Initial Modeling of Hydrothermal Circulation at the Mid-Cayman Spreading Center

Abstract:

Hydrothermal plume data and other observations were used to parameterize the Piccard (PHF) and Von Damm (VDHF) hydrothermal fields on the Mid-Cayman Spreading Center. The PHF is hosted in basalt at a seafloor depth of nearly 5000 m where a number of active sites vent with temperatures as high as 400°C from an area of approximately 10⁵ m². A neutrally buoyant hydrothermal plume occurs at a height of approximately 500 m above the PHF. Buoyant plume theory yields a buoyancy flux of ~4 x10^-2 m⁴/s³ and a heat flux of ~160 MW. Using a single-pass model, this heat output and maximum vent temperature yields a mass flow rate of ~80 kg/s and a crustal permeability of ~2 x 10^-14 m². A neutrally buoyant plume noted in 2010 at a height of approximately 1000 m above the seafloor corresponds to a buoyancy flux of ~ 0.64, yielding a heat output ~ 2.6 GW and a mass flow rate of ~1280 kg/s, which is much larger than any known system to date. We suspect that the plume observed at 1000 m was an event plume rather than a chronic plume. The PHF is likely driven by a subsurface magma source that exhibited a magmatic event in 2010.

The VDHF, hosted in ultramafic rocks near the top of an OCC approximately 13 km west of the neovolcanic zone, lies at a depth of approximately 2300 m. Venting occurs from a conical mound approximately 100 m in diameter. A plume signal at a height of ~300 m above the VDHF yields a buoyancy flux of ~ 5 x 10^-3 m⁴/s³ and a heat output of ~20 MW. Assuming a vent temperature of 150°C based on the presence of anhydrite near the vents, the single-pass model yields a mass flow rate of ~ 30 kg/s and a permeability of 10^-11 m². The low estimated heat output for the VDHF suggests that is likely driven by circulation through a deep-seated fault or fracture system rather than by subsurface magma. The vent temperature, heat output, and permeability estimates, provide useful parameters for constructing mathematical and numerical models of the PHF and the VDHF.