Equilibrium crystallization experiments were carried out on two primitive basaltic
rocks (APR16: Na2O+K2O=4.40 wt%; CM42: Na2O+K2O=2.59 wt%) with the aim to
investigate the amphibole stability in the differentiation processes at deep crustal level,
of primitive alkaline (APR16) and calc-alkaline (CM42) magmas. The experiments were
performed with different initial H2O contents (0-5 wt%), at pressure of 800 MPa, in the
temperature range of 975-1225 °C. For the explored conditions, amphibole crystallization
occurs in both compositions at H2O in the melt >7wt% while the temperature of their
occurrence is lower in the alkaline composition (<1050 °C in APR16 and ≥1050 °C in
CM42). Moreover, amphibole crystallization seems to be influenced by the Na2O/K2O
ratio rather than the absolute Na2O content in the melt. This is evident when experimental
results on the APR16 and CM42 are compared with experimental data obtained from a
primitive ultrapotassic composition (leucite-basanite: Na2O+K2O=4.58 wt%) and with
thermodynamic modelling by the Rhyolite-MELTS algorithm. The comparison shows
that amphibole never saturates the leucite-basanite at any of the investigated/modelled
conditions, even when an extended crystallization increases the Na2O of melts up to
contents like those of calc-alkaline experimental glasses. We conclude that, at pressure
of 800 MPa and hydrous conditions, only primitive liquids with Na2O/K2O ratio ≥0.9
are more prone to crystallize amphibole
