High pressure experiments on kinetic and rheological properties of primitive alkaline magmas: constraints on deep magmatic processes at the Campi Flegrei Volcanic District

Defining the timescales of magma storage and ascent beneath active volcanoes is a fundamental tool in volcanological investigation of the last decade to constrain pre-eruptive magmatic processes and magma chamber dynamics, since it is able to provide the basis for volcanic hazard assessment. This Ph.D. project focuses on the investigation of the deep portion of the Campi Flegrei Volcanic District plumbing system (crustal-mantle boundary; ~25 km of depth), in correspondence of which the presence of a possible crystallization zone has been hypothesized on the basis of melt inclusion studies, seismic data interpretations, gravimetric and petrological modelling and experimental data. The Campi Flegrei Volcanic District, which includes the Campi Flegrei and the islands of Ischia and Procida, represents one of the most active volcanic areas in the Mediterranean region and one of the most dangerous volcanic complexes on Earth owing to the intense urbanization of the area. Many petrological, geochemical and geophysical surveys were carried out in the Campania Active Volcanic Area that have helped to define the main architecture and the development of the sub-volcanic system. Nevertheless, the dynamic processes that operate during the earliest, deepest differentiation steps of primitive magmas that fed all Campi Flegrei eruptions are yet poorly constrained. The knowledge of the dynamics and residence and ascent timescales of magma at deep levels, indeed, may be the key to understand the triggering mechanisms of volcanic eruptions, and are essential for understanding the rates at which magmas are supplied to volcanic complexes. In this thesis, the investigation of the kinetic and rheological properties of a K-basaltic magma at Moho depth, together with the partitioning of trace elements between crystal and melts, has allowed to fill some gaps relative to the knowledge of the deep portion of the Campi Flegrei Volcanic District plumbing system, providing magma residence time and ascent timescales, and models for deep magmatic differentiation processes

Experimental constraints on amphibole stability in primitive alkaline and calc-alkaline magmas

Equilibrium crystallization experiments were carried out on two primitive basaltic rocks (APR16: Na2O+K2O=4.40 wt%; CM42: Na2O+K2O=2.59 wt%) with the aim to investigate the amphibole stability in the differentiation processes at deep crustal level, of primitive alkaline (APR16) and calc-alkaline (CM42) magmas. The experiments were performed with different initial H2O contents (0-5 wt%), at pressure of 800 MPa, in the temperature range of 975-1225 °C. For the explored conditions, amphibole crystallization occurs in both compositions at H2O in the melt >7wt% while the temperature of their occurrence is lower in the alkaline composition (<1050 °C in APR16 and ≥1050 °C in CM42). Moreover, amphibole crystallization seems to be influenced by the Na2O/K2O ratio rather than the absolute Na2O content in the melt. This is evident when experimental results on the APR16 and CM42 are compared with experimental data obtained from a primitive ultrapotassic composition (leucite-basanite: Na2O+K2O=4.58 wt%) and with thermodynamic modelling by the Rhyolite-MELTS algorithm. The comparison shows that amphibole never saturates the leucite-basanite at any of the investigated/modelled conditions, even when an extended crystallization increases the Na2O of melts up to contents like those of calc-alkaline experimental glasses. We conclude that, at pressure of 800 MPa and hydrous conditions, only primitive liquids with Na2O/K2O ratio ≥0.9 are more prone to crystallize amphibole

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

Bibbi93/2021-Tajogaite-eruption---Volcanica-paper: v1.1

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

Influence of Pre-Existing Nuclei on the Crystallization Kinetics of Primitive Alkaline Magmas: Insights on the Deep Feeding System of the Campi Flegrei Volcanic District

Aiming to evaluate the influence of pre-existing nuclei on the clinopyroxene crystallization kinetics, time-series experiments were performed using both natural and vitrified starting materials. Experiments were carried out at pressure of 0.8 GPa, temperatures between 1220 and 1250 °C, and dwell times ranging from 0.16 to 12 h. Clinopyroxene growth rates of the runs performed using the natural starting material containing pre-existing nuclei (~2 × 10−7 to ~6 × 10−8 cm∙s−1) are higher than those of the runs performed using the vitrified one (~3 × 10−7 to ~2 × 10−8 cm∙s−1). In both cases the growth rates decrease with increasing time. Conversely, clinopyroxene nucleation rates are lower in the experiments performed using the natural powder (102 and 10 mm−3 s−1) compared to those performed with the glassy starting material (105 and 103 mm−3∙s−1). The nucleation rates tend to decrease increasing dwell time in all the series up to ~3 h, after which it remains nearly constant. Finally, the combination of the obtained clinopyroxene growth rates with the crystal size analysis of natural clinopyroxenes, allowed to estimate the magma ascent rate and the recharge rate of the Campi Flegrei Volcanic District deep reservoir

