Modeling clinopyroxene and plagioclase growth kinetics at Mt. Etna and Stromboli

Abstract

<p>Textural and compositional changes of clinopyroxene and plagioclase crystals from mafic alkaline magmas are unequivocally related to specific dynamic processes, which extend over a broad range of spatial and temporal scales. Decoding the mechanisms controlling the growth of clinopyroxene and plagioclase crystals may play a key role for interpreting the final crystal cargo of variably undercooled magmas from active alkaline volcanoes. In this context, isothermal-isobaric and undercooling (i.e., cooling rate and decompression) experiments were conducted on primitive basalts from Mt. Etna and Stromboli volcanoes (Sicily, Italy). Clinopyroxene and plagioclase crystals were obtained at variable pressures (30-300 MPa), temperatures (1,050-1,150 °C), volatile contents (0-4.4 wt.% H<sub>2</sub>O and 0-0.2 wt.% CO<sub>2</sub>), and crystallization times (0.25-72 h). Compositional and textural features (i.e., length of major and minor axes, surface area per unit volume, area fraction, and growth rate) of the experimental charges were determined to constrain the key parameters governing the crystallization process. The correlation between growth rate and other system parameters, such as degree of undercooling, crystallization temperature, crystallization time, melt composition/structure (<em>NBO/T</em>) and melt-H<sub>2</sub>O concentration, was investigated via principal component analysis (PCA). Results point out that the crystal growth rate is primary controlled by experimental time and only subordinately by the degree of undercooling. Progressive decay of crystal growth rate over time is due to the transition between diffusion-controlled (skeletal and acicular morphologies) and interface-controlled (blocky, prismatic, and tabular morphologies) growth regimes. The growth rate-time relationship derived in laboratory is interpolated with natural textures from crystal size distribution (CSD) analysis of products recently erupted at Mt. Etna and Stromboli. Results indicate that the crystallization of clinopyroxene and plagioclase microlites is extremely fast, on the order of ~10<sup>0</sup>-10<sup>1</sup> min. This temporal information allows to better constrain the cooling-decompression paths of magmas accelerating within the uppermost part of the plumbing system, providing new insights for the modeling of conduit dynamics.</p>