Differentiation of Magmas Chemical variations - magmas Bulk Upper - - PowerPoint PPT Presentation

Differentiation of Magmas

Figure 16-6. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Chemical variations - magmas

Bulk Upper Crust

Distillation process

• Extract “Crust” from the mantle

Mantle Crust Magma differentiation

Magmatic Differentiation

• Any process by which a magma is able

to diversify and produce a magma or rock of different composition

• Requirements
• 1. Create a compositional difference between

2 or more phases

• 2. Preserves the chemical difference by

physically separating the chemically distinct portions

Magmatic distillation

• Most important process:

– Crystal liquid separation in magma reservoirs

• Others also play a role

– Degassing (gas escape) – Liquid immiscibility

Separating liquids from solids in a gravity field

• 1. At low crystallinity

ü High density crystals settle down

• 2. At high crystallinity:

ü Low density liquid moves up

Crystallization or melting?

• Down temperature = fractional

crystallization

Temperature

Magma A Magma B

• Up temperature = partial melting

Crystallization

Crust Mantle

Physics of phase separation?

• 1. Crystal sinking in a pool of largely liquid

magma

Stokes’ Law

V = the settling velocity (m/s) g = the acceleration due to gravity (9.81 m/s2) r = the radius of a spherical particle (m) ρs = the density of the solid spherical particle (kg/m3) ρl = the density of the liquid (kg/m3) µ = the viscosity of the liquid (Pa s)

V 2gr ( ) 9

2

=

• ρ

ρ µ

s l

Plagioclase in rhyolitic melt (m/years)?

– feldspars (ρl = 2600 kg/m3, r = 0.001 m) – Rhyolitic liquid (µ = 106 Pa s and ρl = 2300 kg/m3)

Olivine settling in basalt (m/years)?

– Olivine (ρs = 3300 kg/m3, r = 0.001 m) – Basaltic liquid (ρl = 2650 kg/m3, µ = 100 Pa s)

Homework

Is crystal settling realistic in magmas? Explain why or why not!

Separating liquids from solids in a gravity field

• 1. At low crystallinity
• Crystal settle down
• 2. At high crystallinity:
• Liquid moves up

Liquid extraction from “mush”

Partial melting of Earth’s crust

• As for the mantle, the Earth’s crust can

melt

• Solidus at much lower T (<700 °C) if

feldspar-quartz-water are present

– Why so low?

The Eutectic Nature of Salt and Water

http://ucalgary.ca/~kmuldrew/cryo_course/cryo_chap6_1.html

Phase diagram with eutectic Ab-Or as a f(PH2O)

Figure 6-17. The Albite-K-feldspar system at various H2O pressures. (a) and (b) after Bowen and Tuttle (1950), J. Geol, (c) after Morse (1970) J. Petrol.

Granitic eutectic

Figure 18-3. The Ab-Or-Qtz system with the ternary cotectic curves and eutectic minima from 0.1 to 3 GPa. Included is the locus of most granite compositions from Figure 11-2 (shaded) and the plotted positions of the norms from the analyses in Table 18-2. Note the effects of increasing pressure and the An, B, and F contents on the position of the thermal minima. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Main source of heat

• 1. Burial
• 2. Intrusion of mafic magma

Burial

Crustal thickening

Figure 18-6. A simple modification of Figure 16-17 showing the effect of subducting a slab of continental crust, which causes the dip of the subducted plate to shallow as subduction ceases and the isotherms begin to “relax” (return to a steady-state value). Thickened crust, whether created by underthrusting (as shown) or by folding or flow, leads to sialic crust at depths and temperatures sufficient to cause partial melting. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure 18-7. Schematic cross section of the Himalayas showing the dehydration and partial melting zones that produced the

• leucogranites. After France-Lanord and Le Fort (1988) Trans. Roy. Soc. Edinburgh, 79, 183-195. Winter (2001) An Introduction to

Igneous and Metamorphic Petrology. Prentice Hall.

Himalayas

Basalt intrusion

Partially molten rocks Migmatites

Extraction pf liquid at high crystal fraction?

McKenzie, 1984

(1 )( )

s l

g w k ! ! " " µ! = # #

1

(1 ) (1 )

h c c h c m

t h h h h t w h h e ! " ! " ! # #

$

= $ = = $ = $

Compaction

Collaboration between melting and crystal fractionation

• Due to latent heat release during

crystallization, the two processes may

• perate in conjunction
• Assimilation Fractional

Crystallization (AFC)

Liquid Immiscibility

• Liquids do not mix with each other.

– E.g., oil and water/vinegar

• Silicate-Carbonatite
• Sulfide liquids in silicate
• Fe-rich basaltic magmas can form two

separate liquids - one felsic and rich in SiO2, and the other mafic and rich in FeO.

Diversification of magma

• Magma mixing: recognized at

least since Bunsen (1851)

Basalt pillows accumulating at the bottom

• f a in granitic magma

chamber, Vinalhaven Island, Maine Comingled basalt-Rhyolite

• Mt. McLoughlin, Oregon

Figure 11-8 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall

Differentiation processes

Magma chambers

Magma chamber