A deep tow magnetic survey of Middle Valley, Juan de Fuca Ridge

Abstract

[1] Abstract: We report here results from a deep tow magnetic survey over Middle Valley, Juan de Fuca Ridge. A series of track lines are combined to generate a high-resolution map of the magnetic field anomaly within a 10 Â 12 km region surrounding the Bent Hill massive sulfide (BHMS) deposit. A uniformly magnetized body (5 A/m) with a cross section approximating the body inferred from Ocean Drilling Program (ODP) drilling can account for the observed near-bottom magnetic anomaly amplitude. Assuming this magnetization is entirely induced, the average susceptibility (0.11 SI) corresponds to $3.5% magnetite + pyrrhotite by volume, consistent with the abundance of these phases observed in drill core samples. However, this uniform magnetization model significantly underestimates the magnetic anomaly measured a few meters above the seafloor by submersible, indicating that the upper portion of the sulfide mound must have a significantly higher magnetization ($10% magnetite + pyrrhotite) than at deeper levels. On a larger scale, the near-bottom magnetic anomaly data show that basement magnetizations are not uniformly near zero, as had been inferred from analysis of the sea surface anomaly pattern. We interpret this heterogeneity as reflecting primarily differences in the degree of hydrothermal alteration. Our results highlight the potential of magnetic anomaly data for characterizing hydrothermal deposits where extensive drill core sampling is not available.

Ages and magnetic structures of the South China Sea constrained by deep tow magnetic surveys and IODP Expedition 349

“…The technique was later extensively applied in surveying global mid‐ocean ridges [ Tivey , ; Perram et al ., ; Hussenoeder et al ., ; Pouliquen et al ., ; Yamamoto et al ., ]. In recent years, deep‐tow magnetic data were further used in discerning hydrothermal vents and their activities [ Gee et al ., ; Tivey and Johnson , ; Zhu et al ., ]. In addition, high‐resolution magnetic anomalies were essential for understanding continental breakup [ Whitmarsh et al ., ], magnetic reversal behaviors [ Tominaga et al ., ], and paleointensity variations [ Sager et al ., ; Gee et al ., ; Granot et al ., ].…”

High‐resolution magnetic signature of active hydrothermal systems in the back‐arc spreading region of the southern Mariana Trough

“…These techniques have allowed the discovery of reduced magnetization associated with basalt‐hosted hydrothermal sites on the Mid‐Atlantic Ridge (MAR) [ Tivey et al ., ; Tivey and Dyment , ], Juan de Fuca Ridge [ Tivey and Johnson , ], Southwest Indian Ridge [ Zhu et al ., ], and Southern Tyrrhenian Sea [ Caratori‐Tontini et al ., ]. Enhanced magnetization zones have been discovered on ultramafic‐hosted sites on the MAR [ Dyment et al ., ; Tivey and Dyment , ; Szitkar et al ., ] and sediment‐hosted sites [ Tivey , ; Gee et al ., ].…”

What causes low magnetization at basalt-hosted hydrothermal sites? Insights from inactive site Krasnov (MAR 16°38′N)

“…Specific geological contexts, such as the presence of abundant sediments or the occurrence of magma‐poor seafloor spreading, result in different processes and therefore different magnetic signatures (e.g., Tivey [] and Gee et al . [] for the sediment‐hosted sites of Middle Valley, Juan de Fuca Ridge; Dyment et al . [], Tivey and Dyment , [], and F. Szitkar et al (Deep‐Sea magnetics unveil mineral‐rich ultramafic‐hosted hydrothermal sites, submitted to Geology , 2014), for the ultramafic‐hosted sites of the MAR).…”

“…Specific geological contexts, such as the presence of abundant sediments or the occurrence of magmapoor seafloor spreading, result in different processes and therefore different magnetic signatures (e.g., Tivey [1994] and Gee et al [2001]…”