Download 1 Magmatic Processes Magmatic Processes

Magmatic Processes
• Some useful definitions:
– Primary magma:
magma magma originating in (mantle) source
directly from melting
– Primitive magma:
magma magma that underwent minimal
differentiation
– Parental magma:
magma least differentiated magma in a series
leading to evolved rocks
• Liquid line of descent:
descent relates series of liquids derived
from single parent magma
Magmatic Processes
Eruption ?
Stagnation
• wallrockwallrock-magma density contrast
• pooling coalescence in chamber
• solidification into pluton?
pluton?
Magma Ascent
Partial melting
From: Wilson (1989) Igneous Petrogenesis
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Magma Differentiation
how do we get from here to there?
• Fractional Crystallization
• Assimilation ± Fractional Crystallization
– crystallization releases latent heat needed to melt rocks
surrounding the magma
• Magma Mixing
• InIn-situ Crystallization, Convective Crystallization
• Flowage differentiation
• Filter Pressing (Compaction)
Magma Differentiation
• Fractional Crystallization
– Separation of crystals from liquid
– Gravitative settling or flotation play a significant role
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Magmatic Processes
Fractional crystallization of basalt
• liquid line of descent: illustrated major element variation
From: Wilson (1989) Igneous Petrogenesis
Fractional crystallization
Quantifying the evolution of derivative magma
and wt.% crystallized in a Harker Diagram
Use the lever rule:
%E removed =
distance PM - DM
x100
distance E - DM
From: Wilson (1989) Igneous Petrogenesis
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Quantifying Magmatic Processes
• Trace element modeling of magma series
• Using Distribution Coefficients (D):
– Equilibrium crystallization
CL
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o =
CL F + D − FD
where:
D=
∑α X α Dα
F is Fraction of liquid remaining
– Rayliegh Fractional Crystallization
CL
= F ( D −1)
CLo
Magmatic Processes
Fractional crystallization of basalt
• liquid line of descent: illustration of trace element variation
From: Wilson (1989) Igneous Petrogenesis
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Magma Differentiation
• Assimilation " Fractional Crystallization
– Reaction/dissolution of wallrock
– Crystallization provides heat for reaction
– Can strongly modify trace element concentrations in magma
– If isotopic contrast
Is large between
wallrock and magma,
isotopic ratios of the
magma change
? Where might this be
Most significant ?
• 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:
C L = C Lo f +
r
• C* (1 − f )
r −1+ D
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:
ε L = ε + (ε * − ε L )(1 −
o
L
C 0L
CL
f)
where εoL is isotope ratio in original magma, ε* is ratio of contaminant
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AssimilationAssimilation-Fractional crystallization of basalt
• liquid line of descent: illustration of trace element variation
incompatible
vs.
compatible
compatible
elements
From: Wilson (1989) Igneous Petrogenesis
Quantifying Magmatic Processes
• Trace element modeling of magma series
– Assimilation-Fractional Crystallization of basalt in the crust
MORB + 15 wt %
crustal rock
Spiderdiagram
Rock / Chondrite
MORB
From: Wilson (1989) Igneous Petrogenesis
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Magma Differentiation
• Magma Mixing
– linear arrays in
Harker diagrams
of major or trace
elements
Magma Differentiation
• InIn-situ Crystallization, Convective Crystallization
– Crystal growth along walls of chamber, esp. roof, sidewalls
– Probably chief mechanism of differentiation of basalt
Wallrocks
Convecting
Basaltic
magma
Sidewall
cooling
xtlln
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