A modelling study of ridge flank hydrothermal circulation globally, constrained by fluid and rock chemistry, and seafloor heat flow
Date
2014-04-28
Authors
Anderson, Brock
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Abstract
Hydrothermal circulation through the seafloor on the mid-ocean ridge flanks is responsible for globally significant fluid, heat and chemical fluxes between the ocean and the oceanic crust. This dissertation investigates the locations of fluid ingress and egress, fluid flow paths within the crust, and the hydrology of the crust. Based on a global compilation of sediment interstitial water chemistry and models of interstitial water chemical transport and reaction, it is found that <10% of the ridge flank hydrothermal fluid flux passes through marine sediments globally. This requires that the large majority of hydrothermal fluid enters and leaves the crust through exposed basement outcropping through the sediment (“outcrops”). A probabilistic model of basement topography and sedimentation was used to quantify the distribution of seafloor outcrops globally, estimating that outcrops are, on average, a few kilometres apart on young crust, increasing to tens of kilometres apart as the crust ages. A model in which fluid travels laterally within the crustal aquifer for kilometres to tens of kilometres between discrete outcrops (“outcrop-to-outcrop flow”) is consistent with the global heat flow data. This finding supports the proposition that outcrop-to-outcrop flow is the dominant mode of ridge flank hydrothermal circulation globally. An alternative model of ridge flank hydrothermal circulation in which fluid circulation occurs by local convection within isolated outcrops is also possible, and is probably the dominant mode of circulation in crust younger than 3-5 Myrs old, on average, where there is insufficient sediment cover to support the lateral pressure gradients required by outcrop-to-outcrop flow. Estimated crystallization temperatures of carbonate minerals in the crust suggest that, at some locations in the aquifer, local convective mixing may be restricted (i.e., the aquifer is poorly mixed), whereas the carbonate data for other locations cannot distinguish between a well mixed and a poorly mixed aquifer. A poorly mixed aquifer requires that vertical permeability is 1.5 - 2.5 orders of magnitude lower than horizontal permeability. This permeability anisotropy may arise from interlaying of different lithological units within the upper crust.
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Keywords
hydrothermal systems, marine geology, numerical modelling