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.