The geological controls on the Von Damm Vent Field

Abstract

Hydrothermal systems at mid-ocean ridges (MOR) are of scientific interest due to their role in oceanic crustal heat loss, global geochemical cycles, and the evolution of chemosynthetic life. Hydrothermal circulation at MOR is usually characterised by high-temperature (up to 400°C), low pH (~3) vent fluids with high concentrations of dissolved base metals and H₂S. The driver of this hydrothermal circulation is magmatic heat, and results in the formation of massive sulphide deposits. Hydrothermal vent fields were originally thought to be sparse at slow spreading ridges due to the low magma supply. However, the emergence of low angle detachment faulting at ridges with spreading rates of < 75 mm yr^-1, indicated that tectonic controls are of equal importance to magmatic activity in generating hydrothermal circulation at slow-spreading MOR. These oceanic core complexes (OCC) expose lower crustal and upper mantle rocks along the flanks, and result in a greater geochemical diversity of hydrothermal deposit at slow spreading MOR. This thesis describes the geological setting, mineralogy and geochemistry of the Von Damm Vent Field (VDVF), which is hosted atop the Mt. Dent OCC at a seawater depth of 2300 metres.

The VDVF was discovered in 2010, and is located in the Caribbean on the ultraslow spreading Mid-Cayman Rise. It is hosted in a gabbro-peridotite basement, 13 km away from the axial rift. The mineralogy is predominantly talc (Mg₃Si₄O₁₀(OH)₂), with accessory microcrystalline silica and sulphides. The talc has a botryoidal and banded habit, textures that are indicative of precipitation from a hydrothermal fluid. Trace element geochemistry indicates a strong, positive europium anomaly in the talc, characteristic of high-temperature vent fluids. 87Sr/86Sr_talc ranges from 0.70613- 0.70913, in between the value of the end-member vent fluid (0.70291) and seawater (0.70917). Hydrothermal vent fluids have low metal concentrations, intermediate pH (5.8) and high concentrations of chloride (667 mmol kg^-1) relative to seawater, and are generated by interaction with the mafic-ultramafic basement at low water-rock ratios. The calculated heat flux of 487±101 MW is comparable to some of the largest hydrothermal vent fields known. This heat and volume flux from the VDVF may represent a previously unrecognised, significant mode of off-axis crustal cooling, and has implications for global geochemical cycles.

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