Smoke Without Fire: How Long Can Thermal Cracking Sustain Hydrothermal Circulation in the Absence of Magmatic Heat?

Abstract

Long‐lived hydrothermal circulation is now well documented along slow and ultraslow spreading mid‐ocean ridges, even though these settings only receive a moderate and intermittent supply of magma. This challenges the notion that hydrothermal convection must be sustained by a continuously replenished magma body. Here we investigate the possibility of sustaining hydrothermal circulation by infrequent magmatic intrusions separated by episodes of downward propagation of small cracks enabling fluids to tap heat from deep hot rocks. We focus on cracks nucleating from grain boundary flaws in response to the buildup of cooling stresses at the base of the convection system. We develop an analytical model describing the stable propagation of a percolation front and the associated heat transfer through hydrothermal circulation. Convection ceases when thermoelastic stresses can no longer overcome lithostatic pressure and hydrothermal circulation can no longer mine heat from underlying units. For lithospheric permeabilities greater than ∼10−15 m2 and over a wide range of grain and flaw sizes, this occurs within ∼100 kyr to ∼1 Myr from the onset of cracking, after the cracking front has moved by a few kilometers. We validate this analytical prediction by developing two‐dimensional numerical models of porous convection subjected to a lower boundary condition representing the dynamics of the cracking front. These models suggest that moderate‐ to low‐temperature hydrothermal venting in off‐axis, ultramafic‐hosted sites does not require an underlying magma sill at all times, but instead repeated sill intrusions every ∼10–100 kyr, which periodically reinvigorate hydrothermal convection without building a continuous crustal section.

Why Is Crustal Underplating Beneath Many Hot Spot Islands Anisotropic?

“…The ascent of magma will induce stresses and small fractures in the crust, surrounding the magma conduit, perhaps re‐opening the thermal‐crack network that Korenaga () argues to be a better choice for seawater infiltration than earthquake faults. Olive and Crone () develop a thermal‐cracking model for the mid‐ocean ridge that cools new oceanic crust, allowing seawater to serpentinize the mantle. If basaltic magma ponds and persists below the Moho, the hot spot mantle will be too hot to produce antigorite, so metasomatic and magmatic underplating cannot coexist.…”

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

“…Two general models have emerged: the first invokes a cracking front that taps heat by cracks migrating downward into the top of the CBL (e.g., Lister, , ) and the second requires replenishment of the magmatic heat source keeping the base of the CBL from thickening downward (e.g., Lowell & Germanovich, ). It is now generally accepted that the latter model is a better representation of magma‐rich environments such as fast‐spreading ridges, whereas the former model may be appropriate for some magma‐poor settings (Olive & Crone, ; Wilcock & Delaney, ). More recent models have explored how hydrothermal cooling effects the evolution of AMLs and how episodic replenishment of AMLs impacts heat transfer into the hydrothermal system (e.g., Fontaine et al, ; Liu & Lowell, ).…”

Brine Formation and Mobilization in Submarine Hydrothermal Systems: Insights from a Novel Multiphase Hydrothermal Flow Model in the System H2O–NaCl

“…This requires a sufficient basal inclination and hot basal temperatures (within the two-phase region). A complementary possibility is that brines may be able to move to deeper crustal levels possibly along pathways newly formed by thermal cracking (Olive and Crone 2018). At T = 600 °C (the approximate temperature at the brittle-ductile transition) and 45 MPa, brines have a density of ~ 1200 kg/m 3 , a salinity of ~ 70 wt%, and a viscosity of ~ 2 × 10 -4 Pa•s (comparable to the viscosity of pure water at 150 °C and 45 MPa).…”