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Magmatic Phases - I
Basalt Melting Relations & Eruption Temps.
Magmatic Phases - II
Conceptual Models of Silicate Melts
From Carmichael et al. 1974
Common Magmatic Volatile Species• Volatiles are defined as those chemical species that at near
atmospheric P and high T appropriate for magmas, exist in a gas or vapor phase.
• Common chemical species include: H2O (steam), CO2, H2, HCl, HF, F, Cl, SO2, H2S, CO, CH4, O2, NH3, S2, and noble gases He and Ar. H2O and CO2 dominate!
• Most volatile species consist of only six low-atomic weight elements: H, C, O, S, Cl, and F. Small but measurable amounts of these elements can be dissolved in both the coexisting melt and crystalline phases.
• Oxygen is the major ion in all three phases in magmatic systems: solid, liquid, and volatile.
Specific Volume of Pure Water
From Burnham et al., 1969
Critical Point
GeothermalGradient
Pure H2O ->218 bars; 371°C
Pure CO2 ->73 bars; 31°C
At magmatic conditions no distinction between liquid and gas phases.Refer to phase as volatile fluids if density < 2 g/cm3.
Specific Volume of Water vs. Pressurespecific volume = 1 /
0.1 g/cm3
magma = 2.2 g/cm3
Magmatic Volatile Reservoirs
Isostatic = Lithostatic pressure
Px = Py = Pz
PH2O < Pf
Water Solubility vs. Pressure
Platinum capsule:contains melt +dissolved volatiles
“Cold-Seal” bomb
Pressure medium
To pump to increasepressure
From Moore et al., 1998
Depolymerization of Silicate Melts
Speciation of Water in Silicate Melts
From Silver et al., 1990
H2O + O2- = 2OH-
in melt in melt
CO2 Solubility in Silicate MeltsaSiO2
Pressure Effects on Volatile-rich Systems
Vesiculation Stages
Bubble Nucleation
New Nucleii and Growth
Froth Saturation
FragmentationFrom: Sparks (1978)
Volatiles and Eruptions
Explosive Eruptions