Experimental determination of plagioclase and clinopyroxene crystal growth rates in an anhydrous trachybasalt from Mt Etna (Italy)

摘要

Plagioclase and clinopyroxene crystal growth rates were experimentally determined in an anhydrous potassic trachy-basalt lava from the 1991–93 Mt Etna eruption. Experiments were performed at atmospheric pressure, in air, at temperatures variable from 1160 to 1240 °C. Growth rates, in potassic trachybasalt to shoshonitic melts, were calculated measuring both the width of overgrowth rims on crystal seeds and the sizes of microlite crystals: they are in the order of 10–8 cm s–1 at low under-cooling (< 20 °C) and decrease down to 10–9 cm s–1 at higher undercoolings. The growth rates, determined for plagioclase are three-four orders of magnitude lower than those determined for this mineral in the same lava sample by other authors under water-saturated condition. Combining experimental data on plagioclase growth rates, water solubility in trachybasalt magmas, and crystal size distributions of Etnean lavas, we modelled the final ascent of magmas from shallow reservoirs located within the basement of the volcano in correspondence with known geological discontinuities. Calculations suggest that the final ascent of magmas occurs in a time span of 1–2 h with final velocities of 0.2–0.8 m s–1, values compatible with actual output rates observed during the quiet effusion of degassed lavas at Mt Etna.