ALBERT

Description: Hydration of the oceanic lithosphere is an important and ubiquitous process which alters both the chemical and physical properties of the affected lithologies. One of the most important reactions that affect the mantle is serpentinization. The process of serpentinization results in a drastic decrease in the density (up to 40%), seismic velocity and brittle strength as well as water uptake of up to 13 wt.% of the ultramafic rock. In this paper, we use numerical models to study the amount and extent of serpentinization that may occur at mid-ocean ridges and its effects on fluid flow within the lithosphere. The two dimensional, FEM model solves three coupled, time-dependent equations: (i) mass-conserving Darcy flow equation, (ii) energy conserving heat transport equation and (iii) serpentinization rate of olivine with feedbacks to temperature (exothermic reaction), fluid consumption and variations in porosity and permeability (volume changes). The thermal structure of the ridge is strongly influenced by rock permeability in addition to the spreading velocity of the ridge. Increased rock permeability enhances hydrothermal convection and results in efficient heat mining from the lithosphere whereas higher spreading velocities result in a higher thermal gradient. Serpentinization of the oceanic mantle, in turn, depends on the aforementioned, competing processes. However, serpentinization of mantle rocks is itself likely to result in strong variations of rock porosity and permeability. Here we explore the coupled feedbacks. Increasing rates of serpentinization lead to large volume changes and therefore, rock fracturing thereby increasing rock porosity/permeability while as serpentinization reaches completion, the open pore space in the rock is reduced due to the relative dominance of mineral precipitation. Although, variations in the relation between porosity and permeability and serpentinization before the reaction reaches completion do not significantly affect the degree of serpentinization, we find that unreasonably large portions of the mantle would be serpentinized if rock closure does not occur at the final reaction stage. The amount of water trapped as hydrous phases within the mantle shows a strong dependency on the spreading velocity of the ridge with water content ranging from 0.18 × 105 kg/m2 to 2.52 × 105 kg/m2. Additionally, two distinct trends are observed where the water content in the mantle at slow-spreading ridges drops dramatically with an increase in spreading velocity. The amount of water trapped in the mantle at fast-spreading ridges, on the other hand, is lower and does not significantly depend on spreading velocity.

Description: The genesis of oceanic crust at intermediate to fast spreading ridges occurs by the crystallization of mantle melts accumulated in at least one shallow melt lens situated below the ridge axis. Seismic reflection data suggest that the depth of this melt lens is inversely correlated with spreading rate and thereby magma supply. The heat released in it by crystallization and melt injection is removed by a combination of hydrothermal cooling and diffusion. Due to the different time scales of hydrothermal cooling and crustal accretion, numerical models have so far focused on only one of the two processes. Here we present the results from a coupled model that solves simultaneously for crustal accretion and hydrothermal cooling. Our approach resolves both processes within one 2D finite-element model that self-consistently solves for crustal, mantle, and hydrothermal flow. The formation of new oceanic crust is approximated as a gabbro glacier, in which the entire lower crust crystallizes in one shallow melt lens. We find that the depth of the melt lens and the shape of hot (potentially molten) lower crust are highly dependent on the ridge permeability structure. The predicted depth of the melt lens is primarily controlled by the permeability at the ridge axis, whereas the off-axis permeability determines the width of hot lower crust. A detailed comparison of the modeling results with observed locations of the melt lens at intermediate to fast spreading ridges shows that only a relatively narrow range of crustal permeabilities is consistent with observations. In addition, we find significant deviations between models that resolve or parameterize hydrothermal cooling: the predicted crustal thermal structures show major differences for models that predict the same melt lens location. This illustrates the importance of resolving hydrothermal flow in simulations of crustal accretion.

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.