Crustal magnetization and the subseafloor structure of the ASHES vent field, Axial Seamount, Juan de Fuca Ridge: Implications for the investigation of hydrothermal sites

Abstract

High‐resolution geophysical data have been collected using the Autonomous Underwater Vehicle (AUV) Sentry over the ASHES (Axial Seamount Hydrothermal Emission Study) high‐temperature (~348°C) vent field at Axial Seamount, on the Juan de Fuca Ridge. Multiple surveys were performed on a 3‐D grid at different altitudes above the seafloor, providing an unprecedented view of magnetic data resolution as a function of altitude above the seafloor. Magnetic data derived near the seafloor show that the ASHES field is characterized by a zone of low magnetization, which can be explained by hydrothermal alteration of the host volcanic rocks. Surface manifestations of hydrothermal activity at the ASHES vent field are likely controlled by a combination of local faults and fractures and different lava morphologies near the seafloor. Three‐dimensional inversion of the magnetic data provides evidence of a vertical, pipe‐like upflow zone of the hydrothermal fluids with a vertical extent of ~100 m.

Progressive Dissolution of Titanomagnetite in High‐Temperature Hydrothermal Vents Dramatically Reduces Magnetization of Basaltic Ocean Crust

“…Thermoremanence is destroyed through progressive dissolution of primary titanomagnetites during hydrothermal alteration when fluid circulates at depth, while CRMs are acquired when hydrothermal deposits precipitate at the up‐flow zones near seafloor. This distinction is important when magnetic isosurfaces are established using survey data to reconstruct the deep structure of seafloor hydrothermal mounds (e.g., Caratori Tontini et al, 2016). Those isosurfaces can be overall effects of both decreased magnetization by hydrothermal alteration and enhanced magnetization by hydrothermal production, rather than either alone as we thought.…”

“…If proved true, these rock magnetic proxies will provide a simple but widely useful tool to quantify the hydrothermal alteration degree. With magnetic signatures increasingly used in reconstructing subseafloor structures (e.g., Caratori Tontini et al, 2016), rock magnetic data can be used to verify inverted magnetization or serve as initial conditions for forward modeling, which are important in linking geophysical interpretations to geological observations of seafloor hydrothermal circulation in mid‐ocean ridges.…”

A deep-towed magnetic survey in the southern Okinawa Trough: Implications for hydrothermal system detection

“…The magnetic amplitude will decrease while increasing altitude above the seafloor (Caratori Tontini et al 2016). To examine the possibility that such small-scale hydrothermal field with a low susceptibility can be detected by magnetic surveys, the magnetic susceptibility contrast between high and low susceptibility zones was assumed to be 10 × 10 -4 S.I.…”

“…Several previous studies (Tivey et al 1993(Tivey et al , 2014Tivey and Johnson 2002;Okino et al 2013;Fujii et al 2015;Szitkar et al 2015a;Honsho et al 2016;Tao et al 2017) have collected high-resolution magnetic data from hydrothermal fields; highlighting the correlation between low crustal magnetization with both active and extinct submarine hydrothermal vents due to hydrothermal alteration (Rona 1978;Caratori Tontini et al 2016). In addition, Okino (2018) analyzed high resolution magnetic data which collected in two sites of different host rocks (rhyolitic and basaltic) in the southern OT.…”

“…In addition, previous observations (Tivey and Johnson 2002;Okino et al 2013;Caratori Tontini et al 2016) pointed that the horizontal scale of a submarine hydrothermal vent or alteration zone is small (typically a few hundreds of meters in diameter or less). Thus, the detection of hydrothermal vents is a technical challenge.…”