GEOCHEMICAL AND ISOTOPIC EVIDENCE FOR SEAWATER CONTAMINATION OF THE HYDROTHERMAL SYSTEM OF TAAL VOLCANO, LUZON, THE PHILIPPINES

The hydrologic structure of Taal Volcano has favored development of an extensive hydrothermal system whose prominent feature is the acidic M ain Crater Lake (pH<3) lying in the center of an active vent complex, which is surrounded by a slightly alkaline caldera lake (Lake Taal). T his peculiar situation makes Taal prone to frequent, and sometimes cat astrophic, hydrovolcanic eruptions. Fumaroles, hot springs, and lake w aters were sampled in 1991, 1992, and 1995 in order to develop a geoch emical model for the hydrothermal system. The low-temperature fumarole compositions indicate strong interaction of magmatic vapors with the hydrothermal system under relatively oxidizing conditions. The thermal waters consist of highly, moderately, and weakly mineralized solution s, but none of them corresponds to either water-rock equilibrium or ro ck dissolution. The concentrated discharges have high Na contents (>35 00 mg/kg) and low SO4/Cl ratios (<0.3). The Br/Cl ratio of most sample s suggests incorporation of seawater into the hydrothermal system. Wat er and dissolved sulfate isotopic compositions reveal that the Main Cr ater Lake and spring discharges are derived from a deep parent fluid ( T approximate to 300 degrees C), which is a mixture of seawater, volca nic water, and Lake Taal water. The volcanic end member is probably pr oduced in the magmatic-hydrothermal environment during absorption of h igh-temperature gases Into groundwater. Boiling and mixing of the pare nt water give rise to the range of chemical and isotopic characteristi cs observed in the thermal discharges, Incursion of seawater from the coastal region to the central part of the volcano is supported by the low water levels of the lakes and by the fact that Lake Taal was direc tly connected to the China sea until the sixteenth century:. The depth to the seawater-meteoric water interface is calculated to be 80 and 1 60 m for the Main Crater Lake and Lake Taal, respectively. Additional data are required to infer the hydrologic structure of Taal, Geochemic al surveillance of the Main Crater Lake using the SO4/Cl, Na/K or Mg/C l ratio cannot be applied straightforwardly due to the presence of sea water in the hydrothermal system.