Original paper

Crystal Size Distribution of amphiboles from Bezow dacites, Urmia, Iran

Modjarrad, Monir


The crystal size distribution (CSD) of amphiboles in dacites from the Bezow dome (Iran) has been studied. All samples show non-linear concave up CSD trends with intercept values of approximately 17 cm-4, forming a population density with two distinct parts. It can be modeled by mixing two crystal populations, here named fine grains and phenocrysts, each with overlapping linear CSDs. The magmas bearing the fine grains and phenocrysts started to crystallize 5-7 and 11-24 years, respectively, before each eruption, assuming a growth rate of 10-10 cm/s. Some magma remained after each eruption and continued to crystallize. Phenocrysts crystallized during long residence times. Subsequently, new magma containing few or no crystals was introduced and the fine grains crystallized from the mixed magma. Eruption followed 5-7 years after mixing. The residence time of the first, older magma batch in the chamber was twice as long as that of the mixed magma. Such model suggests that some porphyritic volcanic rocks erupted from shallow magma chambers that were never fully emptied.


bezowiranamphibolemixingcrystal size distribution (csd)residence time