The Campanian Ignimbrite is a > 200 km(3) 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 (approximate to 6 km depth). The parental melt, reconstructed from melt inclusion and host clinopyroxene compositions, is found to be basaltic trachyandesite liquid (51.1 wt% SiO(2), 9.3 wt% MgO, 3 wt% H(2)O). A significant feature of phase equilibria simulations is the existence of a pseudo-invariant temperature, similar to 883 degrees C, at which the fraction of melt remaining in the system decreases abruptly from similar to 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 similar to 200 Pa s), coupled with a change in the volume fraction of water in magma (from similar to 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 similar to 200 kyr from the beginning of fractionation until eruption, leading to an apparent rate of evolved magma generation of about 10(-3) km(3)/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.

Phase equilibria constraints on the chemical and physical evolution of the Campania Ignimbrite / Fowler, S. J.; Spera, F. J.; Bohrson, W. A.; Belkin, H. E.; DE VIVO, Benedetto. - In: JOURNAL OF PETROLOGY. - ISSN 0022-3530. - STAMPA. - 48 (3):(2007), pp. 459-493. [10.1093/petrology/egl068]

Phase equilibria constraints on the chemical and physical evolution of the Campania Ignimbrite

DE VIVO, BENEDETTO
2007

Abstract

The Campanian Ignimbrite is a > 200 km(3) 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 (approximate to 6 km depth). The parental melt, reconstructed from melt inclusion and host clinopyroxene compositions, is found to be basaltic trachyandesite liquid (51.1 wt% SiO(2), 9.3 wt% MgO, 3 wt% H(2)O). A significant feature of phase equilibria simulations is the existence of a pseudo-invariant temperature, similar to 883 degrees C, at which the fraction of melt remaining in the system decreases abruptly from similar to 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 similar to 200 Pa s), coupled with a change in the volume fraction of water in magma (from similar to 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 similar to 200 kyr from the beginning of fractionation until eruption, leading to an apparent rate of evolved magma generation of about 10(-3) km(3)/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.
2007
Phase equilibria constraints on the chemical and physical evolution of the Campania Ignimbrite / Fowler, S. J.; Spera, F. J.; Bohrson, W. A.; Belkin, H. E.; DE VIVO, Benedetto. - In: JOURNAL OF PETROLOGY. - ISSN 0022-3530. - STAMPA. - 48 (3):(2007), pp. 459-493. [10.1093/petrology/egl068]
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11588/104148
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