Magma Storage and Extraction Associated with Plinian and Interplinian Activity at Santorini Caldera (Greece)

Abstract

Santorini caldera has had a long history of plinian eruptions and caldera collapses, separated by 20-40 kyr interplinian periods. We have carried out a study to constrain magma storage/extraction depths beneath the caldera. We analysed H 2 O in 138 olivine-, pyroxene-and plagioclase-hosted melt inclusions from plinian and interplinian products from the last 200 kyr, and CO 2 , S, Cl, F and dD in various subsets of these. The dataset includes 64 inclusions in products of the Minoan plinian eruption of the late 17th century BCE. All the melt inclusions were ellipsoidal and isolated, with no textural evidence for volatile leakage. Mafic melt inclusions contain 1-4 wt % H 2 O and up to 1200 ppm CO 2 , 1200 ppm S, 2000 ppm Cl and 400 ppm F; silicic inclusions contain 2-7 wt % H 2 O, up to 150 ppm CO 2 , up to 400 ppm S, 2000-6000 ppm Cl and 600-1000 ppm F. The dD values of 27 representative inclusions (-37 to-104%) are intermediate between mantle and slab values and rule out significant H 2 O loss by hydrogen diffusion from olivine-hosted inclusions. H 2 O, S and Cl behave compatibly in melt inclusion suites varying from mafic to silicic in composition, showing that entrapment of many melt inclusions took place under volatile-saturated conditions. Most Santorini melts are saturated in a free COHSCl vapour phase at depths of less than $10 km; the only exceptions are basaltic melts from a single interplinian eruption, which were volatile-undersaturated up to K 2 O contents of $1 wt %. The rhyolitic melt of the Minoan eruption probably contained a free hypersaline liquid phase. H 2 O þ CO 2 saturation pressures were calculated using suitably calibrated solubility models to estimate pre-eruptive magma storage depths. Magmas feeding plinian eruptions were stored at >4 km (>100 MPa) and extracted over depth intervals of several kilometres. Plagioclase phenocrysts in rhyodacitic pumice from the Minoan eruption have cores containing melt inclusions trapped at depths up to 10-12 km (320 MPa), and rims (also orthopyroxene and clinopyroxene) containing inclusions trapped at 4-6 km (100-160 MPa). This records late-stage silicic replenishment of a <2 km thick shallow magma chamber, rather than extraction of melts syneruptively over the entire depth range. The plagioclase cores were carried from depth in the ascending melt, then overgrown by the rims in the shallow chamber. Exsolution of volatiles during ascent may have caused the replenishment melt to inject as a bubbly plume, causing mixing prior to eruption. This would explain (1) the homogeneity of the Minoan rhyodacitic magma, and (2) extraction of melt inclusions from the entire pressure spectrum during the first eruptive phase. Most silicic magmas feeding eruptions of the interplinian periods were stored in reservoirs at shallow depths (2-3 km) compared with those feeding the plinian eruptions (>4 km). Melt inclusions from the AD 726 eruption of Kameni Volcano yield a pre-eruptive storage depth of $4 km, which is similar to that estimated from geodetic data for the inflation...

Multiple timescale constraints for high-flux magma chamber assembly prior to the Late Bronze Age eruption of Santorini (Greece)

“…Cadoux et al (2014) also proposed that the melt co-existed with a H2O+Cl-rich hydrosaline liquid. The MI H2O + CO2 data of Druitt et al (2016) yielded pre-eruptive storage pressures (100-160 MPa; 4-6 km) intermediate between the estimates of the previous authors. Combining the different constraints, the best-fit pre-eruptive conditions for the LBA rhyodacite are 850-880 °C, fO2 ~ NNO, 100-160 MPa (4-6 km depth), melt saturated with 4.0-5.0 wt% H2O, <200 ppm CO2, <150 ppm S and 3000-5000 ppm Cl, a free H2O-rich vapour phase, and possibly a H2O+Cl-rich hydrosaline liquid.…”

“…In both interpretations, however, magma chamber assembly took place on timescales that were geologically very short. Other data have subsequently been published on the LBA magma: micro-Sr isotopic analyses of plagioclase (Martin et al 2010), phase equilibria experiments on Santorini silicic magmas (Cadoux et al 2014) and the basaltic/andesitic magmas parental to them (Andújar et al 2015;, H2O+CO2 melt inclusion barometry (Druitt et al 2016), and the chemistry and petrology of minor juvenile and cognate components in the LBA eruption products (Druitt 2014).…”

“…Melt inclusions (MIs) in An40±3 plagioclase contain 69-75 wt% SiO2 (Online Resource 1). Secondary ion mass spectrometry (SIMS) analysis of >60 isolated, crack-free MIs hosted in plagioclase, opx and cpx revealed volatile concentrations of 4.0-6.9 wt% H2O, 20-200 ppm CO2, 40-100 ppm S, and 3000-4500 ppm Cl (Cadoux et al 2014;Druitt et al 2016). MIs within plagioclase An50±7 cores are more H2O-rich (4.2-6.9 wt%) than those within An40±3 rims (4.0-5.0 wt%) ( Fig.…”

“…2d). Druitt et al (2016) used MI H2O+CO2 contents and the solubility models of Newman and Lowenstern (2002) and Papale et al (2006) to estimate entrapment pressures, which were converted to entrapment depths using a mean upper crust density of 2640 kg.m -3 (Konstantinou, 2010) assuming volatile saturation (Cadoux et al 2014;Druitt et al 2016). MI entrapment in the An40±3 plagioclase occurred at 100-160 MPa (4-6 km depth).…”

Magma Chamber Growth During Intercaldera Periods: Insights From Thermo‐Mechanical Modeling With Applications to Laguna del Maule, Campi Flegrei, Santorini, and Aso

“…These eruptions have been dominantly effusive lavas that built up the Kameni islands in the center of the caldera bay and have ranged in volume from~0.0006 to 0.14 km 3 , with the volume varying in proportion with the repose time (Parks et al, 2012). Deposits from the four major caldera-forming eruptions indicate relatively cold and wet conditions (850-900°C and 2-7 wt% H 2 O) compared to the postcaldera lavas (960-1,012°C and 1-4 wt% H 2 O; Barton & Huijsmans, 1986;Cadoux et al, 2014;Cottrell et al, 1999;Druitt et al, 2016). Petrologic data combined with modeling by Degruyter et al (2016) suggest that the magma chamber was water saturated prior to the Minoan eruption but has been drier and possibly even undersaturated ever since.…”