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Igneous Geochemistry

55 65 7545

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3

4

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2

0

0

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80

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1014

16Al2O3

MgO

CaO

Fe2O3

Na2O

K2O

Wt. % SiO2

B BA A D RD R

OUTLINEReading this week:

White Ch 7

Note: Thu will be in-class exercise (hands-on, will need to hand in, don’t skip it)

Today

1.Finish making the crust

2.Major elements

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Compositions by Goldschmidt’s classes

Split “primitive mantle” to crust, mantle; elements divided:

Lithophiles mostly in crust; ionic bonds; large ions. O, Mg, Fe, Si in mantle too

Chalcophiles

split between

mantle, crust,

core; covalent

Siderophiles

mostly in the

core (metal)

Major Element CompositionsMeasure minerals or glasses:• Electron microprobe

• Electron beam + element electrons = Xray

Measure whole rocks (multiple minerals)• XRF (XRay Fluorescence)

• Xrays excite inner-shell electrons, on return to ground state emit Xrays

• ICP-MS (Inductively-Coupled Plasma Mass Spectrometry)• Measure by “counting” atoms per mass

Few more possible (AA, INAA, Ion Probe)

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Major elements: usually greater than 1%SiO2 Al2O3 FeO* MgO CaO Na2O K2O H2O

Minor elements: usually 0.1 - 1%TiO2 MnO P2O5 CO2

Trace elements: usually < 0.1%everything else

Abundance of the elementsin the Earth’s crust

Crustal composition

Table 8-3. Chemical analyses of somerepresentative igneous rocks

Peridotite Basalt Andesite Rhyolite PhonoliteSiO2 42.26 49.20 57.94 72.82 56.19TiO2 0.63 1.84 0.87 0.28 0.62Al2O3 4.23 15.74 17.02 13.27 19.04Fe2O3 3.61 3.79 3.27 1.48 2.79FeO 6.58 7.13 4.04 1.11 2.03MnO 0.41 0.20 0.14 0.06 0.17MgO 31.24 6.73 3.33 0.39 1.07CaO 5.05 9.47 6.79 1.14 2.72Na2O 0.49 2.91 3.48 3.55 7.79K2O 0.34 1.10 1.62 4.30 5.24H2O+ 3.91 0.95 0.83 1.10 1.57

Total 98.75 99.06 99.3 99.50 99.23

Data exampleMajor elements given as oxides (historical)

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Major elements reflect melts vs crystals

Major elements: make up essential parts of crystal lattice

ÞDistribution between melt and crystals follows stoichiometry of minerals (mineral compositional formula)

ÞMostly: melt and crystals are not identical in composition: separating melt from crystals allows for compositional change

ÞThis works for both partial melting, fractional crystallization

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22

Al2O3

0

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MgO

0

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10FeO*

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Na2O

0

5

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15

CaO

45 50 55 60 65 70 750

1

2

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4

K2O

SiO2

45 50 55 60 65 70 75SiO2

Plotting compositions

Harker diagramfor Crater Lake:Evolving systems create trends.

Notice curves and kinks:Why not straight?

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Effect of crystallizationBalance liquid and crystals

to bulk composition (B) = mass balance

In cooling melt, crystals CONTROL liquid path (arrow points away from crystal composition)

Example for olivine crystallization: melt follows olivine control line.

Melt evolution = liquid line of descent

Key:B=bulk, L=liquid, P-S=crystals

✦Formed crystals sink and separate from melt✦Removing crystal composition

changes melt

Melt will lose what elements?

