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Differentiation conditions of a basaltic magma from Santorini and its bearing on andesitic/dacitic magma production

Abstract : The phase relations of a basalt from Santorini have been investigated experimentally with the aim of determining the conditions of differentiation (pressure, temperature, volatile fugacities) and the parental relations with the andesitic/rhyodacitic constituents. The starting material is a basaltic rock from the Balos volcano (50.7 wt% SiO2, 18.0 wt% Al2O3, 9.2 wt% Fe2O3, 6.7 wt% MgO, 2.8 w% Na2O, 0.5 wt% K2O) that contains phenocrysts of olivine (Fo74-76), pyroxene (En46,Fs12,Wo42, Mg#79) plagioclase (An77-80) and minor traces of magnetite and ilmenite. The rock also contains some xenocrysts of olivine (Fo78) and plagioclase (An90) the latter sometimes displaying core sieved textures and inverse compositional zoning (cores:An55-60, rims An80). The compositional variation of plagioclase suggests the presence of a dynamic system that reached a steady state prior to eruption, making this sample a good starting material for phase equilibria.Previous pre-eruptive constraints for the Minoan rhyodacite suggest initial storage conditions ranging from 400 to 200 MPa, followed by rapid migration to shallow levels (1 km; 50 MPa). Pre-eruptive temperatures were in the range of 825-900°C, with dissolved water contents from 4 to 6 wt%, and fO2 between FMQ-NNO. Accordingly we have performed a series of crystallization experiments at ISTO in the temperature range 1040-975°C, pressure between 100-400 MPa,fO2˜NNO using an IHPV operated vertically, pressurized with Ar-H2 mixtures. XH2O varied from 1 (saturation) to nominally dry conditions by using different H2O/CO2 ratios of the fluid. Experimental minerals are: olivine (ol), pyroxene (px), plagioclase (plag), magnetite (mag), ilmenite (ilm) and amphibole (amph). At 200 MPa the assemblage ol+px+plag+ox is stable from 1040 to 1000°C. Below 1000°C ol+px disappears to form amphibole. At 1000°C amphibole is stable at pressures >300 MPa whereas at lower pressure the stable assemblage is ol+px+plag. Melts compositions vary between 51 up to 60 wt% SiO2, 0.7 to 6.5 wt% MgO and 1.3 to 5.7 FeO/MgO.A decrease in water content increases alkalis, Al and decreases Fe, Ca and Mg, whereas at constant temperature an increase in pressure increases Si, alkalis, Ti and FeO/MgO and decreases Al, Ca, Mg and Mg-number of the melt. The comparison of the experimental melts with the dacitic composition of the Upper Scoriae 1 eruption suggests a range of conditions for melt production between 975-1000°C,pressure between 200 to 400MPa and XH2O between 1.0 to 0.5. The main phases controlling basalt to dacite evolution are amph+mag at 400MPa or amph+mag+ilm+plag at 200MPa. However,the higher experimental FeO/MgO and lower Mg# compared to the natural dacite suggest fO2
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Joan Andújar, Bruno Scaillet, Michel Pichavant, Timothy H. Druitt. Differentiation conditions of a basaltic magma from Santorini and its bearing on andesitic/dacitic magma production. American Geophysical Union, Fall Meeting, 2010, United States. ⟨insu-00838517⟩



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