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.