Hydrothermal alteration of green tuff belt, Japan: Implications for the influence of seawater/volcanic rock interaction on the seawater chemistry at a back-arc basin

Abstract Chemical data on hydrothermally altered volcanic rocks from a green tuff belt in Japan indicate that the average rate of Mg removal from seawater due to seawater cycling through back-arc basins in the circum-Pacific region during the early to middle Miocene (25–15 Ma) is estimated to be 2.6±1 × 10¹³ g/year. This is similar to that through present-day mid-ocean ridges (2.4 × 10¹³ g/year). Hydrothermal fluxes of K, Ca and Si are calculated to be 4.2±1.6 × 10¹³ g/year, 4.3±1.7×10¹³ g/year and 1.0±0.4 × 10¹⁴ g/year, respectively. These calculated results indicate that the seawater/volcanic rocks interaction at subduction-related tectonic settings have to be taken into account in considering the geochemical mass balance of seawater over geologic time.     

Major and trace element geochemistry of neutral and acidic thermal springs at El Chichón volcano,Mexico: Implications for monitoring of the volcanic activity

Four groups of thermal springs with temperatures from 50 to 80 °C are located on the S–SW–W slopes of El Chichón volcano, a composite dome-tephra edifice, which exploded in 1982 with a 1 km wide, 160 m deep crater left. Very dynamic thermal activity inside the crater (variations in chemistry and migration of pools and fumaroles, drastic changes in the crater lake volume and chemistry) contrasts with the stable behavior of the flank hot springs during the time of observations (1974–2005). All known groups of hot springs are located on the contact of the basement and volcanic edifice, and only on the W–SW–S slopes of the volcano at almost same elevations 600–650 m asl and less than 3 km of direct distance from the crater. Three groups of near-neutral (pH ≈ 6) springs at SW–S slopes have the total thermal water outflow rate higher than 300 l/s and are similar in composition. The fourth and farthest group on the western slope discharges acidic (pH ≈ 2) saline (10 g/kg of Cl) water with a much lower outflow rate (< 10 l/s).     

A distinction technique between volcanic and tectonic depression structures based on the restoration modeling of gravity anomaly: a case study of the Hohi volcanic zone, central Kyushu, Japan

In this study, we propose a numerical modeling technique which restores the gravity anomaly of tectonic origin and identifies the gravity low of caldera origin. The identification is performed just by comparing the restored gravity anomalies with the observed gravity anomalies, thus we do not need detailed geophysical and geological information around the buried caldera. The technique has been successfully applied to distinguish the gravity low originated in the buried Shishimuta caldera from other gravity lows in the Hohi volcanic zone, central Kyushu in Japan.