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The assimilation of carbonate rocks by magmas can dramatically change their chemistry and differentiation path, thereby affecting the rheological properties of the derived products. Here we present a set of viscosity measurements exploring the effect of variable degrees of carbonate assimilation on the melt viscosity (η) of a phonotephrite from Vesuvius (Italy). We doped the starting material with different amounts (0, 10, and 20 wt.%) of CaO and CaO+MgO, mimicking the effects of limestone and dolomite assimilation, respectively. Through this approach, we focused on the composition change liquid phase, regardless of the effect of CO2 bubbles produced by the decarbonation on the rheological properties.The high and low temperature liquid viscosity of the decarbonated melts were measured by concentric cylinder viscometry (CC) and differential scanning calorimetry (DSC), respectively. Viscosity data show non-Arrhenian trends, well described by both Vogel-Fulcher-Tammann (VFT) and Mauro-Yue-Ellison-Gupta-Allan (MYEGA) equations. Trends obtained at high-T, low-η differ from those at low-T, high-η conditions. In the high-T regime, all decarbonated melts show lower viscosity than the pristine melt, the effect being more pronounced when only CaO is added. The opposite trend is observed in the low-T-regime, due to different fragility of the investigated melt.,The most recent predictive viscosity models well reproduce the high-T, low-η regime, whereas modeled data are less accurate in the low-T, high-η regime. This discrepancy is apparently caused by the lack of decarbonated melt (i.e., Si poor, Ca-Mg-rich compositions) in the calibration dataset of viscosity models.

After half a century of quiescence, activity at Cumbre Vieja volcano (La Palma, Canary Islands, Spain) restarted with a spectacular flank eruption characterized by both high fire fountaining and effusive activity. The products emitted comprise tephra fall and lava flows, ranging from tephrite to basanite. Between September 19th and December 13th 2021 the lava flows covered ~ 12,5 km2, affecting more than 3000 buildings and paralyzing the viability and the essential activity on the SW sector of the island. This scenario highlights the importance of rheological data deriving from experimental studies of such low viscosity magma to better understand lava flow emplacement dynamics, hazard and mitigate risk.We performed a detailed experimental study to characterize the rheology of the basanitic lava sampled between October 3rd and 7th in a Concentric Cylinder set-up. Starting from a superliquidus state of 1400 °C, a set of isothermal deformation experiments was carried out at different target subliquidus temperatures (from 1225 to 1175 °C) and fixed shear-rate of 10 s-1 to investigate the near equilibrium viscosity. Moreover, a series of cooling deformation experiments were performed at different cooling-rates (ranging from 0.1 to 10 °C/min) and at constant shear-rate of 10 s-1 with the aim to mimic the dynamic evolution of natural flowing lava through controlled cooling-rate conditions. In isothermal deformation experiments, the steady state conditions (i.e., stable crystal contents) were achieved faster at increasing degree of undercooling, showing a progressive increase in the final viscosity values. In cooling deformation experiments, with increasing cooling-rate applied, the onset of crystallization took place at progressively lower temperature over shorter timescales. The experiments performed at cooling-rates from 0.1 to 1 °C/min were interrupted when viscous rupture (i.e., the transition from coherent flow to shear localization and physical separation) was observed. For the experiments conducted at higher cooling-rates (i.e., from 3 to 10 °C/min), the experimental runs were stopped at viscosity values of ~104 (Pa s), when the stress limit of the device was achieved.Preliminary results show that the thermal history plays a fundamental role on the kinetics of the crystallization hence modulating the capacity of lava to flow. The different viscosity paths observed at low and high cooling-rates lead to a rheological decoupling between the slow-cooling core and the fast-cooling external part of the lava flows. This process would be key in promoting the transition from pahoehoe to ‘a‘ā emplacement regimes, ultimately controlling the runout distance of lava flows.

Within subvolcanic plumbing systems, along volcanic conduits and post-eruptive emplacement, mineral textures and compositions are governed by complex kinetic (undercooling) and dynamic (convective) processes that deviate from theoretical models and equilibrium criteria. In this perspective, we have investigated the partitioning of major and trace cations between clinopyroxene and phonotephritic melt under convective stirring conditions at high degrees of undercooling (−ΔTnominal = 30–60 °C) and atmospheric pressure. We have integrated this novel data set with conventional static (no physical perturbation) clinopyroxene-melt compositions obtained under interface- and diffusion-controlled growth regimes. Results show that clinopyroxene growth kinetics and diffusion boundary layers caused by melt supersaturation are partly mitigated by the homogenizing effects of stirring. Because of continuous supply of fresh melt to the advancing crystal surface, the partitioning of major and trace cations is governed by local equilibrium effects, which are interpreted as the extension of equilibrium thermodynamic principles to non-equilibrium bulk systems. Major cations are incorporated into the clinopyroxene structure via the coupled substitution [M1Mg, TSi] ↔ [M1Ti, TAl] and in conformity with the thermodynamic mixing properties of CaMgSiO2, CaAl2SiO6, and CaTiAl2O6 components. The complementary relationship between lattice strain (ΔGstrain) and electrostatic (ΔGelectrostatic) energies of heterovalent substitutions is the most appropriate thermodynamic description for the accommodation of trace cations in the clinopyroxene lattice site (i.e., ΔGpartitioning = ΔGstrain + ΔGelectrostatic). The excess energy of partitioning ΔGpartitioning changes principally with Al in tetrahedral coordination and determines the type and number of charge-balanced and -imbalanced configurations taking place in the structural sites of clinopyroxene. An important outcome from dynamic stirring experiments is that superimposition of convective mass transfer on melt supersaturation phenomena causes the formation of Cr-rich concentric zones under closed system crystallization conditions. However, these Cr-rich zones do not correlate with enrichment in other compatible elements and depletion in incompatible elements, as would be expected in natural open systems characterized by input of more primitive magmas. While the convective transport acts to reduce the diffusive length scale of chemical species in the experimental melt, fresh Cr cations are more easily incorporated into the concentric zones due to crystal field effects. Together, our findings reveal that during magma ascent and emplacement, convective stirring may promote clinopyroxene crystallization and minimize kinetic effects on clinopyroxene zoning.