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Evolution of magmas

Evolution of magmas. 1- Fractional crystallization: minerals formed. 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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Evolution of magmas

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  1. Evolution of magmas 1- Fractional crystallization: minerals formed.

  2. 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

  3. What processes allow magmas to differenciate? • Fractionnal crystallization • Liquid immiscibility • Magma mixing • Country-rock assimilation

  4. 1 - C Systems The system SiO2 After Swamy and Saxena (1994), J. Geophys. Res., 99, 11,787-11,794. AGU

  5. 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.

  6. Amphibole (± Biotite) cumulate in a granite.

  7. Augite forms before plagioclase Gabbro of the Stillwater Complex, Montana This forms on the left side of the eutectic

  8. Plagioclase forms before augite Ophitic texture Diabase dike This forms on the right side of the eutectic

  9. Gravity settling • Cool point a olivine layer at base of pluton if first olivine sinks • Next get ol+cpx layer • finally get ol+cpx+plag • Cumulate texture: • Mutually touching phenocrysts with interstitial crystallized residual melt

  10. Olivine Clinopyroxene Plagioclase Opaque 1250 Liquidus 1200 1150 Temperature oC 1100 Melt Crust 1050 1000 Solidus 950 0 10 0 10 20 30 40 50 0 10 20 30 40 50 10 0 Makaopuhi Lava Lake Minerals that form during crystallization From Wright and Okamura, (1977) USGS Prof. Paper, 1004.

  11. Bowen’s 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

  12. r - r 2 2gr ( ) = s l V h 9 Stoke’s 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)

  13. Olivine in basalt • Olivine (rs = 3.3 g/cm3, r = 0.1 cm) • Basaltic liquid (rl = 2.65 g/cm3, h = 1000 poise) • V = 2·980·0.12 (3.3-2.65)/9·1000 = 0.0013 cm/sec

  14. 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

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

  16. Two other mechanisms that facilitate the separation of crystals and liquid 2. Flow segregation

  17. 1713 Liquid Thermal Divide Tr + L Ne + L Ab + L Ab + L 1070 1060 Ne + Ab Ab + Tr Q Ne Ab

  18. 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

  19. 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. Isobaric polythermal projection

  20. > 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

  21. Bowen’s 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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