Shallow submarine hydrothermal activity with significant contribution of magmatic water producing talc chimneys in the Wakamiko Crater of Kagoshima Bay, southern Kyushu, Japan

• Published in: Journal of volcanology and geothermal research 2013-05, Vol.258, p.74-84
• Main Authors: Yamanaka, Toshiro, Maeto, Kotaro, Akashi, Hironori, Ishibashi, Jun-Ichiro, Miyoshi, Youko, Okamura, Kei, Noguchi, Takuroh, Kuwahara, Yoshihiro, Toki, Tomohiro, Tsunogai, Urumu, Ura, Tamaki, Nakatani, Takeshi, Maki, Toshihiro, Kubokawa, Kaoru, Chiba, Hitoshi
• Format: Article
• Language: English
• Subjects: Aira; Aira caldera; Chimneys; Computational fluid dynamics; Craters; Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Fluid flow; Fluids; Geochemical thermometer; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Kagoshima Bay; Marine; Sea water; Shallow seafloor hydrothermal system; Silicon dioxide; Talc-dominated chimney; Talcs; Thermodynamic calculation
• ISSN: 0377-0273; 1872-6097
• DOI: 10.1016/j.jvolgeores.2013.04.007

Active hydrothermal venting from shallow seafloor (200-m depth) with talc chimneys has been discovered at the Wakamiko Crater floor in the Aira Caldera, southern Kyushu, Japan. The major chemical composition of the fluids suggests that the fluids are supplied from a single reservoir. The fluid is characterized by a low chloride concentration, low δD value, and a high δ18O value, suggesting that the endmember hydrothermal fluid is a mixture of seawater and andesitic water and possibly contribution of meteoric water and/or phase separation. Such noticeable magmatic input may be supported by high helium isotopic ratio (6.77 RA) of fumarolic gas discharging from the crater. Silica and alkaline geothermometers indicate that the fluid–rock interaction in the reservoir occurs in the temperature range of 230 to 250°C. The high alkalinity and high ammonium and dissolved organic matter concentrations in the fluid indicate interaction of the fluid with organic matter in sedimentary layers. At least three hydrothermal vents have been observed in the crater. Two of these have similar cone-shaped chimneys. The chimneys have a unique mineralogy and consist dominantly of talc (kerolite and hydrated talc) with lesser amounts of carbonate (dolomite and magnesite), anhydrite, amorphous silica, and stibnite. The precipitation temperature estimated from δ18O values of talc was almost consistent with the observed fluid temperature. Geochemical modeling calculations also support the formation of talc and carbonate upon mixing of the endmember hydrothermal fluid with seawater and suggest that the talc chimneys are currently growing from venting fluid. •We discovered a unique hydrothermal activity at shallow-water depth in Kagoshima Bay.•The venting fluid is composed mixture of seawater, meteoric water and magmatic water.•The estimated mixing ratio is roughly 2:3:5 based on δD and δ18O isotopic signature.•We found at least three chimneys and two of them have similar cone-like sharp.•The chimneys formed mainly talc with small amount of carbonate and anhydrite.