Bibbi93/2021-Tajogaite-eruption---Volcanica-paper: v1.2

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

Clinopyroxene growth rates at high pressure. Constraints on magma recharge of the deep reservoir of the Campi Flegrei volcanic district (south Italy)

Clinopyroxene growth rates were experimentally determined in a K-basaltic rock from Campi Flegrei Volcanic District (south Italy). The primary objective was to provide constraints on the clinopyroxene crystallization kinetics at high pressure: we carried out a series of experiments at 0.8 GPa, 1030–1250 °C, 1 ≤ H2O ≤ 4 wt.%, with durations of 0.25, 3, 6 and 9 h. Overall, growth rate reaches a maximum value in the shortest experiments (~ 3·10−7 cm·s−1), decreasing to ~ 1·10−8 cm·s−1 in the longest duration runs. Temperature and water content do not seem to significantly affect the growth rate. Moreover, partition coefficients based on the crystal-liquid exchange show that mineral chemistry progressively approaches equilibrium with increasing run duration. Furthermore, to estimate the magma recharge of the deep reservoirs, we combined the determined growth rates with data from thermobarometry and from crystal size distribution analyses of clinopyroxenes in the most primitive scoria clasts of the Campi Flegrei Volcanic District (CFVD). We obtained a minimum residence time of ~ 5 months for the larger clinopyroxene population, and an ascent velocity of ~ 0.5·10−4 m·s−1 for the CFVD K-basaltic magma. The smaller clinopyroxene phenocrysts and microlite populations, however, suggest that the late stage of Procida magma crystallization took place in disequilibrium conditions

Clinopyroxene growth and dissolution rate: constraints on the deep level ascent rate of a K- basaltic magma from the Campi Flegrei Volcanic District

The estimation of the magma ascent rate in the Campi Flegrei Volcanic District (CFVD, south Italy) is of paramount significance in terms of volcanic hazard. Indeed, deeplevel ascent rates may be the key to understand the triggering mechanisms of volcanic eruptions and are essential for understanding the rates at which magmas are supplied to volcanic complexes. Thus, to investigate the CFVD magmas transport at Moholower crust depth and provide an estimate of magma recharge of the deep reservoirs, we assessed the cooling rate and the deeplevel ascent rates of Kbasaltic magmas by combining the clinopyroxene growth rate determined by highpressure crystallization experiments with data from crystal size distribution analyses and thermobarometry of clinopyroxenes occurring in the most primitive scoria clasts of the CFVD. In addition, since only few studies have considered the role played by crystal dissolution phenomena during the crystalmelt interaction and crystal growth, we investigated the role of crystal dissolution in the estimation of magma ascent rate by performing a series of dissolution experiments. In particular, clinopyroxene growth and dissolution rates were experimentally determined in a Kbasaltic rock from Procida island (CFVD) through a series of experiments performed at 0.8 GPa by using the piston cylinder apparatus available at the HPHT Laboratory of the Department of Earth Sciences of Sapienza University of Rome (Italy). Crystallization experiments were carried out at 10301250 °C, 1 ≤ H2O ≤ 4 wt.% and dwell times of 0.25, 3, 6 and 9 hours. Overall, growth rate reaches a maximum value in the shortest experiments (~ 3·107 cm s1) decreasing to ~ 1x108 cm s1 in the longest duration runs. Partition coefficients based on the crystalliquid exchange show that mineral chemistry progressively approaches equilibrium with increasing run duration. Furthermore, the combination of the determined growth rates with data from thermobarometry and from crystal size distribution analyses of clinopyroxenes in the most primitive scoria clasts of the CFVD, suggests that recharge by primitive magma in the deep reservoirs occurs with a relatively high ascent rate of ~ 0.5x104 m s1. Dissolution experiments, instead, were performed at superliquidus temperatures of 1300 and 1350 °C and dwell times between 0.5 and 2 hours. The calculated dissolution rates are in the order of ~105106 cm s1 and results significantly controlled by temperature, while they are not pressure and time dependent. The role of crystal dissolution in the estimation of magma ascent rate has been tested for a natural magmatic system, by interpolating the obtained dissolution rates with the textural data of clinopyroxene crystals from the AgnanoMonte Spina pyroclastic deposits at Campi Flegrei (Italy). Calculations indicate that the time required for partial or complete resorption of these clinopyroxene crystals varies from ~0.5 to ~40 hours, and that the effect of crystal dissolution may be relevant on the estimates of magma residence times if significant dissolution occurs during magma mixing processes