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

<p>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 (&#951;) 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.</p> <p>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-&#951; differ from those at low-T, high-&#951; 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.,</p> <p>The most recent predictive viscosity models well reproduce the high-T, low-&#951; regime, whereas modeled data are less accurate in the low-T, high-&#951; 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.</p>