Igneous Geochemistry. What is magma? What is polymerization? Average compositions (% by weight) and liquidus temperatures of different magmas

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Igneous Geochemistry

• What is magma – phases, compositions, properties

• Major igneous processes

• Making magma – how and where

• Major-element variations

• Classification using a whole-rock analysis

• Fractional crystallization

• Magma mixing

• Trace-element variations

• Compatible vs. incompatible trace elements

• Partition coefficients

• Partial melting

• Crystallization

• Relationship between rock texture and the kinetics of nucleation & growth

What is magma?

- partly or completely molten rock

typically silicate-based (except for carbonities)

- variable properties include composition, temperature, density, viscosity

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- contains:

- silicate liquid (variable composition – felsic, intermediate, mafic) - volatiles (mainly H₂O, CO₂in variable amounts)

- crystals (variable amount, absent if temperature above liquidus) - gas bubbles (presence depends mostly on pressure and gas

content)

Average compositions (% by weight)

and liquidus temperatures of different magmas

Composition: Ultramafic Mafic Intermediate Felsic Volc. rock name: Komatiite (rare) Basalt Andesite Rhyolite

SiO2 47.9 50.9 58.9 73.7

TiO2 0.4 1.7 0.9 0.2

Al2O3 4.1 14.6 17.1 14.3

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2O3 . .6 7. .3

FeO 9.7 14.6 6.2 2.1

MgO 27.5 4.8 3.8 0.3

CaO 7.5 8.7 5.3 1.4

Na2O 0.2 3.1 4.3 5.2

K2O 0.02 0.8 1.2 4.1

Temp (°C) ~1600 ~1200 ~1000 ~900 Eruption temperatures generally are a little lower than liquidus temperatures.

Variation in viscosity with temperature and magma composition

At higher temperatures, magmas have lower viscosities (i.e. they can flow

more easily) because higher temperatures reduce the degree of polymerization

Higher viscosity, 4

thicker/stiffer

degree of polymerization (linkage of SiO4tetrahedra) Felsic magmas also are more

viscous than mafic magmas because they contain more silica and therefore are more

polymerized. Composition plays a greater role than temperature in controlling

viscosity. Lower viscosity,

flows more easily Fig 5.4 from Francis, 1993

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What is polymerization?

Single tetrahedron

Three

In magmas, silicon-oxygen tetrahedra link together by sharing oxygen atoms to form 3-dimensional networks.

The more the tetrahedra link together, the more “polymerized” is a magma and the greater its viscosity or resistance to flow.

Fig 2.16 Understanding Earth

Three polymerized

tetrahedra

Variation in viscosity with water content Adding water reduces the degree of polymerization and thereby reduces the viscosity.

The illustration shows the relationship for a felsic melt.

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Fig 5.5 from Francis, 1993

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Major processes that control the composition of igneous rocks

Rollinson's Using Geochem Data

Making magma

How to make rocks melt

- Increase temperature

- generally, temperature increases with depth within the earth - intrusion of hot, mafic magma can heat the surrounding rocks

and produce melting

least important way of making magmas

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- least important way of making magmas

- Decrease pressure

- bring rocks closer to the surface - extension of the lithosphere

- Change composition by adding fluids, especially water

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Effect of water content on melting a rock of granitic composition

Note that water content varies along the curved line – i.e. the water content increases with pressure along the curve. So, this is not a simple P-T melting diagram along the curved line.

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Brownlow’s Geochemistry

For constant H2O (the straight lines), the melting temperature is be greater at higher pressure as expected.

By adding water, the rock can be made to melt at a lower temperature. Or, the rock can be made to melt to a greater extent at the same temperature.

De

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Deeper

Brownlow's Geochemistry

Igneous rocks and magmas are not pure substances, but rather are complex mixtures of various components.

They do not change from liquid to solid or from solid to liquid all at one temperature. When a rock melts, minerals which have the lowest melting temperature melt first.

Phase diagram for a simple two-component system

“partial melt”

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Melting of “dry” basalt

Dry means that no volatiles like water or carbon dioxide are present.

If water is added, the boundaries change, and melting

all solid

Pure substance

Mixed substance:

(Basalt)

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g , g

happens at lower temperatures than illustrated.

Felsic rocks also melt at lower temperatures than mafic rocks like basalt.

all liquid

Fig 5.1 from Francis, 1993

Where and how are magmas formed?

Mid-ocean ridges – decompression melting mafic (basaltic) magma(primary mantle melt)

Continental rifts and other areas of extension – decompression melting mostly mafic (basaltic) magma(primary mantle melt)

some intermediate and felsic magmas(crustal melts, derivative magmas) 14

Subduction zones – melting largely due to addition of fluids mafic, intermediate, and felsic magmas - often water-rich

Mantle plumes (hot spots) – decompression melting ocean basins: mafic magma (primary mantle melt)

continents: mostly mafic magma, (primary mantle melt)

some intermediate and felsic magmas (crustal melts, derivative magmas)

Locations of magma formation ¹⁵

Winter’s Prin. Ig. Met. Petrol.

