CRYSTAL SIZE DISTRIBUTIONS (CSDs) IN A BASALTIC FLOW AT THE SMALL HASANDAG VOLCANO, CENTRAL TURKEY: COMPARISON OF CALCULATED RESIDENCE TIMES WITH PLAGIOCLASE AND CLINOPYROXENE CRYSTALS

Crystal Size Distribution (CSD) analysis has been used to calculate the residence times of plagioclase and clinopyroxene crystals from a single basaltic flow in Small Hasandag volcano, Central Turkey. In addition to crystal residence time (s), crystal nucleation density, n^o (#.cm^-4) and crystal nucleation rate, J (#.cm^-3.s^-1) have also been evaluated. The CSDs of plagioclase and clinopyroxene crystals from this basaltic flow yields a linear relationship when the natural logarithm of the number of crystals of a given size interval per unit volume is plotted against mean crystal length, L_mean. The slope of this line is controlled by the crystal growth rate, G (cm/s) and the crystal residence time (s) in the magma chamber.For plagioclase crystals the observed slope of -1.5896 cm^-1 and the intercept of 16.454 cm^-4 correspond to a crystal residence time of 1.99 years (±0.60 years) and a nucleation rate of 2.84x10⁶ #.cm^-3.s^-1, assuming that the crystal growth rate of plagioclase is 10^-10 cm/s (Cashman, 1988) . Using the published growth rate of 10^-10 cm/s (Congdon, 1991) for clinopyroxene crystals, the observed slope of -2.9075 cm^-1 and the intercept of 16.676 cm^-4 gives a crystal residence time of 1.09 years (±0.33 years) and a nucleation rate of 4.74x10⁶ #.cm^-3.s^-1. Uncertainty in the residence times is estimated as ±30% by Mangan (1990) and once this uncertainty factor is applied to the calculated values, the residence time by plagioclase and clinopyroxene crystals slightly overlap implying that in calculation of magma residence times, either plagioclase or clinopyroxene of the Small Hasandag volcano can be used.