Differentiation Conditions of a Basaltic Magma from Santorini, and its Bearing on the Production of Andesite in Arc Settings

JOURNAL OF PETROLOGY Journal of Petrology, 2015, Vol. 56, No. 4, 765–794 doi: 10.1093/petrology/egv016 Original Article Differentiation Conditions of a Basaltic Magma from Santorini, and its Bearing on the Production of Andesite in Arc Settings Joan Andújar1,2,3*, Bruno Scaillet1,2,3, Michel Pichavant1,2,3 and Timothy H. Druitt4 1 Université d’Orléans, ISTO, UMR 7327, 45071 Orléans, France, 2CNRS/INSU, ISTO, UMR 7327, 45071 Orléans, France, 3BRGM, ISTO, UMR 7327, BP 36009, 45060 Orléans, France and 4Laboratoire Magmas et Volcans, Université Blaise Pascal–CNRS–IRD, OPGC, 5 Rue Kessler, 63038 Clermond-Ferrand, France *Corresponding author. Telephone: (þ33) 2 38 25 53 87. Fax: (þ33) 02 38 63 64 88. E-mail: Received March 21, 2014; Accepted March 23, 2015 ABSTRACT Santorini volcano in the Aegean region (Greece) is characterized by andesitic- to silicic-dominated explosive activity and caldera-forming eruptions, sourced from magmatic reservoirs located at various structural levels beneath the volcano. There is a good understanding of the silica-rich magmatism of the island whereas the andesite-dominated volcanism and the petrogenesis of the parental mafic magmas are still poorly understood. To fill this gap we have performed crystallization experiments on a representative basalt from Santorini with the aim of determining the conditions of differentiation (pressure, temperature, volatile fugacities) and the parental magma relationship with the andesitic eruptive rocks. Experiments were carried out between 975 and 1040 C, in the pressure range 100–400 MPa, fO2 from QFM to NNO þ 35 (where QFM is quartz– fayalite–magnetite and NNO is nickel–nickel oxide), with H2Omelt contents varying from saturation to nominally dry conditions. The results show that basalt phenocrysts within the basalt crystallized at around 1040 C in a magma storage reservoir located at a depth equivalent to 200–400 MPa pressure, with 3–5 wt % dissolved H2O, and fO2 around QFM. Comparison with the xenocryst and phenocryst assemblages of the Upper Scoria 1 andesite shows that andesitic liquids are produced by fractionation of a similar basalt at 1000 C and 400 MPa, following 60–80 wt % crystallization of an ol þ cpx þ plag þ Ti-mag þ opx 6 pig–ilm assemblage, with melt water contents around 4–6 wt %. At Santorini, the andesitic low-viscosity and water-rich residual liquids produced at these depths segregate from the parent basaltic mush and feed the shallow magma reservoirs, eventually erupting upon mixing with resident magma. Changes in prevailing oxygen fugacity may control the tholeiitic–calc-alkaline character of Santorini magmas, explaining the compositional and mineralogical differences observed between the recent Thyra and old eruptive products from Akrotiri. Key words: phase equilibria; basalt; andesite; liquid line of descent; experimental petrology; Santorini INTRODUCTION Caldera-forming eruptions are among the most hazardous phenomena on Earth. They are characterized by the emission of huge amounts of magma and gas (e.g. Mt Mazama: Bacon & Druitt, 1988; Druitt & Bacon, 1989; Tambora: Self et al., 2004; Krakatoa: Mandeville et al., 1996; Fish Canyon Tuff: Bachmann et al., 2002; Bishop Tuff: Hildreth & Wilson, 2007) and can affect the climate at a global scale and be harmful to human life and related infrastructure. This is the case of Santorini volcano, in the Aegean region of Greece, the explosive activity of which has led to several caldera-forming C The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: V 765 766 events. The last and largest of these eruptions occurred at around 36 ka (the so-called Minoan eruption; Heiken & McCoy, 1984; Druitt, 2014), with 30–60 km3 of ejected rhyodacitic magma. Although several studies have focused on the petrogenesis of the silicic magmas (Cottrell et al., 1999; Druitt et al., 1999; Gertisser et al., 2009; Cadoux et al., 2014), less attention has been paid to the more mafic compositions, even though the volcanic activity at Santorini has involved several large andesite-dominated explosive eruptions (i.e. the Upper Scoria eruptions; Druitt et al., 1999) and effusive activity involving magmas ranging from basalt to dacite in composition. The conditions of evolution from basalt to more evolved compositions are therefore still unclear. To improve our knowledge of andesitic volcanism at Santorini, and at volcanic arcs in general, we have investigated experimentally the phase relationships of a representative basalt from Santorini, with the aim of determining the conditions of differentiation (pressure, temperature, volatile fugacities) that generate the andesitic magmas erupted there. To achieve this objective, we have identified the conditions under which basaltic-andesite to andesite liquids are generated by fractional crystallization of the basalt, and have refined this estimate by comparing the phenocryst and xenocryst populations of a typical Santorini andesite (Upper Scoria 1 eruption) with the experimental phases of basaltic charges with residual andesitic liquids. Our results show that mantle-derived basalts stagnate at depths of 12–14 km (400 MPa), where they fractionate to yield basaltic andesite liquids (55–58 wt % SiO2) with 4–6 wt % H2Omelt at around 1000 C and at an f O2 near QFM (quartz–fayalite–magnetite; 603 log units). This is achieved by crystallizing about 60 wt % of olivine (ol), clinopyroxene (cpx), plagioclase (plag), orthopyroxene (opx), Ti-rich magnetite 6 pigeonite (pig), and ilmenite (ilm). GEOLOGICAL SETTING The geology and tectonic setting of Santorini have been the focus of many studies, which are briefly summarized below. The Santorini islands belong to the Cyclades archipelago. They constitute the most active volcanic complex of the South Aegean volcanic arc, which is related to the NE-directed subduction of the African plate beneath the southern margin of the Eurasian plate (Le Pichon & Angelier, 1979; Jolivet & Faccenna, 2000; Papazachos et al., 2000; Reilinger et al., 2010). The volcanic island is built on a 23 km thick continental crust made of an upper layer of Mesozoic to Cenozoic marbles and phyllites, and a lower layer of Precambrian to Paleozoic garnet-bearing gneisses and mica schists (Nicholls, 1971b; Druitt et al., 1999). The volcanic history of Santorini can be summarized as follows (Druitt et al., 1999): activity began at about 650 ka with the eruption of hornblende-bearing rhyolites, rhyodacites and minor andesites of calc-alkaline affinity. From 550 ka onwards, the volcano erupted magmas Journal of Petrology, 2015, Vol. 56, No. 4 ranging from basalt to rhyodacite in composition, hornblende became scarce as a phenocryst phase, and the magmas became tholeiitic to transitional tholeiitic– calc-alkaline in character. Around 360 ka, the activity became highly explosive, with the alternation of plinian eruptions with interplinian periods (...truncated)