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 Systems1 - C SystemsThe system SiOThe system SiO22

Stishovite

Coesite

- quartz

- quartz

Liquid

TridymiteCristobalite

600 1000 1400 1800 2200 2600

2

4

6

8

10P

ress

ure

(GP

a)

Temperature oC

After Swamy and Saxena (1994), J. Geophys. Res., 99, 11,787-11,794. AGU

2-C Eutectic Systems2-C Eutectic Systems Example: Diopside - AnorthiteExample: Diopside - Anorthite

No solid solutionNo solid solution

1274

Di 20 40 60 80 An

1200

1300

1400

1500

1600

T oC

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.

a b

OrthocumulateAccumulated minerals in liquid

Amphibole (± Biotite) cumulatein a granite.

Augite forms before plagioclaseAugite forms before plagioclase

This forms on the This forms on the leftleft side of the eutectic side of the eutectic

Gabbro of Gabbro of the the Stillwater Stillwater Complex, Complex, MontanaMontana

Plagioclase forms before augitePlagioclase forms before augite

This forms on the This forms on the rightright side of the eutectic side of the eutectic

OphiticOphitic texture texture

Diabase dikeDiabase 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:Cumulate texture:Mutually touching Mutually touching phenocrysts with phenocrysts with interstitial crystallized interstitial crystallized residual meltresidual melt

Minerals that form during crystallizationMinerals that form during crystallization1250

1200

1150

1100

1050

1000

9500 0 0 010 10 20 10 102030 40 3050 40 50

Liquidus

Melt

Crust

Solidus

Olivine Clinopyroxene Plagioclase OpaqueT

emp

erat

ure

oC

Makaopuhi Lava LakeMakaopuhi Lava Lake

From Wright and Okamura, (1977) USGS Prof. Paper, 1004.

olivine Calcic plagioclase

Mg pyroxene

Mg-Ca pyroxene

amphibole

biotite(S

pin

el)

Te

mp

era

ture

potash feldspar muscovite quartz

alkalic plagioclase

Calci-alkalic plagioclase

alkali-calcic plagioclase

Bowen’s Reaction SeriesBowen’s Reaction Series

DiscontinuousDiscontinuousSeriesSeries

ContinuousContinuousSeriesSeries

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)

s = the density of the solid spherical particle (g/cm3)

l = the density of the liquid (g/cm3)

= the viscosity of the liquid (1 c/cm sec = 1 poise)

V2gr ( )

9

2

s l

Olivine in basalt

– Olivine (s = 3.3 g/cm3, r = 0.1 cm)

– Basaltic liquid (l = 2.65 g/cm3, = 1000 poise)

– V = 2·980·0.12 (3.3-2.65)/9·1000 = 0.0013 cm/sec

Rhyolitic melt = 107 poise and l = 2.3 g/cm3

– hornblende crystal (s = 3.2 g/cm3, r = 0.1 cm)

• V = 2 x 10-7 cm/sec, or 6 cm/year

– feldspars (l = 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

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

Ne Ab Q

1070 1060

1713

Ab + Tr

Tr + L

Ab + LNe + L

Liquid

Ab + L

Ne + Ab

ThermalDivide

Diopside-Albite-Anorthite

Di - An EutecticDi - An EutecticDi - Ab EutecticDi - Ab EutecticAb - An solid solutionAb - An solid solution

Figure 7-5. Figure 7-5. Isobaric Isobaric diagram illustrating diagram illustrating the liquidus the liquidus temperatures in the temperatures in the system diopside-system diopside-anorthite-albite at anorthite-albite at atmospheric pressure atmospheric pressure (0.1 MPa). After (0.1 MPa). After Morse (1994)Morse (1994), Basalts , Basalts and Phase Diagrams. and Phase Diagrams. Krieger PublushersKrieger 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

olivine Calcic plagioclase

Mg pyroxene

Mg-Ca pyroxene

amphibole

biotite(S

pin

el)

Te

mp

era

ture

potash feldspar muscovite quartz

alkalic plagioclase

Calci-alkalic plagioclase

alkali-calcic plagioclase

Bowen’s Reaction SeriesBowen’s Reaction Series

DiscontinuousDiscontinuousSeriesSeries

ContinuousContinuousSeriesSeries