Magma water content of Pico Volcano (Azores Islands, Portugal): a clinopyroxene perspective
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(2020) 175:87
ORIGINAL PAPER
Magma water content of Pico Volcano (Azores Islands, Portugal): a clinopyroxene perspective S. Nazzareni1 · V. Barbarossa2 · H. Skogby3 · V. Zanon4 · M. Petrelli1 Received: 4 December 2019 / Accepted: 11 August 2020 © The Author(s) 2020
Abstract 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 2O 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 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 Fe3+/Fetot 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. Keywords Mantle · Intraplate magmatism · FTIR spectroscopy · Azores islands · Magma water content · OIB · Cpx geobarometry
Introduction
Communicated by Gordon Moore. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00410-020-01728-7) contains supplementary material, which is available to authorized users. * S. Nazzareni [email protected] 1
Department of Physics and Geology, University of Perugia, Perugia, Italy
2
Institute of Environmental Sciences, Leiden University, Leiden, The Netherlands
3
Department of Geological Sciences, Natural History Museum, Stockholm, Sweden
4
Instituto de Investigação Em Vulcanologia E Avaliação de Riscos, Universidade Dos Açores, Rua Mãe de Deus, 9501‑801 Ponta Delgada, Portugal
The Azores region is located at the triple junction between the North American, Eurasian and Nubian lithospheric plates, a complex region where a mantle plume intersects with the mid-Atlantic ridge (MAR) and is associated to the presence of the Terceira Rift, a slow-spreading oceanic rift system on a thick and relatively old lithosphere (Fig. 1). The Azores islands are well-known for their chemical heterogenei
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