Phase equilibria constraints on the chemical and physical evolution of basalt to rhyolite magmas

Phase equilibria constraints on the chemical and physical evolution of the Campanian Ignimbrite: 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 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. % SiO 2, 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 (1700 Pa s to ∼200 Pa s), coupled with a change in the volume fraction of water in magma (∼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/yr. 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 trigger for explosive volcanic eruptions: A mechanism for explosive volcanic eruptions based on multicomponent phase equilibria modelling of four explosive volcanic systems is proposed. In each system, either isochoric or isobaric crystallization, where either crystals or crystals and exsolved fluid are chemically fractionated from melt, leads inevitably to near-solidus dynamical instability culminating in violent explosive eruption. Driving this instability is a dramatic increase in the volume fraction of fluid bubbles in the magma exceeding the limit for magma fragmentation. This phenomenon is independent of magma decompression. Country rock may be weak, allowing magma to remain in lithostatic equilibrium with its host rock, or strong, leading to divergence of magma and lithostatic pressure. Bubbles may be retained or expelled during crystallization. Instability is the inevitable outcome of crystallization at shallow levels in the crust for all four systems, regardless of the mechanical state of host country rock. We speculate that this phase equilibria mechanism driving explosive eruptions has general significance.;Conceptual model for small-volume alkali basalt petrogenesis: implications for volcanic hazards at the proposed Yucca Mountain nuclear waste repository: Assessment of volcanic hazards for nuclear facilities requires a robust understanding of the processes resulting in magmatism, essentially the petrotectonic environment. In this chapter we focus on continental extensional regimes such as Basin Range of the USA. A model is developed for magmatism from source-to-surface that is consistent with tectonic setting, thermo- and magma-dynamics. For example, in this chapter we explore the thermodynamics of partial melting of peridotite to study conditions of magma generation (petrogenesis) as well as dynamics of magma ascent. Transport history adds additional complexities that can be addressed from the perspective of magma physics. A major point of the chapter is that a comprehensive picture of the processes that give rise to magmatism provides a necessary context for models of rates and volumes, and types of volcanic activity that are used in hazard and risk assessments.