Magma formation at subduction zones

The addition of water to the mantle overlying the subducting plate causes partial melting. When enough melting has occurred, the liquid separates from the residual solid and rises buoyantly because it is less dense than the solid.

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General Controls on Magma Compositions

During Melting

The composition of the resulting melt depends upon:

(a) temperature

(b) pressure: greater pressure produces more alkaline melts with lower silica content

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lower silica content

(c) volatile content (e.g. amount of water present)

(d) mineral composition of the source rock (e.g. peridotite in the mantle melts to produce mafic magma like basalt; melting of mafic rock in the crust can produce a felsic melt)

(e) amount or degree of melting: with more melting, the magma composition becomes more like the source material composition

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DataRollinson's Using Geochem

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Use the X-Y values for creating this

chart in Excel

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Fe oxides 27

Alkalis

Na2O + K2O MgO

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8 10 12 14 16

+ K2O

Igneous Classification

control points for chart Columbia River basalts Mount Rainier

0 2 4 6

35 40 45 50 55 60 65 70 75 80

Na2O

SiO2

Basalt Basaltic Andesite

Andesite

Dacite Rhyolite

Primary vs. derivative magmas

A magma which has not been changed after leaving its

source region is considered to be “primary.”

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If it has been modified after leaving the source area, it is

a “derivative” or secondary magma.

Processes which change magma compositions

Fractional crystallization – removal of crystals from the magma More FC = more evolved or more differentiated

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Mixing

with another magma (magma mixing)

with fragments of wall rock (assimilation)

with a partial melt from the wall rock

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Fractional crystallization - part 1 Fig 4.9 Understanding Earth

Potential complexities of magma differentiation

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differentiation and mixing

Bowen’s reaction series When a magma crystallizes, those minerals with the highest melting temperatures form first. The starting point depends on the composition of the magma. For example, rhyolite doesn’t crystallize olivine.

When melting a rock, the sequence takes place in reverse. Minerals like quartz and mica melt before pyroxene, for example.

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Rollinson's Using Geochem Data Crystallization

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Crystallization

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Analyses of glass in volcanic ash and pumice (tephra) by electron microprobe

Kuehn-Negrini, in press

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Trace Elements

Show a range of behaviors so can be sensitive indicators of igneous processes

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e.g. amount of fractional crystallization & minerals involved

% partial melting of source rocks type of source rock (minerals present) tectonic setting (e.g. subduction-related or not)

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Classification of the trace-elements

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Rare-earth element abundances are often presented as normalized values to

simplify interpretation.

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Rare-earth element abundances of mafic rocks by type

Mean trace element abundances by rock type 44

Classification of the trace-elements

Compatible – prefer the solid phases during melting/crystallization

Incompatible – prefer the liquid phase during melting/crystallization

Whether an element is compatible or incompatible depends upon

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Whether an element is compatible or incompatible depends upon which solid phase(s) are present.

During partial melting, the first liquid is enriched in incompatible elements relative to the source rock.

During crystallization, the liquid becomes progressively enriched in incompatible elements and progressively depleted in compatible as crystallization proceeds.

Distribution coefficients

(a.k.a. partition coefficients)

A type of equilibrium constant.

Used to describe the distribution of trace elements between two phases.

Most typically used for distribution between solid & liquid phases,

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but sometimes also used for distribution between two solids.

liquid solid

D

c

K = c

Like other equilibrium constants, this relationship is affected by both pressure and temperature. It is also affected by the composition of the liquid and often by oxygen fugacity.

concentration of the trace element in a specific solid phase

concentration of the same trace element in the liquid phase

Distribution coefficients

If multiple solid phases are present, a bulk distribution coefficient may be calculated for each trace element of interest.

D = x₁Kd₁+ x₂Kd₂+ x₃Kd₃ etc.

x1x2and x3represent the relative proportions of the individual phases

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x1, x2, and x3represent the relative proportions of the individual phases (e.g. 0.2, 0.2, and 0.6)

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Variation in distribution coefficients with melt composition

All of these but Eu have a 3+ charge, so the observed pattern largely

reflects ionic size.

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Variation in KDwith melt composition

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Variation in distribution coefficients ionic radius and charge

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Effect of pressure on K_D

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Effect of oxygen fugacity on K_D

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Enrichment/

depletion with partial melting

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Crystallization 5% liquid

Enrichment with fractional crystallization

Brownlow's Geochemistry 90% liquid

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Trace elements during fractional

crystallization

Plagioclase fractional crystallization vs.

accumulation

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Trace elements during fractional

crystallization

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Extra Stuff

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