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

Description

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 (ΔT), 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 ΔT 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 ΔT 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 ΔT 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 Xray 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 ΔT. In particular, as the degree of ΔT 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 ΔT 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.

Period	24 May 2022
Event title	EGU General Assembly 2022
Event type	Conference
Location	Vienna, AustriaShow on map
Degree of Recognition	International