EXPERIMENTAL STUDY OF THE EFFECT OF OXYGEN FUGACITY ON LOW PRESSURE PHASE RELATIONS OF ALKALI BASALT FROM THE ACATLAN VOLCANIC FIELD, JALISCO, MEXICO

The Acatlán Volcanic Field is part of the Mexican Neovolcanic Axis and is located near a tectonic ridge-ridge-ridge triple junction south of Guadalajara, Mexico. It consists of Quaternary basalt and andesite lava flows, andesite cinder cones, rhyolite and dacite domes, and a zoned (andesite and rhyolite) ignimbrite. Phase relations of an alkali basalt from this region were determined experimentally in order to determine whether or not a magma of this composition could be parental to more evolved lavas at the site. Since volcanic rocks can sometimes form under more oxidizing conditions than plutonic equivalents, phase relations were determined at two oxygen fugacity conditions. One set of experiments were conducted at the quartz-fayalite-magnetite oxygen fugacity buffer and the other at the nickel-nickel oxide buffer. Experiments were conducted at one atmosphere pressure in a vertical tube furnace. Oxygen fugacity was controlled by mixing H2 and CO2 gases.