Magmatic Phases I

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Magmatic Phases - I
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Basalt Melting Relations Eruption Temps.
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Magmatic Phases - II
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Conceptual Models of Silicate Melts
From Carmichael et al. 1974
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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.

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Specific Volume of Pure Water
Geothermal Gradient
Pure H2O -gt 218 bars 371C
Pure CO2 -gt 73 bars 31C
Critical Point
At magmatic conditions no distinction between
liquid and gas phases. Refer to phase as volatile
fluids if density lt 2 g/cm3.
From Burnham et al., 1969
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Specific Volume of Water vs. Pressure
specific volume 1 / r
0.1 g/cm3
rmagma 2.2 g/cm3
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Magmatic Volatile Reservoirs
PH2O lt Pf
Px Py Pz
Isostatic Lithostatic pressure
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Water Solubility vs. Pressure
Cold-Seal bomb
Pressure medium
Platinum capsule contains melt dissolved
volatiles
To pump to increase pressure
From Moore et al., 1998
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Depolymerization of Silicate Melts
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Speciation of Water in Silicate Melts
H2O O2- 2OH- in melt in
melt
From Silver et al., 1990
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CO2 Solubility in Silicate Melts
aSiO2
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Pressure Effects on Volatile-rich Systems
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Vesiculation Stages
Bubble Nucleation
Froth Saturation
New Nucleii and Growth
Fragmentation
From Sparks (1978)
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Volatiles and Eruptions
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Explosive Eruptions