MAGMA WATER CONTENT OF PICO VOLCANO (AZORES ISLANDS, PORTUGAL): A CLINOPYROXENE PERSPECTIVE

Clinopyroxenes from the Pico Volcano (Pico Island, Azores Archipelago) have been used as a proxy to define the water content of primitive magmas and the volcanological history of the erupted rocks. This very young volcano (53 ±5 ka) is at a primordial stage of its evolution in comparison with the other volcanoes of the Azores. Clinopyroxenes from Pico Volcano suffered important dehydration processes and after annealing experiments under H2 gas flux a pre-eruptive H2O content between 93 and 182 ppm was recovered. A moderately high cooling rate for the cpx-host lavas expressed by the clinopyroxene closure temperature (Tc = 755–928°C ±20°C) correlates with the dehydration suggesting that this process may have occurred during magma ponding at the Moho Transition Zone (17.3–17.7 km) and/or after the eruption. By applying an IVAl-dependent partition coefficient to the measured H amount in clinopyroxene, the pre-eruptive water content of the parental magma was calculated to vary between 0.71 and 1.20 (average 1.0) wt%. Clinopyroxene geobarometry performed by combining X-ray diffraction with mineral chemistry points to a general crystallisation from the mantle lithosphere (~8-9 kbar) to the oceanic mantle/crust boundary (~4-5 kbar). The similar major and trace chemistry, water content and Fe3+/Fetot ratio of clinopyroxene, suggest similar condition of oxygen fugacity, water content and fractional crystallisation of the magma from which clinopyroxene core crystallised over the Pico Volcano central eruptions from 40 ka to historical times.