ALBERT

Description: 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. Little is known about the quantity of water that is retained in the slab during mantle serpentinization. Recent studies using thermodynamical and/or experimental models of subduction zone processes have assumed that the mantle is uniformly serpentinized to a depth determined from the equilibrium stability of serpentine minerals in P-T space. This approach yields an incomplete picture of the pattern of serpentinization that may occur during bending-related faulting; an initial state that is essential for quantifying subsequent dehydration processes. In order to provide further constraints on the pattern of hydration and the amount of water trapped in the subducting mantle, we build a 2-D reactive-flow model incorporating the kinetic rate-dependence of serpentinization based on experimental results. After simulating hydration processes at the trench outer-rise, we find that the water content in serpentinized mantle strongly depends on the age of the subducting lithosphere and subduction rate, with values ranging between 1.8x105 and 4.0x106 kgm-2 reactive water uptake into the subducting mantle column. Serpentinization also results in a reduction in surface heat flux towards 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. 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. The depth-dependent pattern of serpentinization including reaction kinetics predicts a separation between the two Benioff planes consistent with seismic observations.

Description: In this paper, we constrain the input and output fluxes of H2O, Cl and S into the southern-central Chilean subduction zone (31°S–46°S). We determine the input flux by calculating the amounts of water, chlorine and sulfur that are carried into the subduction zone in subducted sediments, igneous crust and hydrated lithospheric mantle. The applied models take into account that latitudinal variations in the subducting Nazca plate impact the crustal porosity and the degree of upper mantle serpentinization and thus water storage in the crust and mantle. In another step, we constrain the output fluxes of the subduction zone both to the subcontinental lithospheric mantle and to the atmosphere–geosphere–ocean by the combined use of gas flux determinations at the volcanic arc, volume calculations of volcanic rocks and the combination of mineralogical and geothermal models of the subduction zone. The calculations indicate that about 68 Tg/m/Ma of water enters the subduction zone, as averaged over its total length of 1,480 km. The volcanic output on the other hand accounts for 2 Tg/m/Ma or 3 % of that input. We presume that a large fraction of the volatiles that are captured within the subducting sediments (which accounts for roughly one-third of the input) are cycled back into the ocean through the forearc. This assumption is however questioned by the present lack of evidence for major venting systems of the submarine forearc. The largest part of the water that is carried into the subduction zone in the crust and hydrated mantle (accounting for two-thirds of the input) appears to be transported beyond the volcanic arc.