Anhydrite precipitation and the relationship between focused and diffuse flow in seafloor hydrothermal systems

Abstract

[1] The mechanisms that control the style of high-temperature hydrothermal discharge at ocean ridge crests are not well understood. Sites of vigorous high-temperature focused venting are often accompanied by nearby regions of low-temperature diffuse flow. Moreover, at some sites, high-temperature fluids are not focused at all and emerge as lowtemperature diffuse flow as a result of conductive cooling and/or mixing with cooler seawater, whereas at others discharge has evolved from low-temperature diffuse flow to high-temperature focused flow over a number of years. To better understand the mechanisms that control these discharge styles, we construct a two-branch single-pass hydrothermal circulation model to investigate the role of anhydrite precipitation in mixing between deep-seated, high-temperature, sulfate-free upflow and cooler, sulfate-rich seawater circulating through permeable pillow basalts. We also estimate the width of the impermeable anhydrite barrier between focused and diffuse discharge. The model results suggest that focusing can occur in a matter of years provided the permeability in the deep part of the discharge zone is greater than or equal to that of the mixing zone and the permeability of the mixing zone is initially high ($10 À11 -10 À12 m 2 ). If one of the previous conditions is not met, only diffuse flow venting will occur. The width of the impermeable barrier resulting from the precipitation of anhydrite ranges between 1 and 100 m, thus permitting diffuse flow to occur in close proximity to focused discharge.INDEX TERMS: 3015 Marine Geology and Geophysics: Heat flow (benthic) and hydrothermal processes; 3035 Marine Geology and Geophysics: Midocean ridge processes; 5114 Physical Properties of Rocks: Permeability and porosity; 8135 Tectonophysics: Hydrothermal systems (8424); KEYWORDS: seafloor hydrothermal systems, anhydrite precipitation, permeability, diffuse venting, black smokers Citation: Lowell, R. P., Y. Yao, and L. N. Germanovich, Anhydrite precipitation and the relationship between focused and diffuse flow in seafloor hydrothermal systems,

Lithium and Li-isotopes in young altered upper oceanic crust from the East Pacific Rise

“…If this is true, then the observed focussed venting on the seafloor requires focussing of fluid flow to occur near the base of the lava pile. Focused discharge observed at black smokers may be related to the existence of high permeability conduits within the lavas due to faulting (e.g., Haymon et al, 1991;Saccocia and Gillis, 1995) or precipitation of secondary minerals that locally decreases the permeability in the wall rocks around upflow zones (e.g., Lowell et al, 2003). In locations where focussing of upwelling fluids does not occur, upwelling high-temperature fluids exiting the sheeted dike complex would mix with ambient fluid in the lava pile and could lead to the widespread existence of warm fluid in the lavas.…”

Subsurface conditions in hydrothermal vents inferred from diffuse flow composition, and models of reaction and transport

“…where φ is the difference between the porosity and the percola- For comparison, Lowell et al (2003) use scale analysis to show that the sealing time can be given approximately by the expression…”

“…Lowell and coworkers have developed a number of approaches to modeling diffuse flow near mid-ocean ridges (Lowell et al, 2003(Lowell et al, , 2015Crowell et al, 2008;Craft and Lowell, 2009). For this study, a steady-state temperature field for an axial basalthosted hydrothermal system is simulated by solving the coupled equations of conservation of mass, momentum and energy, assuming Darcy flow using the numerical simulator FISHES (Lewis and Lowell, 2009) in a two-dimensional model domain 2 km wide × 1.5 km deep divided into 25 m × 25 m grid cells (Fig.…”

“…Equation (4) is modified slightly from equation (18) in Lowell et al (2003) to take into account that the width of the mixing zone may differ from that of the downgoing limb of the induced convection cell in layer 2A.…”

Hydrothermally-induced melt lens cooling and segmentation along the axis of fast- and intermediate-spreading centers

“…In this respect, it is worth noting that our model considers a uniform permeability hydrothermal layer. Permeability contrasts between upflow and downflow zones are poorly constrained, but a higher recharge than discharge permeability due to mineral precipitation/dissolution (e.g., anhydrite or sulphurs [Lowell et al, 2003;Fontaine et al, 2001]) will proportionally decrease AMC cooling time, and vice-versa.…”