Magma residence time, ascent rate and eruptive style of the November ash-laden activity during the 2021 Tajogaite eruption (La Palma, Spain)

We combined compositional analyses, crystal size distributions and geothermobarometry of tephra erupted during the 2021 Tajogaite eruption (La Palma, Spain), focusing on samples collected in November 2021 associated with a period of abundant ash emission characteristic of the second half of the eruption (from October onwards). Magma erupted in November exhibits a more primitive basanitic composition than the earlier magma. Crystallization temperatures range between ~1100-1160 °C (H2O = 1-3 wt.%) for phenocrysts and microphenocrysts, with corresponding pressures indicating depths from ~10 to ~30 km. Crystal size distribution analysis reveals short (minutes) residence times for plagioclase. Finally, magma ascent velocities (~0.01-0.3 m/s) suggest acceleration and fragmentation in the shallowest part of the conduit. Our results suggest that the trigger of the November explosive activity can be attributed to complex feedback between gas emission rates, changes in conduit geometry, and magma ascent rate

Spherulites growth in trachytic melts: a textural quantitative study from synchrotron X-ray microtomography and SEM data

This study shows the first textural data on synthetic alkali-feldspar spherulites grown in trachytic melts during cooling and decompression experiments with water-saturated conditions. Previous textural studies have shown the shape evolution and the growth process of spherulites as a function of undercooling (deltaT) and water content, although just in basaltic and rhyolitic melts [1-3]. Spherulites are spherical clusters of polycrystalline aggregates that occur commonly in rhyolitic melts under highly non-equilibrium conditions [3-4]. Cooling and decompression experiments have been carried out on trachytic melts in order to investigate crys- tallization kinetics of alkali feldspars and the implications for magma dynamics during the ascent towards the surface. Experiments have been conducted using cold seal pressure vessel apparatus at pressure range of 30 - 200 MPa, temperature of 750 - 850 degrees C and time of 2 - 16 hours, thereby reproducing pre- and syn-eruptive conditions of the Campi Flegrei volcanoes. This study presents quantitative data on spherulite morphologies obtained both by scanning electron microscopy (SEM) and synchrotron X-ray microtomography. Size, aspect ratio, number and crystallographic misorientation of alkali feldspar crystals will be measured. Furthermore, experiments performed at different durations could allow us to follow the growth and the evolution of spherulites. The shape of spherulites changes as a function of delta T and experimental durations. Two kind of spherulites occured during experiments: open spherulites and close spherulites. The open spherulites are characterized by an structure with large (generally rectangular prismatic), widely spaced fibers with main axis converging towards a central nucleus, in agreement with previous observations [5-6]. Instead, the close spherulites consist of acicular and tiny fibers radially aggregated around a nucleus and single crystals are hardly distinguishable. First preliminary results show: a) spherulites grow between 70-200 MPa, thus the nucleation process was favored at higher water contents; b) open spherulites seem to be favored at low deltaT, whereas close spherulites were favored in experiments at higher delta T and long durations; c) estimated growth rates of spherulites were of 10-7 cm/s

Crystallization Kinetics of Alkali Feldspar in Peralkaline Rhyolitic Melts: Implications for Pantelleria Volcano

Peralkaline rhyolites, associated with extensional tectonic settings, are medium to low viscosity magmas that often produce eruptive styles ranging from effusive to highly explosive eruptions. The role of pre-eruptive conditions and crystallization kinetics in influencing the eruptive style of peralkaline rhyolitic magmas has been investigated and debated considering equilibrium conditions. However, experimental constraints on the effect of disequilibrium in crystallization in such magmas are currently lacking in the literature. Therefore, we performed isobaric cooling experiments to investigate alkali feldspar crystallization kinetics in peralkaline rhyolitic melts. Experiments were performed under water-saturated, water-undersaturated, and anhydrous conditions between 25 and 100 MPa, at 670–790°C and with experimental durations ranging from 0.5 to 420 h. Here we present the first data on crystallization kinetics of alkali feldspar, which is the main crystal phase in peralkaline rhyolitic melts, in order to improve our understanding of the evolutionary timescales of these melts and their ability to shift between effusive and explosive activity. Our experimental results indicate that the alkali feldspar nucleation delay can range from hours to several days as a function of undercooling and H2O content in the melt. Thus, a peralkaline rhyolitic magma can be stored at the pre-eruptive conditions for days without important variations of its crystal fraction. This suggests that crystallization may not necessarily play the main role in triggering fragmentation during explosive eruptions of peralkaline rhyolitic magmas

Phase equilibrium experiments and thermodynamic simulations to constrain the pre-eruptive conditions of the 2021 Tajogaite eruption (Cumbre Vieja volcano, La Palma, Canary Islands)

Phase relations of a tephritic lava erupted at La Palma, Canary Islands, the 2nd of October 2021, during the eruption of Cumbre Vieja volcano were determined experimentally in the temperature/water content space, T (◦C)/H2O (wt%). The crystallisation experiments were carried out at a fixed pressure of 275 MPa for tempera- tures varying from 1000 to 1200 ◦C and with H2O added contents ranging from 0 to 5 wt%. Results from the synthetic products are compared with those obtained from thermodynamic simulations and with the natural phases to constrain the pre-eruptive conditions of the tephritic magma stored in the shallow reservoir. The major phases (clinopyroxene + olivine + oxides) occurring in the natural products were reproduced. The experimental results combined with thermodynamic modelling allow us to constrain the pre-eruptive temperature to ~1100 ◦ C considering H2O to 3 wt% and to restrict the crystallisation of plagioclase at low pressures (< 20 MPa). The results indicate that, under the presence of a pure H2O fluid, amphibole is stable at pressures higher than 300 MPa and temperatures ~1000 ◦C. Our results on pre-eruptive conditions of the 2021 Tajogaite eruption provide important constraints on mechanisms of storage and transport in primitive alkaline magmas

The effect of diffusive re-equilibration time on trace element partitioning between alkali feldspar and trachytic melts

We present new experimental data on major and trace element partition coefficients between alkali feldspar and trachytic melt. Experiments were conducted at 500 MPa, 870 890 {\deg}C to investigate through short disequilibrium and long near equilibrium experiments the influence of diffusive re-equilibration on trace element partitioning during crystallization. Our data show that Ba and Sr behave compatibly, and their partition coefficients are influenced by re-equilibration time, orthoclase (Or) content, growth rate and cation order-disorder. High field strength elements (HFSE) and rare earth elements (except Eu) are strongly incompatible, but alkali feldspar efficiently fractionates light (LREE) from heavy rare earth elements (HREE). Our crystallization experiments reveal a strong influence of disequilibrium crystal growth on the partitioning of Ba and Sr. In particular, short-duration experiments show that rapid alkali feldspar crystal growth after nucleation, promotes disordered growth and less selectivity in the partitioning of compatible trace elements that easily enter the crystal lattice (e.g., Ba and Sr)...

A combined petrological and numerical approach to investigate basaltic Plinian eruptions at Masaya Caldera, Nicaragua

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