HR: 1340h
AN: V13B-0550    [Abstracts]
TI: Evolution of the Campanian Ignimbrite Magmatic System I: Constraints on Compositional Zonation and Eruption Probability Imposed By Phase Equilibria
AU: * Fowler, S
EM: fowler@umail.ucsb.edu
AF: Earth Science, UCSB, Santa Barbara, CA 93106
AU: Spera, F
EM: spera@geolucsb.edu
AF: Earth Science, UCSB, Santa Barbara, CA 93106
AU: Bohrson, W
EM: bohrson@geology.cwu.edu
AF: Geol. Sci., CWU, Ellensburg, WA 98926
AU: Belkin, H
EM: hbelkin@usgs.gov
AF: USGS, 956 National Center, Reston, VA 20192
AU: DeVivo, B
EM: bdevivo@unina.it
AF: Dipt Geo Vul, Univ Naples, Naples, 80134 Italy
AB: The eruption and deposition of the ~39.3 ka Campanian Ignimbrite (CI), a large volume (~200 km³ DRE) trachytic to phonolitic ignimbrite, is the dominant event in the history of the Campi Flegrei volcanic field near Naples, Italy. In an effort to comprehend its petrological evolution, we have conducted ~~110 MELTS (Ghiorso, 1997) phase equilibria simulations of the major element evolution of parental CI magma. The goals of this work are to approximate oxygen fugacity (f_O2), initial dissolved water content and pressure at which isobaric closed system fractional crystallization of parental melt most accurately captures the observed liquid line of descent and to study the implications of heat extraction from parental CI magma with respect to the origin of compositional zonation and the probability of explosive eruption. Although the CI magma body did not evolve as a perfectly closed system, this assumption allows quantitative insight into magma-host rock mass exchange using trace element and isotopic data (see companion contribution by Bohrson et al.). The parental melt composition was reconstructed using data for melt inclusions trapped within CI clinopyroxene phenocrysts reported by Webster et al. (2003), while allowing for reaction between parental melt and clinopyroxene host. The inferred parental melt is a basaltic trachyandesite. The search space for pressure, (f_O2) and initial dissolved H₂O was 0.1-0.5 in 0.05 GPa increments, QFM-1 to QFM+3 and 1, 2 and 3 wt. % H2O, respectively. The criteria used to judge the quality of a simulation include correspondence of the MELTS prediction with CI liquid and phenocryst compositions. Results indicate that a good first-order model involves evolution from a basaltic trachyandesite parent by isobaric (~0.15 GPa) crystal fractionation initially containing ~3 wt% dissolved H₂O along the QFM+1 buffer. H₂O first saturates at 1127°C at 0.15 GPa when the dissolved water content is ~4 wt %. A striking result is the discovery of a pseudo-invariant point at ~883°C (T_ip) and 0.15 GPa. The fraction of melt changes abruptly from ~0.5 to ~0.1 at T_ip due to the simultaneous crystallization of alkali feldspar, plagioclase, spinel, biotite and apatite. At T_ip, there is a dramatic decrease in the viscosity of melt (by a factor of four) and magma density (~5%) and an increase in the dissolved H₂O content of the melt (from 4.4-5.1 wt%) and in the volume fraction,θ, of supercritical fluid in the multiphase system. In particular, θ increases from ~0.05 at 885°C to ~0.6 at 882°C. The liquid composition also changes discontinuously at T_ip with Si, Na, and H₂O increasing and K and Al decreasing as temperature falls below T_ip. The marked variations in composition and properties of volatile-saturated melt and magma were the trigger that led to the catastrophic eruption and formation of the compositionally-zoned CI magma. Because phase equilibria modeling provides information on the enthalpy changes associated with fractional crystallization and because the dimensions of the CI magma chamber and heat extraction rate can be approximated, a time scale for CI magmatic evolution can be derived. The estimated crystallization duration (τ) is10-100 ka and 75% of τ is spent at or near T_ip.
DE: 1749 Volcanology, geochemistry, and petrology
DE: 3618 Magma chamber processes (1036)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005