HR: 1340h
AN: V13B-0525    [Abstracts]
TI: Phase Equilibria Experiments on Nisyros, Greece:``the Right Stuff" in -``the Right Stuff" out
AU: * Pyle, D M
EM: dmp11@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AU: Hunt, C A
EM: Carol.Hunt@uea.ac.uk
AF: School of Environmental Science, University of East Anglia, Earlham Road, Norwich, NR4 7TJ United Kingdom
AU: Barclay, J
EM: j.barclay@uea.ac.uk
AF: School of Environmental Science, University of East Anglia, Earlham Road, Norwich, NR4 7TJ United Kingdom
AU: Charlier, B L
EM: b.l.a.charlier@open.ac.uk
AF: Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AB: To extract meaningful information from phase equilibrium experiments the starting composition must be made of ``the right stuff'' and the results compared with the ``real phenocrysts''. Nisyros island is one of the most active volcanic centres on the South Aegean Volcanic Arc. Pleistocene to recent volcanism has evolved from submarine to sub-aerial in style, culminating in the large caldera-forming eruptions at ca. 33 ka. Post caldera activity has been confined to the extrusion of viscous domes, which fill almost half of the caldera, and hydrothermal eruptions. Studies on the sub-volcanic plumbing dynamics of Nisyros have shown that it is a complex and poorly constrained system, and the need to constrain these parameters is gaining impetus given that Nisyros is showing signs of unrest. The domes are dacitic (SiO2 68 wt. %). Phenocryst phases in decreasing order of abundance are plagioclase, clinopyroxene, orthopyroxene and Fe-Ti oxides. Amphibole is present only as a xenocryst phase, derived from the disaggregation of andesitic magmatic inclusions which are prominent in the dome rocks. There are no eruption dates for the post-caldera activity, but the domes are probably Holocene in age. Age differentiation between the domes is based solely on the extent of weathering and position relative to adjacent domes. Preliminary U/Th investigations of zircons from the ca. 33 ka caldera phase by ion probe have shown that these are inherited, with ages of 92, +35/-27 ka. Low zircon saturation temperatures (731-769 °C) for the post-caldera domes suggest similarly that any zircons present will have been inherited. Extensive geochemical evidence for magma mixing indicates the need for caution in choosing an appropriate starting composition for the phase equilibria experiments. Simple major element modelling suggests limited bulk mixing between silicic and andesitic end-members. There is also little evidence to suggest that the dacite has been hybridised by the andesitic inclusions, therefore a natural composition (SiO2 69 wt. %) is being used as a starting composition. This composition represents that of the most recent volcanic eruption on the island. A series of phase equilibria experiments are currently being conducted in Cold Seal Pressure Vessels over a temperature range of 750-900 °C, at varying pressures and water contents and a range of oxygen fugacities. Results from these will be presented in comparison with the natural assemblage in a search for the real storage conditions for the post-caldera dacites.
DE: 8412 Reactions and phase equilibria (1012, 3612)
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005