Journal of Petrology Advance Access originally published online on December 13, 2006
Journal of Petrology 2007 48(3):459-493; doi:10.1093/petrology/egl068
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Phase Equilibria Constraints on the Chemical and Physical Evolution of the Campanian Ignimbrite
1Department of Earth Science and Institute for Crustal Studies, University of California, Santa Barbara, Santa Barbara, CA 93106, USA
2Department of Geological Sciences, Central Washington University, Ellensburg, WA 98926, USA
3956 National Center, US Geological Survey, Reston, VA 20192, USA
4Dipartimento di Scienze Della Terra, Università Di Napoli Federico II, 80134 Napoli, Italy
RECEIVED FEBRUARY 22, 2006; ACCEPTED OCTOBER 27, 2006
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Abstract |
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The Campanian Ignimbrite is a > 200 km3 trachyte–phonolite pyroclastic deposit that erupted at 39·3 ± 0·1 ka within the Campi Flegrei west of Naples, Italy. Here we test the hypothesis that Campanian Ignimbrite magma was derived by isobaric crystal fractionation of a parental basaltic trachyandesitic melt that reacted and came into local equilibrium with small amounts (5–10 wt%) of crustal rock (skarns and foid-syenites) during crystallization. Comparison of observed crystal and magma compositions with results of phase equilibria assimilation–fractionation simulations (MELTS) is generally very good. Oxygen fugacity was approximately buffered along QFM + 1 (where QFM is the quartz–fayalite–magnetite buffer) during isobaric fractionation at 0·15 GPa (
6 km depth). The parental melt, reconstructed from melt inclusion and host clinopyroxene compositions, is found to be basaltic trachyandesite liquid (51·1 wt% SiO2, 9·3 wt% MgO, 3 wt% H2O). A significant feature of phase equilibria simulations is the existence of a pseudo-invariant temperature, 
883 °C, at which the fraction of melt remaining in the system decreases abruptly from 0·5 to < 0·1. Crystallization at the pseudo-invariant point leads to abrupt changes in the composition, properties (density, dissolved water content), and physical state (viscosity, volume fraction fluid) of melt and magma. A dramatic decrease in melt viscosity (from 1700 Pa s to 200 Pa s), coupled with a change in the volume fraction of water in magma (from 0·1 to 0·8) and a dramatic decrease in melt and magma density acted as a destabilizing eruption trigger. Thermal models suggest a timescale of 200 kyr from the beginning of fractionation until eruption, leading to an apparent rate of evolved magma generation of about 10–3 km3/year. In situ crystallization and crystal settling in density-stratified regions, as well as in convectively mixed, less evolved subjacent magma, operate rapidly enough to match this apparent volumetric rate of evolved magma production.
KEY WORDS: assimilation; Campanian Ignimbrite; fractional crystallization; magma dynamics; phase equilibria
*Corresponding author.Telphone: (805) 893-4880. E-mail: fowler{at}umail.ucsb.edu
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