A new test for equilibrium based on clinopyroxene-melt pairs: Clues on the solidification temperatures of Etnean alkaline melts at post-eruptive conditions

Abstract

We have performed new global regression analyses to calibrate a model of equilibrium between clinopyroxene and co-existing melt. Then we have applied this model to a restricted but important range of clinopyroxene and melt compositions from Mt. Etna volcano. The degree of disequilibrium is determined through the comparison between components “predicted” for clinopyroxene via regression analyses of clinopyroxene-liquid pairs in equilibrium conditions, with those “measured” in the analyzed crystals. The model is tested using compositions not included into the calibration dataset, i.e., clinopyroxene-melt pairs obtained from equilibrium and cooling rate experiments conducted at ambient pressure on an Etnean trachybasalt. The experiments were duplicated at the NNO+1.5 and QFM oxygen buffering conditions estimated for magmas at Mt. Etna. Both equilibrium and disequilibrium clinopyroxene-melt pairs from the experiments were also used as input data for one of the most recent thermometers based on the Jd-DiHd exchange reaction. Results from calculations indicate that, under rapid cooling rate conditions, clinopyroxenes do not equilibrate with the melt. Consequently, the thermometers predict higher crystallization temperatures compared to the final experimental temperature, prior to rapid quenching of the experiment. The systematic difference between expected and measured compositions and temperatures allows us to calibrate a model that describes undercooling based on disequilibrium exchange reactions. We use this new tool to estimate the thermal history of naturally cooled lava flows and dikes at Mt. Etna volcano.