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