Cooling kinetics experiments on different Stromboli lavas: Effects on crystal morphologies and phases composition

Controlled cooling experiments at atmospheric pressure and fO2 of the NNO buffer were performed on three Stromboli lavas of calcalkaline (CA), high-K calcalkaline (HKCA) and shoshonitic (SHO) affinities. The experiments were conducted at cooling rates of 900 °C/h (fast cooling rate—FCR) and 1 °C/h (slow cooling rate—SCR), respectively, to investigate the kinetic factors governing textural and compositional features such as phase growth morphology, phase relations and phase compositions. The experiments produced different types of textures as both the cooling rate and the quenching temperatures were varied. In general, FCR experiments show a decrease of crystal size as well as a change in crystal shape from euhedral, to granular, to skeletal up to dendritic morphology with decreasing temperatures. SCR experiments produce less variable crystal size and crystal morphology. Measurements of plagioclases indicate that the crystal size and calculated crystal growth are functions of cooling rate; crystal growth rates are about two orders of magnitude higher in FCR experiments than SCR ones. Such textural responses are in agreement with the empirical calibration based on the plagioclase size dependence on cooling rate developed by Cashman (1993).Experimental phase relations display three main features of the crystallization process: (1) the delay in the crystallization of the liquidus phase in the FCR experiments with respect to the SCR ones; (2) crystallization, controlled by the multiphase cotectic relations (plagioclase + pyroxenes ± olivine), is more enhanced for FCR experiments compared to that of SCR ones at the lowest experimental temperature (1100 °C); (3) crystallization of augite and pigeonite is sensitive to cooling rate and system compositions; pigeonite, not found in the natural rocks, crystallizes in the CA sample across SCR and FCR runs, and in the HKCA sample just in the SCR experiments.FCR and SCR experiments produce similar liquid lines of descent for both CA and HKCA starting materials suggesting that the processes of nucleation and growth play reciprocal roles through mass balance as cooling rate varies. This is not true for the SHO composition where the trends of melt composition appear to be especially influenced by the different amounts of olivine crystallization at the different cooling rates. Data on mineral compositions show that plagioclase and clinopyroxene are the phases more strongly affected by the cooling gradients; crystals from SCR experiments and, to a lesser extent, from FCR experiments, show zoning patterns mainly at the lowest experimental temperature.