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

Abstract

There is increasing recognition that both textural and compositional changes of clinopyroxenes 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 upon variable pressures (30-300 MPa), temperatures (1,050-1,100 °C), volatile contents (0-5 wt.% H 2 O and 0-0.2 wt.% CO 2 ), and equilibration times (0.25-72 h). By integrating experimental data and thermodynamic modeling, the

transition between interface-controlled (euhedral morphologies) and diffusion- controlled (anhedral morphologies) growth regimes has been determined at an

undercooling threshold value of ~33 °C. Early melt supersaturation causes the fast growth of tiny clinopyroxenes with strong disequilibrium shapes, whereas an increasing relaxation time leads to the slow growth of large clinopyroxenes showing textural equilibration. According to these kinetic principles, both growth rate and relaxation time have been parameterized in relation to the crystal size distribution (CSD) analysis of naturally undercooled clinopyroxenes erupted during 2011-2012 lava fountain episodes at Mt. Etna volcano. Results indicate that the crystallization of microlites and microphenocrysts takes place under (dis)equilibrium growth conditions, in the order of ~10 0 -10 1 min (large undercooling, short equilibration time) and ~10 1 -10 2 h (small undercooling, long equilibration time), respectively. This temporal information allows to disentangle the cooling and 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 (~10 0 -10 1 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.