Origin of Cross-Arc Trends and Temporal Geochemical Variations in Lavas from Saurajima, Aso, and Unzen Volcanoes, Kyushu, Japan

Ashley Nagle ('05)

    Sakurajima, Aso, and Unzen volcanoes are located 0, 20, and 70 km behind the volcanic front of Kyushu arc respectively. Because of their different locations relative to the front, it is possible to examine geochemical cross-arc trends and their implications for regional tectonics. With distance from the front, fluid-mobile element concentrations in lavas decrease, whereas incompatible element concentrations increase. Consequently, the influence of slab-derived fluids appears to decrease away from the trench. The geochemical signatures of all three volcanoes can be linked to tectonic processes. Whereas Sakurajima and Aso volcanism is directly related to subduction processes, Unzen is located in the back-arc. Nevertheless, Unzen preserves a strong subduction zone signature. Mantle wedge convection likely carries subduction-contaminated material into the back-arc region. Extensional processes in the Central Kyushu rift zone then cause upwelling and melting of this contaminated mantle through decompression, resulting in Unzen magmatism.

Lavas erupted in the past 12,000 years a Sakurajima become increasingly mafic over time in two distinct cycles. Between these two cycles, magma compositions become progressively more evolved. Linear correlations of major and trace elements with SiO₂ and disequilibrium phenocryst compositions indicate that magma mixing is contributing to the chemical changes observed in these lavas. Two distinct mixing cycles are evident between the historic and prehistoric samples, and in each case the rate of mixing in the historic samples increases with time. Calculated mixing lines reveal that Sakurajima lavas have incorporated up to 55% mafic material into an initially dacitic magma. The existence of two discrete mixing cycles is likely caused by repeated mafic recharge into a felsic magma chamber. In order to accommodate this increased volume of magma, melt may be stored in a second, shallower magma chamber beneath the volcano. In between mixing cycles, fractional crystallization and crustal assimilation cause the magmas in the upper chamber to become more dacitic, and likely occurs during periods of decreased eruptive frequency.