LAVA-FLOW AND SUBVOLCANIC MAGMA RESERVOIR COMPOSITION TRENDS IN THE CA-POOR PYROXENES OF HAKONE VOLCANO, JAPAN

The temperatures at which pigeonite began to crystallize in tholeiitic basalts and andesite of Hakone volcano range from 1123 to 1019-degrees-C based on the three-pyroxene geothermometer of Ishii (1975). These values are lower than the temperatures of other tholeiitic magmas of similar solidification index, such as the magmas of Funagata-yama, O-shima, and Akita-komagatake volcanoes and the Skaergaard intrusion. The cores of Ca-poor pyroxene phenocrysts formed in the Hakone subvolcanic magma reservoir are invariably orthopyroxene; the groundmass and microphenocryst pyroxenes crystallized inthe lava during ascent and extrusion of magma are generally pigeonite, but orthopyroxene is also present occasionally. The subvolcanic temperature-composition path [T - X(Fe) path; X(Fe) = Fe/(Mg + Fe), atomic ratio] of the Ca-poor pyroxenes does not interest the pigeonite eutectoid reaction line (PER-line) of Ishii & Takeda (1974), whereas the lava-flow paths characteristically intersect it at X(Fe) values between 0.31 and 0.52. These two different kinds of T - X(Fe) paths may be interpreted as reflecting different solidus temperatures, and the content of volatiles, particularly water, appears to be the controlling factor. It is suggested that the water content of the magma increased because of fractional crystallization of anhydrous minerals from a relatively hydrous magma in the subvolcanic reservoir; then each erupted batch of the fractionated magma had its water content lowered by degassing as it ascended to the volcano. Pigeonite is common in the groundmass of basaltic rocks throughout the world, both extrusive and intrusive, but it is rare as phenocrysts in the former, a feature that may also be explained by the different solidus temperatures. Calc-alkali rocks in Hakone volcano appear to be derived by fractionation of hydrous tholeiitic magma.