Index Geophysics

Seafloor hydrothermal alteration affecting magnetic properties of abyssal basaltic rocks: insights from back-arc lavas of the Okinawa Trough

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.

Masakazu Fujii

Hiroshi Sato

Eri Togawa

Kazuhiko Shimada

Jun-ichiro Ishibashi

Magnetic properties

Hydrothermal alteration

Titanomagnetite

Back-arc basaltic andesite

Okinawa Trough

Geography. Anthropology. Recreation

G

Geodesy

QB275-343

Geology

QE1-996.5

SpringerOpen

2018-12-01T00:00:00Z

article

10.1186/s40623-018-0958-6

1880-5981

https://doaj.org/article/5fedb7a708fb409cb86f3202dbda6d68

Earth, Planets and Space, Vol 70, Iss 1, Pp 1-14 (2018)

EN

http://link.springer.com/article/10.1186/s40623-018-0958-6

https://doaj.org/toc/1880-5981

Journal Licence: CC BY