Clinopyroxene compositions as an indicator of magmatic water content: insight from high pressure experiments on trachybasaltic magmas

Water contents in magmas affect liquidus temperatures, crystal fractionation trends, melt rheology and, in turn, the dynamics of ascent and eruption of magmas. Hence, it is of broad interest to determine the initial concentration of water in magmas and its variations during magma ascent and differentiation. Analysis of melt inclusions and experimental studies allowed to define the qualitative effects of water on magmatic processes; however, approaches for the quantitative estimation of water content in magmas are still scarce. In this regard, clinopyroxene (cpx) can be used as a tracer of crystallization processes since it is among the earliest and most abundant phase in basaltic rocks and contains the most complete record of crystallization history of a magma. In particular, since cpx abundances and compositions are controlled by magma water solubility, this phase has been successfully used as empirical hygrometer in trachybasaltic magmas of Mt. Etna (Armienti et al., 2011). Experimental data available in literature on hydrous liquids representative of Mt. Etna volcanics, used to calibrate the empirical method, cover a wide range of pressure (0.2≤P≤0.8 GPa). However, very few runs were performed at high pressure (P≥0.5 GPa) up to now. For this reason, new HP experimental data on Etnean trachybasalts are fundamental for the implementation of the Cpx-hygrometer. Experiments were conducted in a piston-cylinder apparatus at a pressure of 0.8 GPa in the presence of 0-6 wt% of added H2O. Preliminary results indicate that cpx is the liquidus phase in both dry and hydrated conditions, followed by plagioclase and olivine. At a given temperature (e.g. 1150°C), the addition of 1 wt% of H2O produces a significant decrease of crystallization and appears also to influence the size and the number of crystals; the addition of 6 wt% of H2O induces nearly 100°C decrease of the liquidus temperature, down to about 1050°C. In terms of end-members (DiHd, EnFs, CaTs, Jd, CaTi), it appears that the main effect on cpx composition ascribable to an increase of water content in the melt is the increase of EnFs and Jd components coupled with a decrease of Ca-Tschermak end-member. References Armienti P., Perinelli C. & Putirka K.D. (2011). An empirical hygrometer for trachybasaltic melts: applications to the kinetics of magma ascent at Mt. Etna. EGU2011, Vol. 13, 10220.