Magma water content of Pico Volcano (Azores Islands, Portugal): a clinopyroxene perspective

• Published in: Contributions to mineralogy and petrology 2020-08, Vol.175 (9), Article 87
• Main Authors: Nazzareni, S., Barbarossa, V., Skogby, H., Zanon, V., Petrelli, M.
• Format: Article
• Language: English
• Subjects: Archipelagoes; Cooling rate; Crystallization; Dehydration; Den föränderliga jorden; Earth; Earth and Environmental Science; Earth Sciences; Fugacity; Geology; Lava; Lithosphere; Magma; Mantle; Mineral Resources; Mineralogy; Moho; Moisture content; Oceanic crust; Original Paper; Petrology; Ponding; The changing Earth; Transition zone; Volcanic eruptions; Volcanoes; Water content; X-ray diffraction
• ISSN: 0010-7999; 1432-0967; 1432-0967
• DOI: 10.1007/s00410-020-01728-7

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 underwent important dehydration processes and after annealing experiments under H 2 gas flux, a pre-eruptive H 2 O content between 93 and 182 ppm was recovered. A moderately high cooling rate for the cpx-host lavas expressed by the clinopyroxene closure temperature ( T c  = 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 IV Al-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 of 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 Fe 3+ /Fe tot ratio of clinopyroxene, suggest similar conditions of oxygen fugacity, water content and fractional crystallisation of the magma from which clinopyroxene cores crystallised during the Pico Volcano central eruptions from 40 ka to historical times.