Plagioclase crystal size distributions, growth and nucleation rates in an anhydrous arc basaltic andesite
摘要
We experimentally investigated plagioclase nucleation and growth in anhydrous arc basaltic andesite at 1 atm and Ni-NiO equilibrium. After equilibration at 1190 °C (15 °C above the liquidus) for 24 h, experiments were cooled at 1, 3, or 9 °C/h and quenched at 1175–1000 °C. New plagioclase grains nucleated near the liquidus, followed by minor amounts of Fe–Ti oxides and pyroxene below 1120 and 1050 °C, respectively. Plagioclase shapes varied from 2D tabular/elongated (1 and 3 °C/h) to hopper and swallowtail textures (9 °C/h), suggesting a transition from interface- to diffusion-controlled growth. Crystal shapes and sizes were correlated, with the smallest and largest having equant/elongated and tabular/bladed 3D shapes, respectively. To identify the most suitable method for inferring storage timescales in natural magmas, we calculated nucleation (J) and growth rates (G) with different methods: Gmax from the average size of the 10 biggest crystals, Gmean from the entire crystal population, Jbatch and Gbatch from the number and proportion of plagioclase estimated by point counting, and JCSD and GCSD from the crystal size distribution (CSD). J and G were greatest near the liquidus and decreased during cooling; the decrease was minimal at slow cooling rates, making G nearly constant. G decreased with decreasing cooling rates (from 10−7 to 10−9 cm/s at 9 and 1 °C/h, respectively), stabilizing after ~ 20 h of cooling. These variations of G principally resulted from differences in experimental conditions, more than the calculation method considered. Given the uncertainties of CSD theory in closed systems and the size and crystallographic axis-dependence of growth rates, combining Gₘₑₐₙ and Gₘₐₓ appears to be the most effective method for experimentally determining growth rates. However, the batch method (JBatch) still provides a good estimate of J.