Sustainability of Outcrop-to-Outcrop Hydrothermal Siphons in Three Dimensions

Abstract:

Hydrothermal circulation in the igneous ocean crust on ridge flanks (far from spreading centers) is limited by thick accumulations of low-permeability sediment, except where basement outcrops that penetrate through the sediment provide high-permeability conduits for flows of fluid, heat and solutes. Hydrothermal siphons can form between recharging and discharging outcrops, driven by density differences between cooler and warmer fluids, although the physics controlling the pattern, magnitude, and direction of flow in these systems are not well understood. We present the first three-dimensional coupled flow models of a self-sustaining, outcrop-to-outcrop hydrothermal siphon that operate between seamounts separated by tens of kilometers. Parametric tests of these models show how physical properties determine flow behavior and control the sustainability and flow rates of hydrothermal siphons. Model construction was guided by physical characteristics, and subseafloor geochemical, thermal, and pressure data, from 3.5 M.y. old seafloor on the eastern flank of the Juan de Fuca Ridge, one of the best-studied examples of a hydrothermal siphon. One set of models generated results that were consistent with field observations from this area, including little to no regional heat flow suppression and siphon throughflow across a distance of 50 km of ≤17 kg/s. Model results suggest that bulk permeability (k) and the area of seafloor exposure (A_oc) of an outcrop have similar effects on siphon sustainability, and that sustaining a siphon requires that the product of these two terms (T_oc=k x A_oc; outcrop transmittance) must differ by two orders of magnitude at sites of recharge and discharge. In addition, models that include a good match to observations include basement permeability that is consistent with direct measurements made with a borehole packer, but is one to two orders of magnitude lower than inferred from earlier studies (based on two dimensional models and tidal analyses). Finally, model results suggest that siphon flow is only sustainable in one direction: recharging through outcrops with higher T_oc, and discharging at those with lower T_oc. This finding is consistent with field observations that generally favor discharge at smaller outcrops.