Crystal size distribution (CSD) of plagioclase phenocryst-microphenocryst and the calculation of crystal resident times in the continuous central eruption sequences of Mount Lasem, Central Java, Indonesia

Abstract

Mount Lasem is an extinct volcano in Central Java. Based on field work, five continuous central lava eruption sequences were identified. The rocks sampled from those lava units were classified as andesite which exhibit sign of magma mixing based on textural features from optical microscopy observation. With consideration of the importance of plagioclase in magma cooling reconstruction, a quantitative approach using crystal size distribution (CSD) of plagioclase phenocryst (long axis >=0.5 mm) and microphenocryst (long axis 0.1 – <0.5 mm) from five lava units was conducted. This method, which has never been applied to the eruption products of this volcano, revealed variation of magma resident time during the continuous central eruption phase of Mount Lasem. By using G=10-10 mm/s (growth rate) for plagioclase phenocryst and microphenocryst, residence times of the magma for both populations were predicted at 63 – 120 years and 8 – 18 years respectively. This results show that there are different time and mechanism constrains in the crystallization of the two populations prior to eruption events.