Magma Storage Conditions of Large Plinian Eruptions of Santorini Volcano (Greece)

JOURNAL OF PETROLOGY VOLUME 55 NUMBER 6 PAGES 1129^1171 2014 doi:10.1093/petrology/egu021 Magma Storage Conditions of Large Plinian Eruptions of Santorini Volcano (Greece) ANITA CADOUX*,1,2,3,4,5,6, BRUNO SCAILLET1,2,3, TIMOTHY H. DRUITT4,5,6 AND ETIENNE DELOULE7 UNIVERSITE¤ D’ORLE¤ANS, ISTO, UMR 7327, 45071 ORLE¤ANS, FRANCE 2 CNRS, ISTO, UMR 7327, 45071 ORLE¤ANS, FRANCE 1 BRGM, ISTO, UMR 7327, BP 36009, 45060 ORLE¤ANS, FRANCE 4 CLERMONT UNIVERSITE¤, UNIVERSITE¤ BLAISE PASCAL, LABORATOIRE MAGMAS ET VOLCANS, BP 10448, F-63038 3 CLERMONT-FERRAND, FRANCE 5 CNRS, UMR 6524, LMV, F-63038 CLERMONT-FERRAND, FRANCE 6 IRD, R 163, LMV, F-63038 CLERMONT-FERRAND, FRANCE 7 CRPG-CNRS, BP20, 54501 VANDOEUVRE LES NANCY, FRANCE RECEIVED JULY 8, 2013; ACCEPTED APRIL 3, 2014 The intensive variables of dacitic^rhyodacitic magmas prior to four large Plinian eruptions of Santorini Volcano over the last 200 kyr (Minoan, Cape Riva, Lower Pumice 2 and Lower Pumice 1) were determined by combining crystallization experiments with study of the natural products, including the volatile contents of melt inclusions trapped in phenocrysts. Phase equilibria of the silicic magmas were determined at pressures of 1, 2 and 4 kbar, temperatures of 850^9008C, fluid (H2O þ CO2)-saturation, XH2O [¼ molar H2O/(H2O þ CO2)] between 0·6 and 1 (melt H2O contents of 2^10 wt %), and redox conditions of FMQ (fayalite^magnetite^ quartz buffer) or NNO þ1 (where NNO is Ni^NiO buffer). Experiments were generally successful in reproducing the phenocryst assemblage of the natural products.The phase relationships vary significantly among the investigated compositions, revealing a sensitivity to small variations in whole-rock compositions. Our results show that the pre-eruptive storage conditions of the four silicic magmas were all very similar. The magmas were stored at T ¼ 850^9008C and P  2 kbar, under moderately reduced conditions (
NNO ¼ 0·9 to 0·1), and were poor in fluorine (500^800 ppm) and sulphur (100 ppm), but rich in water and chlorine (5^6 wt % and 2500^3500 ppm, respectively). In all cases, the melts were slightly undersaturated with respect to H2O, but most probably saturated with respect to H2O þ Cl  CO2 and a brine. The Santorini magma plumbing system appears to be dominated by a large, longlived (200 kyr) predominantly silicic magma storage region situated at 8 km depth, from which crystal-poor melt batches were extracted during the largest caldera-forming eruptions of the volcanic system. *Corresponding author. Present address: Universite¤ d’Orle¤ans, ISTO, UMR 7327, 45071 Orle¤ans, France. Telephone: 00 33 þ2 38 25 53 83. E-mail: ß The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oup.com KEY WORDS: magma storage; melt inclusions; phase equilibrium; Santorini; volatiles I N T RO D U C T I O N It has been increasingly recognized that dramatic changes in magma storage conditions can occur over very short periods of time at a single volcano. This was shown at, for example, Mt. St. Helens and Mt. Vesuvius, where magma storage depths vary over time-scales of several hundred years (e.g. between the Pompeii and Pollena eruptions at Mt. Vesuvius; Scaillet et al., 2008) to a few years (between the Wn and We events at Mt. St. Helens; Gardner et al., 1995; Blundy et al., 2008; Rutherford & Devine, 2008). These variations of magma ponding level may be attributed to differences in the volatile content and buoyancy of the JOURNAL OF PETROLOGY VOLUME 55 ascending magmas. They might also be related to changes in stress imposed on the crustal plumbing system by the overlying edifice (e.g. Pinel & Jaupart, 2003; Pinel et al., 2010), following either caldera formation (Ventura et al., 1999) or volcano spreading (Borgia et al., 2005). It is remarkable indeed that, for these two volcanic systems, the largest and fastest pressure drops recorded by the magmas coincide with the partial destruction of the edifice subsequent to a major explosive eruption. Strain relaxation in the upper crust resulting from a decrease of edifice load could allow accumulation of magmas at shallower depths (Ventura et al., 1999). Variations in magma storage conditions have also been correlated with changes in eruptive dynamics at Mt. Pele¤e, Mt. Vesuvius and Tenerife (e.g. Martel et al., 1998; Scaillet et al., 2008; Andu¤jar & Scaillet, 2012a, 2012b). For instance, Andu¤jar & Scaillet (2012b) showed that the explosive^effusive style of phonolitic magmas correlates with the amount of volatiles, the degree of water-undersaturation and the depth of magma storage, the explosive character generally increasing with pressure, depth and dissolved water content. Determining magma storage conditions over time is thus crucial to understanding volcano behaviour, and may significantly contribute to the evaluation of volcanic hazards during periods of unrest. Santorini volcano (South Aegean Arc, Greece), which has recently shown signs of seismic and geodetic unrest (2011^2012; e.g. Newman et al., 2012), is an ideal target for unraveling these potential relationships, as its history is marked by recurrent large-scale Plinian eruptions (roughly every 20^30 kyr). Some of these eruptions triggered caldera collapses, alternating with edifice construction and minor interplinian eruptions (Fig.1).This study focuses onthe silicic products of the four major Plinian eruptions of Santorini that have occurred over the last 200 kyr (Fig. 1): the Lower Pumice 1, Lower Pumice 2, Cape Riva and Minoan eruptions. Magmas of Lower Pumice1, Lower Pumice 2 and Minoan were dominantly rhyodacitic, and that of Cape Riva was dacitic.To define precisely the P,T, fO2 and volatile (H2O, CO2, Cl, F, S) pre-eruptive storage conditions of these magmas, we carried out phase equilibrium crystallization experiments coupled with a petrological and geochemical study of the natural products. We present the stability fields of the phases for each eruption over the T^P^fO2^XH2O conditions explored: T ¼ 850^9008C; P ¼1, 2 and 4 kbar; fO2 ¼ FMQ (fayalite^magnetite^ quartz buffer) and NNO þ1 (where NNO is Ni^NiO buffer); XH2O [¼ moles of H2O/(H2O þ CO2)] between unity (i.e. H2O-saturated) and 0·6 (H2O-undersaturated). The phase assemblages, abundances and compositions of the experimental and natural products are compared, and their implications for pre-eruptive conditions and for our understanding of the Santorini plumbing system are discussed. NUMBER 6 JUNE 2014 G E O L O G I C A L B AC KG RO U N D Eruptive history Volcanism at Santorini began c. 650 kyr ago and subsequently involved two major explosive cycles between 360 and 3·6 ka that formed the bulk of the volcanic deposits (Druitt et al., 1999). Twelve major Plinian eruptions occurred during the two cycles, with ejected volumes ranging from a few km3 to several tens of km3. The first explosive cycle (360^172 ka) included five major P (...truncated)