The effect of CaO and CaO+MgO on the viscosity of a phonotephritic melt

The assimilation of carbonate rocks by magmas can dramatically change their chemistry and differentiation path, thereby affecting the rheological properties of the derived products. Here we present a set of viscosity measurements exploring the effect of variable degrees of carbonate assimilation on the melt viscosity (η) of a phonotephrite from Vesuvius (Italy). We doped the starting material with different amounts (0, 10, and 20 wt.%) of CaO and CaO+MgO, mimicking the effects of limestone and dolomite assimilation, respectively. Through this approach, we focused on the composition change liquid phase, regardless of the effect of CO₂ bubbles produced by the decarbonation on the rheological properties.

The high and low temperature liquid viscosity of the decarbonated melts were measured by concentric cylinder viscometry (CC) and differential scanning calorimetry (DSC), respectively. Viscosity data show non-Arrhenian trends, well described by both Vogel-Fulcher-Tammann (VFT) and Mauro-Yue-Ellison-Gupta-Allan (MYEGA) equations. Trends obtained at high-T, low-η differ from those at low-T, high-η conditions. In the high-T regime, all decarbonated melts show lower viscosity than the pristine melt, the effect being more pronounced when only CaO is added. The opposite trend is observed in the low-T-regime, due to different fragility of the investigated melt.,

The most recent predictive viscosity models well reproduce the high-T, low-η regime, whereas modeled data are less accurate in the low-T, high-η regime. This discrepancy is apparently caused by the lack of decarbonated melt (i.e., Si poor, Ca-Mg-rich compositions) in the calibration dataset of viscosity models.