Evolution of magmas

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Evolution of magmas

• 1- Fractional crystallization minerals formed.

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Magmatic Differentiation

• Two essential processes
• 1. Creates a compositional difference in one or
  more phases
• 2. Preserves the chemical difference by
  segregating (or fractionating) the chemically
  distinct portions

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What processes allow magmas to differenciate?

• Fractionnal crystallization
• Liquid immiscibility
• Magma mixing
• Country-rock assimilation

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1 - C Systems
The system SiO2
After Swamy and Saxena (1994), J. Geophys. Res.,
99, 11,787-11,794. AGU
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2-C Eutectic Systems

• Example Diopside - Anorthite
• No solid solution

1600
1553
Liquid
Liquidus
1500
T
C
o
1400
Anorthite Liquid
1392
Diopside Liquid
1300
1274
1200
Diopside Anorthite
Di
20
40
60
80
An
Wt. Anorthite
Isobaric T-X phase diagram at atmospheric
pressure (After Bowen (1915), Amer. J. Sci. 40,
161-185.
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Amphibole ( Biotite) cumulate in a granite.
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Augite forms before plagioclase
Gabbro of the Stillwater Complex, Montana
This forms on the left side of the eutectic
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Plagioclase forms before augite
Ophitic texture
Diabase dike
This forms on the right side of the eutectic
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• Gravity settling
• Cool point a ? olivine layer at base of pluton if
  first olivine sinks
• Next get olcpx layer
• finally get olcpxplag

Cumulate texture Mutually touching phenocrysts
with interstitial crystallized residual melt
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Makaopuhi Lava Lake

• Minerals that form during crystallization

From Wright and Okamura, (1977) USGS Prof. Paper,
1004.
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Bowens Reaction Series
olivine
Calcic plagioclase
(Spinel)
Mg pyroxene
Calci-alkalic plagioclase
Continuous Series
Mg-Ca pyroxene
alkali-calcic plagioclase
Discontinuous Series
amphibole
alkalic plagioclase
biotite
Temperature
potash feldspar
muscovite
quartz
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Stokes Law

• V the settling velocity (cm/sec)
• g the acceleration due to gravity (980
  cm/sec2)
• r the radius of a spherical particle (cm)
• rs the density of the solid spherical particle
  (g/cm3)
• rl the density of the liquid (g/cm3)
• h the viscosity of the liquid (1 c/cm sec 1
  poise)

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• Olivine in basalt
• Olivine (rs 3.3 g/cm3, r 0.1 cm)
• Basaltic liquid (rl 2.65 g/cm3, h 1000 poise)
• V 29800.12 (3.3-2.65)/91000 0.0013 cm/sec

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• Rhyolitic melt
• h 107 poise and rl 2.3 g/cm3
• hornblende crystal (rs 3.2 g/cm3, r 0.1 cm)
• V 2 x 10-7 cm/sec, or 6 cm/year
• feldspars (rl 2.7 g/cm3)
• V 2 cm/year
• 200 m in the 104 years that a stock might cool
• If 0.5 cm in radius (1 cm diameter) settle at
  0.65 meters/year, or 6.5 km in 104 year cooling
  of stock

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• Two other mechanisms that facilitate the
  separation of crystals and liquid
• 1. Compaction

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Two other mechanisms that facilitate the
separation of crystals and liquid 2. Flow
segregation
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Diopside-Albite-Anorthite
Figure 7-5. Isobaric diagram illustrating the
liquidus temperatures in the system
diopside-anorthite-albite at atmospheric pressure
(0.1 MPa). After Morse (1994), Basalts and Phase
Diagrams. Krieger Publushers
Di - An Eutectic Di - Ab Eutectic Ab - An solid
solution
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Isobaric polythermal projection
Figure 7-5. Isobaric diagram illustrating the
liquidus temperatures in the system
diopside-anorthite-albite at atmospheric pressure
(0.1 MPa). After Morse (1994), Basalts and Phase
Diagrams. Krieger Publishers.
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gt 4 Components
Figure 7-13. Pressure-temperature phase diagram
for the melting of a Snake River (Idaho, USA)
tholeiitic basalt under anhydrous conditions.
After Thompson (1972). Carnegie Inst. Wash Yb. 71
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Bowens Reaction Series
olivine
Calcic plagioclase
(Spinel)
Mg pyroxene
Calci-alkalic plagioclase
Continuous Series
Mg-Ca pyroxene
alkali-calcic plagioclase
Discontinuous Series
amphibole
alkalic plagioclase
biotite
Temperature
potash feldspar
muscovite
quartz