The role of water in the 1944 Vesuvius eruption

Abstract

The P-T path of magma associated with the 1944 Vesuvius eruption has been outlined on the basis of probe mineralogy and the relationships between the crystallising phases. Equilibrium P-T values, obtained from the reactions:

• 1. CaMgSi₂O_6(liq) = CaMgSi₂O_6(cpx)

• 2. NaAlSi₃O_{8 (liq)} = NaAlSi₃O_{8 (plag)}

• 3. CaAl₂Si₂O_{8 (plag)}=CaAl₂SiO_6(cpx)+SiO_2(liq) have been established for three intracrustal crystallisation stages: I) 8.0 kbar and 1255 °C; II) 4.0 kbar and 1178 °C; III) 0.5 kbar and 1105 °C.

The H₂O content in the magma has been estimated from an experimental calibration of $a_{^{CaMgSi_2 O_6 } }^{liq}$ as a function of $X_{H_2 O}^{liq}$ at P _tot = 2 kbar. The estimated water contents of the magma for the three stages, I) 0.7%; II) 0.9%; III) 1.1%, are consistent with the pattern of activity of the 1944 Vesuvius eruption and with the relationship between the lavas.

The shallow depth of H₂O-saturation of the magma, 0.24 kbar at 1100 °C, is consistent with the eruption sequence of lava flows followed by lava fountain activity.