be described by an equation of the form where A is a constant equal to 4 × 10 −33 and d is the grain size in meters. Equation gives the strain rate due to compaction via decrease in the pore volume.…”