magmatic processes - richard...

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Magmatic Processes

• Some useful definitions:–– Primary magmaPrimary magma: magma originating in (mantle) source

directly from melting

–– Primitive magmaPrimitive magma: magma that underwent minimal differentiation

–– Parental magmaParental magma: least differentiated magma in a series leading to evolved rocks

•• Liquid line of descentLiquid line of descent: relates series of liquids derived from single parent magma

Magmatic Processes

From: Wilson (1989) Igneous Petrogenesis

StagnationStagnation•• wallrockwallrock--magma density contrastmagma density contrast•• pooling coalescence in chamberpooling coalescence in chamber•• solidification into solidification into plutonpluton??

Partial melting

Magma Ascent

Eruption ?

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Magma DifferentiationMagma Differentiationhow do we get from here to there?how do we get from here to there?

•• Fractional CrystallizationFractional Crystallization•• Assimilation ± Fractional CrystallizationAssimilation ± Fractional Crystallization

–– crystallization releases latent heat needed to melt rocks crystallization releases latent heat needed to melt rocks surrounding the magmasurrounding the magma

•• Magma MixingMagma Mixing

•• InIn--situsitu Crystallization, Convective CrystallizationCrystallization, Convective Crystallization•• Flowage differentiationFlowage differentiation

•• Filter Pressing (Compaction)Filter Pressing (Compaction)

•• Fractional CrystallizationFractional Crystallization– Separation of crystals from liquid – Gravitative settling or flotation play a significant role

Magma DifferentiationMagma Differentiation

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Magmatic Processes

From: Wilson (1989) Igneous Petrogenesis

Fractional crystallization of basaltFractional crystallization of basalt•• liquid line of descent: illustrated major element variationliquid line of descent: illustrated major element variation

From: Wilson (1989) Igneous Petrogenesis

Fractional crystallization Fractional crystallization

Quantifying the evolution of derivative magmaQuantifying the evolution of derivative magma

and wt.% crystallized in a and wt.% crystallized in a HarkerHarker DiagramDiagram

Use the lever rule:Use the lever rule:

100DM-EdistanceDM-PM distance removed E% x=

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• Trace element modeling of magma series• Using Distribution Coefficients (D):

–– Equilibrium crystallizationEquilibrium crystallization

where:

F is Fraction of liquid remaining

–– RaylieghRayliegh Fractional CrystallizationFractional Crystallization

Quantifying Quantifying MagmaticMagmatic ProcessesProcesses

CC F D FD

L

Lo =

+ −1

D X D= ∑ α αα

CC

FL

Lo

D= −( )1

Magmatic Processes

From: Wilson (1989) Igneous Petrogenesis

Fractional crystallization of basaltFractional crystallization of basalt•• liquid line of descent: illustration of trace element variationliquid line of descent: illustration of trace element variation

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•• Assimilation Assimilation "" Fractional CrystallizationFractional Crystallization– Reaction/dissolution of wallrock– Crystallization provides heat for reaction– Can strongly modify trace element concentrations in magma

– If isotopic contrastIs large between wallrock and magma,isotopic ratios of the magma change

? Where might this beMost significant ?

Magma DifferentiationMagma Differentiation

• Quantifying crustal contamination (AFC) – Amount of assimilation limited by thermal energy of

magma— latent heat of xtlln —maximum is 10-20% – For crustal magma chamber undergoing fractional

crystallization, trace element behavior is described by:

where: CoL is conc. of TE in original magma, C* is conc. of TE in

contaminant, r is ratio of rate of assimilation to rate of fractional xtlln, D is the bulk distribution coefficient for the fractionating assemblage, and f is F –[(r-1+D)/(r-1)] and F is fraction of magma remaining.

– For any radiogenic isotope, ratio in the magma will be:

where εoL is isotope ratio in original magma, ε* is ratio of contaminant

)1(1 * fC

DrrfCC o

LL −•+−

+=

)1)((0

* fCC

LL

oLL

L−−+= εεεε

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From: Wilson (1989) Igneous Petrogenesis

AssimilationAssimilation--Fractional crystallization of basaltFractional crystallization of basalt•• liquid line of descent: illustration of trace element variationliquid line of descent: illustration of trace element variation

incompatible vs. compincompatible vs. compatible elementsatible elements

• Trace element modeling of magma series– Assimilation-Fractional Crystallization of basalt in the crust

Quantifying Quantifying MagmaticMagmatic ProcessesProcesses

From: Wilson (1989) Igneous Petrogenesis

MORBMORB

MORB + 15 wt %MORB + 15 wt %crustalcrustal rockrock

SpiderdiagramRock / Chondrite

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•• Magma MixingMagma Mixing–– linear arrays in linear arrays in

HarkerHarker diagrams diagrams of major or trace of major or trace elements elements

Magma DifferentiationMagma Differentiation

•• InIn--situsitu Crystallization, Convective CrystallizationCrystallization, Convective Crystallization– Crystal growth along walls of chamber, esp. roof, sidewalls– Probably chief mechanism of differentiation of basalt

Sidewall coolingxtlln

Wallrocks ConvectingBasalticmagma

Magma DifferentiationMagma Differentiation