Validation of the BASALT model for simulating off‐axis hydrothermal circulation in oceanic crust

Abstract

Fluid recharge and discharge between the deep ocean and the porous upper layer of off‐axis oceanic crust tends to concentrate in small volumes of rock, such as seamounts and fractures, that are unimpeded by low‐permeability sediments. Basement structure, sediment burial, heat flow, and other regional characteristics of off‐axis hydrothermal systems appear to produce considerable diversity of circulation behaviors. Circulation of seawater and seawater‐derived fluids controls the extent of fluid‐rock interaction, resulting in significant geochemical impacts. However, the primary regional characteristics that control how seawater is distributed within upper oceanic crust are still poorly understood. In this paper we present the details of the two‐dimensional (2‐D) BASALT (Basement Activity Simulated At Low Temperatures) numerical model of heat and fluid transport in an off‐axis hydrothermal system. This model is designed to simulate a wide range of conditions in order to explore the dominant controls on circulation. We validate the BASALT model's ability to reproduce observations by configuring it to represent a thoroughly studied transect of the Juan de Fuca Ridge eastern flank. The results demonstrate that including series of narrow, ridge‐parallel fractures as subgrid features produces a realistic circulation scenario at the validation site. In future projects, a full reactive transport version of the validated BASALT model will be used to explore geochemical fluxes in a variety of off‐axis hydrothermal environments.

Evaluating the Physics of Outcrop‐To‐Outcrop Flow With Hydrothermal Flow Models

“…We did not address the effect of the thickness of the aquifer on the resulting flow since this has already been investigated in previous studies (Farahat et al., 2017; Lauer et al., 2018; Stein & Fisher, 2003; Winslow et al., 2016). Those studies found that outcrop‐to‐outcrop flow through a thicker aquifer causes enhanced siphon flow rates.…”

“…However, previous studies have successfully performed numerical simulations of outcrop‐to‐outcrop flow in 2‐D (Anderson et al., 2012; Davis et al., 1997; Farahat et al., 2017; Hutnak et al., 2006; Stein & Fisher, 2003) and 3‐D (Lauer et al., 2018; Price et al., 2022; Winslow & Fisher, 2015; Winslow et al., 2016). These models converge on the prediction that outcrop‐to‐outcrop flow requires high aquifer permeabilities >10 −13 m 2 (Fisher, Davis, et al., 2003; Winslow et al., 2016).…”

Transport of Heat by Hydrothermal Circulation in a Young Rift Setting: Observations From the Auka and JaichMaa Ja'ag' Vent Field in the Pescadero Basin, Southern Gulf of California

“…Although the fault model seems appropriate and straightforward for the Pescadero vents, it does seem to be the exception compared to other known sediment‐hosted vent fields in the Pacific, which require entirely different boundary conditions than those observed in the Pescadero Basin. Convection at ridge flanks usually is sealed off from the ocean by a layer of fine‐grained sediments of the deep ocean which typically have a permeability much lower than that of the upper volcanic crust resulting in little fluid exchange through seafloor sediments (e.g., Farahat et al., 2017; Fisher & Wheat, 2010). In this model, seamounts play a fundamental role in facilitating the exchange of fluids, heat, and solutes between the oceanic lithosphere and the overlying ocean by providing pathways for discharge and recharge.…”

Transport of Heat by Hydrothermal Circulation in a Young Rift Setting: Observations From the Auka and Jaichmaa Ja'ag' Vent Field in the Pescadero Basin, Southern Gulf of California

“…Although the fault model seems appropriate and straightforward for the Pescadero vents, it does seem to be the exception compared to other known sediment-hosted vent fields in the Pacific, which require entirely different boundary conditions than those observed in the Pescadero Basin. Convection at ridge flanks usually is sealed off from the ocean by a layer of fine-grained sediments of the deep ocean which typically have a permeability much lower than that of the upper volcanic crust resulting in little fluid exchange through seafloor sediments (e.g., Farahat et al, 2017;Fisher & Wheat, 2010). In this model, seamounts play a fundamental role in facilitating the exchange of fluids, heat, and solutes between the oceanic lithosphere and the overlying ocean by providing pathways for discharge and recharge.…”