Fractional crystallization

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Harker diagram– Tight (smooth) trends– Model with 3 assumptions:1 Rocks are related2 Trends = liquid line of

descent (mineral control)3 The basalt is the parent

magma from which the others are derived

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1014

16Al2O3

MgO

CaO

Fe2O3

Na2O

K2O

Wt. % SiO2

B BA A D RD RMagma Evolution

B=basalt, BA=basaltic-andesite, A=andesite, D=dacite, RD=rhyo-dacite, R=rhyolite

Cumulates deposited in layers (+/- cross-bedding)Cumulate texture:

Mutually touching phenocrysts with interstitial crystallized residual melt

Gravity settling and cumulates

http://www.geol.lsu.edu/henry/Geology3041/lectures/12LayeredMafic/Fig12-15.jpg

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Other ways: Magma Mixing● End member mixing for a suite of rocks:

combine 2 liquids to 1 new one

+ =

0

5

10

15

45 50 55 60 65 70 75

SiO

End-members

Magma Mixing - chemistry●Variation on a straight line between 2 end-members:●Mixtures:% of each +concentration:

Example:20%*10 +80%*1 = 2.8

80%

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Other ways: Assimilation

Surrounding material absorbed into melt: melt composition moves toward solid

Chemical classification of volcanic rocks

Winter Figure 2-4. A chemical classification of volcanics based on total alkalis vs. silica. After Le Bas et al. (1986) J. Petrol., 27, 745-750. Oxford University Press.

Þpartial melting, fractional crystallization are simplest ways to move around in this plot

partial melting

fractional crystallization

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2

35 40 45 50 55 60 65

%SiO2

Alkaline

Subalkaline

Groupings: alkaline (hotspots, continent) and subalkalinesubalkaline: tholeiitic @ ridges, hotspots (or calc-alkaline @arc)

Magma series -- melting

Partial Melting

• Often <20% of the source is melted• Separation of a partially melted liquid

from the solid residue

ÞMelting/Crystallization good for separation

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The ability to form an interconnected film is dependent upon the dihedral angle (q) a property of the melt

Figure 11-1 After Hunter (1987) In I. Parsons (ed.), Origins of Igneous Layering. Reidel, Dordrecht, pp. 473-504.

Partial melt and grain boundaries

● Need minimum for/to: ✦ Separation of melt from the residue requires a

critical melt %✦ Form a continuous, interconnected film✦ Have more than just wetting the grains

Minimum amount of melt

http://www.ldeo.columbia.edu/~benh/matos/portfolio/index_rocks.html http://www.whoi.edu/oceanus/viewImage.do?id=4981&aid=2390

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Differences in ocean basins

Table 10-1 Common petrographic differences between tholeiitic and alkaline basalts

Tholeiitic Basalt Alkaline BasaltUsually fine-grained, intergranular Usually fairly coarse, intergranular to ophitic

Groundmass No olivine Olivine common

Clinopyroxene = augite (plus possibly pigeonite) Titaniferous augite (reddish)

Orthopyroxene (hypersthene) common, may rim ol. Orthopyroxene absent

No alkali feldspar Interstitial alkali feldspar or feldspathoid may occur

Interstitial glass and/or quartz common Interstitial glass rare, and quartz absent

Olivine rare, unzoned, and may be partially resorbed Olivine common and zoned

Phenocrysts or show reaction rims of orthopyroxene

Orthopyroxene uncommon Orthopyroxene absent

Early plagioclase common Plagioclase less common, and later in sequence

Clinopyroxene is pale brown augite Clinopyroxene is titaniferous augite, reddish rims

after Hughes (1982) and McBirney (1993).

Tholeiitic Basalt and Alkaline Basalt

Bianco et al., 2008

Tholeiites and alkali basalts: both made in Hawai‘iHigh % melt in the middle = tholeiite, periphery (lower %) = alkalic

(Ribe & Christiansen 1999)

=> Major elements depend on P,T thus location in plume

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Dickin’s book

Tholeiites and alkali basalts: both made in Hawai‘iDecompression in hotspots limited by plate thickness; due to high T, melt relatively deep up to plate’s bottom

(Ribe & Christiansen 1999)

At ridge, plate = crust @ axis, so decompression up to ~8 km: melt from less deep, but all the way to ~8 (instead of ~80) km: makes high degree melt tholeiites