Kinetic crystallization of a high-K basalt melt undercooled in laboratory: Implications for modeling open conduit dynamics at Stromboli volcano

Magma crystallization is a fundamental process driving the evolution of magmas in the crust and influencing the style of volcanic eruptions. Crystallization occurs through either (near) equilibrium or kinetically-controlled mechanisms driving the solidification of magmas and the final textural and chemical characteristics of igneous rocks. Among other factors, the degree of undercooling (∆T), expressed as the difference between the liquidus temperature and the actual temperature of solidifying magma, plays a key role. Experimental investigations on the effect of ∆T are extremely important to reconstruct the crystallization of basaltic melts under kinetic conditions which are frequently encountered in open conduit volcanoes.

Stromboli (Sicily, Italy) is a reference example for these types of volcanic systems, due to its persistent activity and periodic changes of eruptive style, from normal, mild strombolian activity to effusive events or sudden, short-lived, more violent explosions (paroxysms). In this study, we examined the effect of ∆T on the crystallization path of basaltic magmas erupted at Stromboli. The starting material is a high-K basaltic glass obtained from a low-porphyritic (LP) pumice erupted during the paroxysm of April 5, 2003. Undercooling crystallization experiments were performed in a non-end loaded piston cylinder apparatus at 350-500 MPa, 1050-1150 °C, anhydrous and hydrous (2 wt.% H₂O added to the experimental charge) conditions, and NNO +1.5 buffer. The degree of ∆T imposed to the system ranges from 10 to 162 °C. Textural features and chemical composition of the experimental charges were investigated by combining synchrotron radiation X-ray microtomography (SR-µCT) for the 3D reconstruction of crystal morphologies, scanning electron microscopy (FE-SEM) and electron probe microanalysis (EPMA).

Clinopyroxene represents the main mineral phase crystallized in all the experimental charges, and shows a remarkable textural and chemical dependence on the degree of ∆T. In particular, as the degree of ∆T increases, clinopyroxene morphology evolves from prevalently skeletal to dendritic, and the crystal composition becomes enriched in incompatible elements (Ti and Al), with a simultaneous depletion in compatible elements (Si and Mg). According to this cation exchange, the degree of ∆T can be parameterized to derive a new predicting model for high-K basaltic melts and based on clinopyroxene composition only. Modeling results using natural clinopyroxene crystals open new perspectives for the interpretation of open conduit dynamics at Stromboli.