Abstract
Seafloor hydrothermal systems in the back-arc region of the Okinawa Trough have been viewed as a modern analogue to the Kuroko-type volcanogenic massive sulfide deposits. Detection of magnetic signatures is widely utilized and assumed to facilitate the understanding of geological controls on hydrothermal system genesis. However, the magnetic properties of seafloor volcanic rocks are still poorly understood because of the difficulties of sample acquisition. Here, we report rock magnetic data along with linked geochemical and petrological data of volcanic rock samples obtained from the Irabu knolls of the southern Okinawa Trough. Both fresh and hydrothermally altered basaltic andesites were successfully obtained from the seafloor via submersible. A fresh sample, with single-domain titanomagnetite grains, is strongly magnetized with NRM intensity of up to 100 A/m. Minute skeletal and dendritic titanomagnetite grains are also observed. A second fresh sample, with multi-domain titanomagnetite grains, contains a greater amount of titanomagnetite grains, but exhibits NRM intensity ~ 10 A/m at most. In contrast to the fresh samples, hydrothermally altered samples show extremely low NRM intensities along with low saturation magnetization and certain contribution of paramagnetic minerals. Grain assemblages of pyrite and chalcopyrite grains appear along cracks in the groundmass. Our results indicated that fine titanomagnetite grains in groundmass within back-arc lava flows are altered due to hydrothermal processes. The recorded primary remanent magnetization of the lava flows is thus partly removed by hydrothermal alteration. Magnetization reduction related to hydrothermal activity produces local crustal magnetization lows and thus enables us to detect hydrothermal alteration zones by utilizing magnetic field measurements in space. In particular, the lavas we examined (via their resultant basaltic andesites) have high Curie temperatures greater than 400 °C, which is significantly higher than those indicated by mid-ocean ridge basalts, suggesting that the thermal effect for crustal magnetization may be less in back-arc settings.
Highlights
Hydrothermal activity in the back-arc region of the Okinawa Trough has been recognized as a modern analogue to the Kuroko-type volcanogenic massive sulfide deposits since its first discovery in 1989 (Halbach et al 1989)
Our analyses of basaltic andesites from the Irabu knolls clearly demonstrate that titanomagnetite grains in groundmass within back-arc lava flows are consumed in hydrothermal processes
Our results present natural remanent magnetization (NRM) and susceptibility data for fresh and altered volcanic rock samples, providing an important benchmark for understanding marine magnetic anomaly related to seafloor hydrothermal circulation
Summary
Hydrothermal activity in the back-arc region of the Okinawa Trough has been recognized as a modern analogue to the Kuroko-type volcanogenic massive sulfide deposits since its first discovery in 1989 (Halbach et al 1989). Clues for understanding the behavior of magnetic minerals relative to hydrothermal activity within oceanic crust were obtained from a massive sulfide body in the Troodos Ophiolite complex in Cyprus. This investigation was based on the results of a magnetic survey that revealed a strong, low magnetic field over a stockwork zone hosted within in a basaltic sequence (Johnson et al 1982). Though reduced magnetization related to hydrothermal processes within basaltic rock is well established and utilized for characterizing hydrothermal systems, studies that use the alteration of magnetic rocks to understand such magnetic lows are limited for mid-ocean ridges. Our newly generated and comprehensive dataset clearly reveals a relationship between the magnetic characteristics of lava flows and sulfide mineralization due to hydrothermal alteration
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