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

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

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

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 rate: experimental investigation at crustal-mantle boundary level in an alkaline basalt from the Campi Flegrei Volcanic District (South Italy)

The kinetics of crystal nucleation and growth are fundamental for the interpretation of thermal history of a magma during its ascent to the surface, and to constrain timescales of magmatic processes. In basaltic systems clinopyroxene (Cpx) is a common phenocryst, and due to its wide crystallization range, it contains the most complete record of evolutionary history of a magma. Nevertheless, to date, experimental studies addressed to obtain measurements of Cpx growth rate are few and limited to low pressure (≤ 0.5 GPa). In this experimental work, we investigated the effects of temperature, water content and time on Cpx growth rate in an alkaline basalt (APR16 sample) from Procida island, representative of the least evolved rocks of the whole Campi Flegrei Volcanic District (Italy). The starting material is an anhydrous glass prepared by melting the APR16 natural powder in a gas mixing furnace at 1400 °C and atmospheric pressure. Experiments were performed at isobaric pressure (0.8 GPa) by using the piston cylinder apparatus at the HP-HT Laboratory of the Earth Sciences Department, Sapienza, University of Rome. We performed a total of 24 experiments divided into three series. Experiments of series 1 were carried out at anhydrous conditions at 1250°C and 1200° C and dwell time of 0.25, 3, 6 and 9 hours. Experiments of series 2 and 3, instead, were carried out at hydrous conditions (2 and 4 wt% H2O added to the starting material, respectively), 1220° C and 1170° C (series 2) and 1080° C and 1030° C (series 3), and the same dwell time of series 1. Results show that crystal sizes increase increasing the duration of the experiments with values between 5 and 20 μm. Moreover, we noted a strong growth rate dependence on time in all the runs, while temperature and water content play a minor role. Obtained Cpx growth rate values range from 10-7 to 10-8 cm/s, varying of an order of magnitude increasing the duration of the experiments

Experimental time constraints on the kinetic and chemistry of amphibole at deep crustal levels

Aiming to improve the current knowledge about amphibole growth kinetics at deep crustal levels, new amphibole growth rate data are provided. Our findings, indeed, may be useful to correctly interpret the textural features of amphibole-bearing mafic cumulates and rocks, and for a better constraining of the timescales of magmatic processes at upper mantle-lower crustal depths. Experiments were performed to determine the amphibole growth rates in a primitive alkaline basalt from Procida island (Campi Flegrei Volcanic District, southern Italy) at the following conditions: temperature of 1030 and 1080 °C, pressure of 0.8 GPa, water content in the range 3.3-4.2 wt%, and variable dwell time from 0.25 to 9 h. Amphibole growth rates range from 1.5·10-7 to 2.9·10-8 cm·s-1 with increasing the duration of the experiments. It is observed that, keeping a constant dwell time, an increase of the experimental temperature or of the water content results in comparable growth rate increase. Coexisting synthetic amphibole and clinopyroxene show a faster growth rates in favour of amphibole regardless of the dwell time, since the chemical and structural similarities of these minerals cause kinetic competition. Moreover, the chemical composition of amphibole is influenced mainly by the experimental time; in detail, in the shortest (≤3 h) and low temperature runs edenite is the prevailing composition whereas the magnesiohastingsitic term becomes dominant at higher temperature and longer run duration. Based on the interpretation of the Fe-Mg exchange coefficient values between amphibole and coexisting liquid, the magnesiohastingsitic amphibole is considered to be the stable term at the investigated run conditions. Finally, the resulting growth rates have been applied to constrain the crystallization time of natural amphiboles and clinopyroxenes from the Oligo-Miocene cumulates of north-western Sardinia (i.e., Capo Marargiu Volcanic District, Italy), yielding crystallization times in the range 1.46-3.12 yr

Amphibole growth from a primitive alkaline basalt at 0.8 GPa: Time-dependent compositional evolution, growth rate and competition with clinopyroxene

Amphibole growth rates were experimentally determined at hydrous (3.3-4.2 wt% H2O), isobaric (0.8 GPa) conditions, variable temperature (1030 and 1080 °C) and dwell time (0.25, 3, 6, and 9 h), using as starting material a primitive alkaline basalt from Procida island (Campi Flegrei Volcanic District, south Italy). Amphibole growth rates decrease from 1.5·10-7 to 2.9·10-8 cm·s-1 as the duration of our experiments increase from 0.25 to 9 hours. Moreover, increasing both temperature and water content leads to similar growth rate increase at constant dwell time. The comparison between amphibole and clinopyroxene growth rates determined at the same experimental conditions reveals for amphibole a faster growth relatively to the coexisting clinopyroxene, regardless of the dwell time. Furthermore, the experimental time appears to be a critical parameter for the composition of synthetic amphiboles; specifically, edenite is the dominant composition in short experiments (≤3 h), particularly, at low temperature (1030 °C), whereas the magnesiohastingsitic amphibole becomes progressively more important shifting towards longer duration and higher temperature run conditions. The magnesiohastingsite, on the basis of the amphibole-liquid Fe-Mg exchange coefficient values, results to be the compositional term representative of amphibole-melt equilibrium at the investigated P-T-H2O conditions. Finally, experimental growth rates from this study have been used to investigate the crystallization time of natural amphiboles and clinopyroxenes from the Oligo-Miocene cumulates of north-western Sardinia (i.e. Capo Marargiu Volcanic District, Italy), yielding crystallization times of 1.46-3.12 yr