Parameterization of clinopyroxene growth kinetics via crystal size distribution (CSD) analysis. Insights into the temporal scales of magma dynamics at Mt. Etna volcano

There is increasing recognition that both textural and compositional changes of clinopyroxene crystallizing from mafic alkaline magmas are the direct expression of complex dynamic processes extending over a broad range of spatial and temporal scales. Among others, supersaturation and relaxation phenomena play a key role in controlling the final crystal cargo of variably undercooled magmas erupted from active alkaline volcanoes. Following this line of reasoning, we have carried out isothermal-isobaric, decompression, and cooling rate experiments on a basalt interpreted as the parental magma of mafic alkaline eruptions at Mt. Etna volcano (Sicily, Italy). The main purpose is to reconstruct and quantify the textural changes (i.e., length of major and minor axes, surface area per unit volume, area fraction, and maximum growth rate) of clinopyroxene at variable pressures (30-300 MPa), temperatures (1,050-1,100 °C), volatile contents (0-5 wt.% H2O and 0-0.2 wt.% CO2), and equilibration times (0.25-72 h). Melt supersaturation, corresponding to a degree of undercooling variable from 14 to 125 °C, drives the formation of clinopyroxene crystals with different textures and sizes as a function of growth rate and relaxation time. By integrating experimental data and thermodynamic modeling, the transition between interface-controlled (euhedral morphologies) and diffusion-controlled (anhedral morphologies) growth regimes has been determined to occur at degrees of undercooling higher than 30 °C. The decrease of clinopyroxene growth rate with increasing the equilibration time is combined with the crystal size distribution (CSD) analysis of naturally undercooled clinopyroxene crystals erupted during 2011-2012 lava fountain episodes at Mt. Etna volcano. Results indicate that the crystallization of microlites and microphenocrysts is on the order of ~100-101 min (large undercooling, short equilibration time) and ~101-102 h (small undercooling, long equilibration time), respectively. This temporal information allows to better constrain the cooling-decompression paths of Etnean magmas rising and accelerating along a vertically extended, highly dynamic plumbing system. While clinopyroxene microlites develop during the fast ascent of magmas (~100-101 m s-1) within the uppermost part of the conduit or immediately before ejection from the vent, the onset of microphenocryst crystallization occurs at depth and continues within the plumbing system during the slow ascent of magmas (~10-2 m s-1) that migrate through interconnected storage regions.