Experimental constraints on alkali mobility in the Campi Flegrei magmatic system

The Na2O/K2O ratio of Campi Flegrei (CF) magmas varies from ~2 to ~0.2 with no clear correlation with the MgO content (2≤MgO≤6 wt.%). Such a significant alkali variation cannot be explained by a mere crystal fractionation process in closed system, but it can be more correctly related to an increase of H2O in the system. This would affect the stability and proportions of the crystallizing minerals and the composition of residual melts. Moreover, the presence of a free fluid phase could have an influence on the liquid line of descent of the system and in particular on alkali mobility. To test the water effect on alkali mobility in the CF magmatic system, we have performed equilibrium experiments on a primitive alkaline (Salkali=4.32 wt.%) basalt at 800 MPa and different initial water contents (1≤H2Oi≤6 wt.%). The experiments confirm the fundamental role played by water content on the liquidus phases (i.e. clinopyroxene vs clinopyroxene+olivine). However, the change of liquidus phases alone does not explain the TAS differentiation trends shown by the experimental melts at different water contents. In particular, the experimental subalkaline glasses produced under hydrous conditions (4≤H2Oi≤6 wt.%) cannot be explained invoking a closed melt+crystal system. In particular, the low alkali content (Salkali=1.48 and 3.26 wt.% at 1050°C and 1000°C, respectively) characterising these experiments is inconsistent with the type (clinopyroxene+olivine+orthopyroxene±amphibole±spinel) and abundance of crystals in the charges. The occurrence of bubbles in these hydrous charges indicates that melt and crystals were in equilibrium with free aqueous fluid phase and that alkali were distributed in the coexisting phases (fluid+melt+crystal). These experimental results and the Na2O/K2O ratio variability could indicate that the CF magmatic system is characterised by a relatively early and deep degassing process able to remove different proportion of Na and K from the melt