A Review of the Geological Constraints on the Conductive Boundary Layer at the Base of the Hydrothermal System at Mid‐Ocean Ridges

Abstract

Models of high‐temperature hydrothermal systems at intermediate‐ to fast‐spreading mid‐ocean ridges have a conductive boundary layer (CBL) separating the magmatic heat source from the convecting hydrothermal fluid. Paleo‐CBLs preserved in the geological record provide a means to test theoretical models of the thermal, mechanical, and petrological evolution of this boundary. CBLs occur as metamorphic contact aureoles at or near the dike‐gabbro boundary where axial magma lenses (now plutonic rocks) intruded into the basal dikes, leading to their transformation into low permeability granulite and hornblende hornfels at >800 °C. Paleo‐CBLs are well documented in the Troodos and Oman ophiolites, and evidence of similar lithologies has been found at every location where the dike‐gabbro transition has been mapped and/or sampled in modern fast‐spreading crust (Hess and Pito Deeps, IODP Hole 1256D). Evaluation of the geothermometers used in studies of the thermal evolution of the CBL shows a lack of consistency that can be understood in terms of compositional controls. Peak temperatures are >900 °C in all areas at the base of the CBL, leading to partial melting, stoping, and disaggregation that facilitates thinning from below as required to maintain the high heat fluxes necessary to drive active black smoker systems. However, such thin CBLs are not the norm; steady state conditions must have thicker CBLs and smaller heat fluxes. In turn, global estimates of properties such as chemical fluxes for normal hydrothermal conditions may lead to substantially different element/heat fluxes than those based on active systems.

Quantifying the Axial Magma Lens Dynamics at the Roof of Oceanic Magma Reservoirs (Dike/Gabbro Transition): Oman Drilling Project GT3 Site Survey

“…An apparent temperature gradient (or metamorphic field gradient; gray box) of ∼3°C.m −1 can be estimated from our data. Similar apparent temperature gradient estimates obtained by Gillis and Coogan (2019) in the Troodos ophiolite (dashed lines, and corresponding circles), and by Koepke et al. (2008) at ODP Hole 1256D (dotted lines, and corresponding triangles) are reported for comparison.…”

“…Our results highlight that the maximum recorded temperature is decreasing up section from the contact (maximum temperatures up to 1040°C) to less recrystallized dikes at 50 meters from the contact (maximum temperatures < 900°C; Figure 7d). This corresponds to an apparent temperature gradient of ~3°C.m -1 which is in the same order of magnitude as the apparent T gradient obtained by Koepke et al (2008) in ODP Hole 1256D (~3°C.m -1 ), and by Gillis & Coogan (2019) in the Troodos ophiolite (~4 °C.m -1 ). This temperature gradient is here considered apparent as it records the maximum temperature reached at each point of the contact metamorphism aureole, but none of these maximum temperatures were reached at the same time as heat transfer by diffusion takes time.…”

“…Field, structural, and petrological observations were interpreted in the frame of a dynamic magma lens model where the dynamic AML can move up or down, and potentially intrude and reheat the root-zone of the SDC (e.g., Coogan et al, 2003;Gillis, 2008;France et al, 2009France et al, , 2014. Although very high prograde temperatures are recorded in the reheated dikes (1045°C in Koepke et al, 2008; 1030°C in France et al, 2009; 1040°C in Gillis & Coogan, 2019) there is currently no data available on the temperature range of the reheating stage (from which low temperature are those dykes reheated?). This temperature range could be of tens or hundreds of degrees, and is a key parameter if we are to fully understand the interactions between the magmatic and hydrothermal systems.…”

“…formed at fast-spreading centers contain both amphibole-bearing and amphibole-free domains (e.g., Gillis, 2008;Gillis & Coogan, 2019;Koepke et al, 2008). In contrast, granoblastic assemblages associated with dikes intruding gabbros at slow-spreading ridges are all amphibole-rich (Dick et al, 2019).…”

“…It is clear from (d) that the 3°C.m −1 value obtained for the apparent temperature gradient in Figure 7d can neither be used as a proxy for geothermal gradient, nor in discussions aiming at comparing geothermal gradients to heat fluxes at active vents as it has been sometimes done in previous studies. et al (2004), Gillis and Coogan (2019), Lesher and Spera (2015), Spear and Peacock (1990) with a thermal conductivity of 3.0 W/m/K, a rock density of 3,000 kg/m 3 , a specific capacity of 1,200 J/kg/K , a latent heat of crystallization of 400 kJ/kg. When considering the equilibrium temperatures obtained from geothermometers at the gabbro/dike contact (1,000°C-1,040°C), models show that the intruding melt temperature should be of at least 1,200°C, casting for a relatively primitive basaltic melt.…”