Presentation on theme: "Evolution of magmas 1- Fractional crystallization: minerals formed."— Presentation transcript:
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
What processes allow magmas to differenciate? Fractionnal crystallization Liquid immiscibility Magma mixing Country-rock assimilation
1 - C Systems The system SiO 2 After Swamy and Saxena (1994), J. Geophys. Res., 99, 11,787-11,794. AGU
2-C Eutectic Systems Example: Diopside - Anorthite No solid solution 1274 Di 2040 60 80 An 1200 1300 1400 1500 1600 T o C Anorthite + Liquid Liquid Liquidus Diopside + Liquid Diopside + Anorthite 1553 1392 Wt.% Anorthite Isobaric T-X phase diagram at atmospheric pressure (After Bowen (1915), Amer. J. Sci. 40, 161-185.
Augite forms before plagioclase This forms on the left side of the eutectic Gabbro of the Stillwater Complex, Montana
Plagioclase forms before augite This forms on the right side of the eutectic Ophitic texture Diabase dike
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
Minerals that form during crystallization Makaopuhi Lava Lake From Wright and Okamura, (1977) USGS Prof. Paper, 1004.
Stoke’s Law V= the settling velocity (cm/sec) g= the acceleration due to gravity (980 cm/sec 2 ) r = the radius of a spherical particle (cm) s = the density of the solid spherical particle (g/cm 3 ) l = the density of the liquid (g/cm 3 ) = the viscosity of the liquid (1 c/cm sec = 1 poise) V 2gr() 9 2 sl
Olivine in basalt –Olivine ( s = 3.3 g/cm 3, r = 0.1 cm) –Basaltic liquid ( l = 2.65 g/cm 3, = 1000 poise) –V = 2·980·0.1 2 (3.3-2.65)/9·1000 = 0.0013 cm/sec
Rhyolitic melt – = 10 7 poise and l = 2.3 g/cm 3 – hornblende crystal ( s = 3.2 g/cm 3, r = 0.1 cm) V = 2 x 10 -7 cm/sec, or 6 cm/year – feldspars ( l = 2.7 g/cm 3 ) V = 2 cm/year = 200 m in the 10 4 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 10 4 year cooling of stock
Two other mechanisms that facilitate the separation of crystals and liquid 1. Compaction
Two other mechanisms that facilitate the separation of crystals and liquid 2. Flow segregation
Diopside-Albite-Anorthite Di - An Eutectic Di - Ab Eutectic Ab - An solid solution 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
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.
> 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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