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

S Nazzareni,V Zanon,V Barbarossa,H Skogby,M Petrelli

doi:10.1007/s00410-020-01728-7

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 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 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.

Highlights

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)
Heterogeneities have been for a long time ascribed to the interactions of a hot-spot-associated, deep-rooted mantle plume with the magmas ascending at the MAR (e.g. Schilling 1975; Cannat et al 1999)
The basalts emplaced along the MAR segments intersecting the Azores region are thought to be originated from dry (50–200 ppm H2O) mantle source melting, whereas a wet (300–1000 ppm H2O) mantle source was initially postulated for the Ocean Island Basalt (OIB) magmas of the Azores (Hirschmann 2006)

Summary

Introduction

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). Heterogeneities have been for a long time ascribed to the interactions of a hot-spot-associated, deep-rooted mantle plume with the magmas ascending at the MAR In the Azores region, a close association of different magmas coexist, namely the mid ocean ridge basalt (MORB) from the MAR and Ocean Island Basalt (OIB) from the Azores volcanoes. Geochemical data modelling indicated that the Azores magmas were produced by melting of a source with a minimum H 2O content of 200 ppm at temperature lower than that of a dry mantle (Asimow et al 2004; Beier et al 2012; Métrich et al 2014). Few data are available on the water content of Azores magmas (Métrich et al 2014; Turner et al 2017), a detailed study on the pre-eruptive magma water content during the magmatic evolution of each single island is lacking

Methods

Results

Conclusion