Kinetic crystallization of clinopyroxenes from alkaline basalts: growth rate experiments at high pressure

The kinetics of crystal nucleation and growth are fundamental for the interpretation of rock textures in terms of thermal history of a magma during its ascent to the surface, and to constrain timescales of magmatic processes. In basaltic systems 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 quantitative measurements of Cpx crystallization kinetics (e.g. crystal growth rate) are few and limited to low pressure conditions (≤ 500 MPa). In this experimental work, we investigated the effects of temperature, water content and dwell time on clinopyroxene (Cpx) crystal growth from an alkaline basalt of the Campi Flegrei Volcanic District (CFVD; Italy) at high pressure. With the aim to provide data on Cpx crystallization kinetics at high pressure, we experimentally investigate a primitive alkaline basalt (APR16 sample, mg# = 0.66 and Na2O+K2O = 4.4 wt.%) from Procida island (Italy), representative of the leastevolved rocks of the whole CFVD volcanic products at both anhydrous and hydrous conditions. Experiments were performed at isobaric pressure (800 MPa) as function of temperature (T), water content and dwell time by using the ½ inch 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 temperatures of 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), temperatures of 1220 °C and 1170 °C (series 2) and 1100 °C and 1050 °C (series 3), and the same dwell time of series 1. Preliminary results show a strong growth rate dependence of temperature and dwell time in both the anhydrous and hydrous experiments. As expected, the charges run at low temperature of each series are more crystallized than those run at high T; moreover, the low-T clinopyroxene grains are generally smaller in size than those formed at high-T. This behavior is noticeable already in the shortest dwell time (0.25 hours) experiments. As regards time, instead, we noted an increase of crystal size with increasing the dwell time (from 0.25 to 9 hours) and a decrease of the number of crystals: this is particularly evident in the hydrous runs (series 2 and 3). Finally, it was possible to note a variation in crystals shape that changes from anhedral/subhedral in the shorter runs to euhedral in the longer ones.