P, T, X magma storage conditions of the dominantly silicic explosive eruptions from Santorini volcano (Aegean Arc, Greece)
Résumé
It has been increasingly recognized that dramatic changes in magma storage conditions can occur over very short periods of time at a single volcano and might be in close relationships with stress variations imposed on the crustal plumbing by the overlying volcano as it changes shape and volume over time. The Santorini volcano (South Aegean Arc) is an ideal target to unravel these potential relationships as its history is marked by alternating episodes of edifice construction and caldera collapses and the chronostratigraphy is well constrained. We focused our study on the products of the four major, dominantly silicic, explosive eruptions of Santorini: the Lower Pumice 1 and 2 (200 to 180 ka; 1st explosive cycle) and, the Cape Riva and the Minoan (~ 21 to 3 ka, 2nd explosive cycle). In order to precisely define the P, T, fO2, X (X for volatiles) storage conditions of the silicic magmas prior to these eruptions, we carried out a detailed micro-petrological and geochemical study on natural samples combined with an experimental work. The selected silicic components of the four eruptions are dacite to rhyodacite (SiO2 = 67-70 wt.%) with similar mineral paragenesis (plagioclase, orthopyroxene, clinopyroxene, ilmenite, magnetite, apatite ± pyrrhotite) and crystallinity < 20%. High resolution BSE images of plagioclase and pyroxene phenocrysts and EMPA profiles reveal a complex crystallization history. Plagioclases display fine-scale oscillatory normal zoning, resorbtion zones where melt inclusions (MI) of rhyolitic compositions were trapped, and An-rich sieved cores. Clinopyroxenes also show zoning patterns and include rhyolitic MI. Both interstitial glass and MI are Cl-rich (~3000 ppm) while F and S are less abundant (F ≤ 700 ppm, S ≤ 100 ppm). Determination of H2O contents by SIMS is in progress at the CRPG-Nancy (previous measurements from the literature gave ~ 5 wt.% H2O in the Minoan rhyodacite and ~4 wt.% in the Lower Pumice 2). Ilmenite-magnetite geothermometry indicates temperatures ranging from ~850 to 900°C and deltaNNO ~ -1. For experimental work, the natural rocks were melted twice at 1400°C and 1 atm; the resulting dry glasses were used as starting materials. Each composition was loaded in Au capsules as powdered glass together with known amounts of water and CO2 (added as silver oxalate) keeping the XH2Oloaded [=moles of H2O/(H2O+CO2) ] in the range 1 - 0.6. We run crystallization experiments by using an Internally Heated Pressure Vessel (ISTO-CNRS) pressurised by an Ar-H2 mixture. The investigated conditions were: T = 850°C and 900°C with P = 2 and 4 kbar and fO2 ~ FMQ, for a total of 20 charges per experiment. The experimental products are analysed by SEM and then by EMPA for major elements compositions of the glass and minerals. Dissolved water content will be determined using the "by difference" method and the SIMS. The experimental work is still in progress; the results of this set of experiments will be presented in our communication.