Near-seafloor magnetic signatures unveil serpentinization dynamics at ultramafic-hosted hydrothermal sites

Abstract

A near-seafloor magnetic and bathymetric survey conducted by the autonomous underwater vehicle AutoSub 6000 over intermediatetemperature, ultramafic-hosted Von Damm Vent Field (Mid-Cayman spreading center, Caribbean Sea) revealed a moderate positive magnetic anomaly, in accordance with the magnetic response of other known ultramafic-hosted hydrothermal vent fields. However, compared with low-temperature ultramafic-hosted hydrothermal activity, the magnetic signature of this intermediate-temperature site indicates a slightly stronger magnetization contrast between the hydrothermal system and its host, but it remains considerably weaker than at high-temperature ultramafic-hosted hydrothermal vent fields. This observation highlights the nonlinear increase of magnetization production with temperature, as iron partitions into weakly magnetic brucite under 200 °C, but magnetite dominates above this temperature, leading to a sudden increase in the magnetic signature of a site. Our study is consistent with recent laboratory experiments and unveils the dynamics of the serpentinization reaction, enabling fine tuning of the magnetic technique for remotely locating hydrothermal systems. In addition to refining our understanding of the magnetic behavior of hydrothermal vent fields, these new results also reveal the orientation of the fluid pathway feeding the hydrothermal site and indicate the nonvertical structure of the complex network of fissures within the host rock and its associated tectonic feature-an oceanic core complex.

Origin of Long‐Wavelength Magnetic Anomalies at Subduction Zones

“…There is good evidence that the temperature of serpentinization exerts a strong control on the magnetic properties of the resulting serpentinite. This has been observed in seafloor hydrothermal vents of different temperatures (Szitkar & Murton, ), Ocean Drilling Program samples of serpentinized peridotites (Klein et al, ), and ophiolites (Bonnemains et al, ). In all cases the critical temperature is 200 °C, below which Fe partitions into brucite rather than magnetite.…”

Magmatism at oceanic core complexes on the ultraslow Southwest Indian Ridge: Insights from near-seafloor magnetics

“…Although our knowledge on the serpentinization and the magnetic properties of the newly-formed magnetite remains quite limited, these observations suggest that the slow and progressive alteration of outcropping peridotite does not result in magnetite bearing a significant remanent magnetization but mostly an induced magnetization. Conversely, serpentinites formed at high temperature in highly fractured and permeable environments, such as at hydrothermal sites, do locally exhibit a strong remanent magnetization (e.g., Szitkar and Murton, 2018). The occurrence of such serpentinite is rare and only local.…”

Petromagnetic Characteristics of Serpentinization and Magnetite Formation at the Zedang Ophiolite in Southern Tibet

“…The magnetic signature of mantle serpentinization has crucial importance in interpreting the origin of high‐amplitude magnetization anomalies at ultramafic‐hosted hydrothermal settings (e.g., Blakely et al, 2005; Fujii et al, 2016; Szitkar & Murton, 2018). However, different groups of serpentinized peridotites from either oceanic lithosphere or ophiolite complexes on land often show large variations in the abundance of magnetite (e.g., Bonnemains et al, 2016; Oufi et al, 2002).…”