Adiabatic
decompression
melting
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Adiabatic Decompression and Magma Generation: Tholeiitic vs. Alkaline Basalt, Slides of Geochemistry
A summary of the magma generation process, focusing on the differences between tholeiitic and alkaline basalt. It includes temperature-pressure data, crystal phase diagrams, and explanations of the crystallization processes for each basalt type.
Typology: Slides
2012/2013
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Adiabatic
decompression
melting
Temperature (°C)
Pressure (kb)
1280°C
1380°C
90km
mantle solidus 700ppm H 2
O
1480°C
1580°C
20% melting
1% melting
MORB
Plume Core
Melting "edge"
dry
Magma generation and composition summary
• Two main types of basalt
– Tholeiitic
– Alkaline
• Tholeiitic basalt forms at lower pressure and with higher melt
fractions
• Alkali basalt forms only at higher pressure and with small melt
fractions
• Andesite forms in island arcs at high pressure because of the
abundacne of H 2 O
(P.S. The following is qualitative, but at least it may be understandable)
Olivine Orthopyroxene Quartz
Clinopyroxene
P
Low Pressure, dry
Pressure < 1kbar
CPX
OLIVINE
OPX QUARTZ
Tholeiitic
Crystallization
(OL, OPX,
OPX+CPX,
CPX+Q)
Tholeiitic basalt will crystallize OPX and with extensive
fractional crystallization can eventually crystallize quartz
Diopside
Olivine Orthopyroxene Quartz
Clinopyroxene
P
Low Pressure, dry
Pressure < 1kbar
CPX
OLIVINE
OPX QUARTZ
Alkaline
Crystallization
(OL, OL+CPX)
Alkali basalt will not crystallize OPX or Quartz
Diopside
Olivine Orthopyroxene Quartz
Clinopyroxene
P
High Pressure, dry Pressure 20kbar
CPX
OL
OPX QUARTZ
Mantle
Peridotite
Mantle peridotite is OL-rich. This version of the diagram
shows approximate SiO 2 contents of rocks
Diopside