Magmatic Processes - Richard...
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Magmatic Processes • Some useful definitions: – Primary magma Primary magma: magma originating in (mantle) source directly from melting – Primitive magma Primitive magma: magma that underwent minimal differentiation – Parental magma Parental magma: least differentiated magma in a series leading to evolved rocks • Liquid line of descent Liquid line of descent: relates series of liquids derived from single parent magma Magmatic Processes From: Wilson (1989) Igneous Petrogenesis Stagnation Stagnation • wallrock wallrock- magma density contrast magma density contrast • pooling coalescence in chamber pooling coalescence in chamber • solidification into solidification into pluton pluton? Partial melting Magma Ascent Eruption ?
Transcript of 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
