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 1 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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