DEPTH OF MAGMA CHAMBER DETERMINED BY EXPERIMENTAL PETROLOGIC METHODS

Depth of magma chamber (or young intrusive body) is one of the most important parameters that determine the thermal structure and the circulation pattern of fluid around a geothermal area. The depth is also important when we recognize the magma chamber as a deep-seated geothermal resource itself. Here, we introduce an experimental petrologic method for determining the depth of magma chamber. There are several methods in order to determine the depth (pressure) of a magma chamber. Among them, a melting experiment for the rock erupted from the magma chamber is the most reliable method. The method consists of the following procedure: (1) Select a sample (volcanic rock) which is representativ e of magma in the chamber; (2) Give a petrographic description of the sample; (3) Try to reproduce the petrographic description experimentally by melting and re-equilibrating the sample at various conditions (pressure, temperature, oxygen fugacity, etc.) using a high-temperatu re, high-pressure apparatus. If the trial results in success, the conditions of the experiment that reproduces the petrographic description are considered to be those of the magma chamber. In this way, we determined the depth of magma chamber beneath Usu Volcano, Japan. We selected Us-b pumice (the major product of the 1663 A.D. eruption; homogeneous rhyolitic pumice with small amount of phenocrysts) as the sample. We conducted a series of melting experiments for the sample using internally heated pressure vessels. As a result, the petrographic description (pl+opx+mt/ilm, 95% melting, An=42-44, about 780 °C) of the pumice was reproduced at about 250 MPa. Thus, the depth of the magma chamber at the 1663 A.D. eruption is estimated to be about 10 km. We also determined the depth at other eruptions, using the dependence of plagioclase composition on temperature and water pressure. After detailed petrographic descriptions for the eruptive products and application of our experimental results, we revealed that the 250-MPa magma chamber still existed after the 1663 A.D. eruption and that another magma chamber formed at about 4 km in depth after the 1663 A.D. eruption. Using the above method, we can estimate depths of magma chambers (or intrusive bodies) beneath volcanoes where their eruptive products are available.