Amphibole crystallization from a water-saturated andesite melt: Experimental data at P = 2 kbar

Abstract

This paper reports the results of an experimental investigation of the influence of iron oxidation state in the water-saturated andesite melt of Shiveluch Volcano (Kamchatka) on amphibole composition. Experiments with durations of 2–4 h were carried out at a pressure of 2 kbar and temperatures close to the amphibole liquidus (950°C). The degree of iron oxidation (NNO and NNO + 1) in the initial glass was determined by Mossbauer spectroscopy. The analyses of experimental amphiboles were recalculated to the distribution of elements among crystallographic sites with the estimation of the degree of iron oxidation on the basis of the IMA-97 method (Leake et al., 1997). A negative correlation was established between AlVI and the sum (Fe3+ + Ti). In the most oxidized amphiboles, (Fe3+ + Ti) reaches 1.8 atoms per formula unit (a.f.u.). The kinetics of growth affects the amphibole composition more significantly than pressure (according to the data of Schmidt, 1992). Quenching (supercooling by more than 100°C) produces amphibole crystals with AlVI = 1.3 a.f.u. and Fe3+ = Ti = 0. The obtained results allowed us to interpret the conditions of amphibole growth in the andesites of Shiveluch Volcano.