Thermal evolution of silicic magma chambers after basalt replenishments

In order to understand the governing factors of petrological features of er upted magmas of island-are or continental volcanoes, thermal fluctuations o f subvolcanic silicic magma chambers caused by intermittent basalt replenis hments are investigated from the theoretical viewpoint. When basaltic magma s are repeatedly emplaced into continental crust, a long-lived silicic magm a chamber may form. A silicic magma chamber within surrounding crust is com posed of crystal-melt mixtures with variable melt fractions. We define the region which behaves as a liquid in a mechanical sense ('liquid part') and the region which is in the critical state between liquid and solid states ( 'mush') collectively as a magma chamber in this study. Such a magma chamber is surrounded by partially molten solid with lower melt fractions. Erupted magmas are considered to be derived from the liquid part. The size of a si licic magma chamber is determined by the long-term balance between heat sup ply from basalt and heat loss by conduction, while the temperature and the volume of the liquid part fluctuate in response to individual basalt inputs . Thermal evolution of a silicic magma chamber after each basalt input is d ivided into two stages. In the first stage, the liquid part rapidly propaga tes within the magma chamber by melting the silicic mush, and its temperatu re rises above and decays back to the effective fusion temperature of the c rystal-melt mixture on a short timescale. In some cases the liquid part no longer exists. In the second stage, the liquid part ceases to propagate and cools slowly by heat conduction on a much longer timescale. The petrologic al features of the liquid part, such as the amount of unmelted preexisting crystals, depend on the intensity of individual pulses of the basalt heat s ource and the degree of fractionation during the first stage, as well as th e bulk composition of the silicic magma.