hons06 igpet l1 major

8/12/2019 Hons06 IgPet L1 Major

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Using geochemical data in

igneous petrology


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Useful books

Title borrowed fromH. RollinsonUsing geochemical data

(Longman, London, 1993)

Chronically out of print; ca. US$60-$100 onwww.amazon.com

See also F. AlbardeIntroduction to geochemical

modelling(quite arduous) & Geochemistry M. WilsonIgneous petrology, a global

tectonic approach


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1. Some background information

2. Major elements

3. Major elements behaviour duringmagmatic processes (FC, PM, mixing)

4. Trace elements5. Trace elements behaviour during

magmatic processes

6. Geochemical models7. Useful software


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1. Some background concepts

1. Getting geochemical data: the hardware

2. Major and trace elements

3. Earth structure and geochemistry

4. Cosmochemistry and elements abundance

2. Major elements

1. Why using wt%?

2. Norms

3. Magmatic series

4. Some diagrams with major elements


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1.1 Analytical methods

Spectrometry (electromagnetic waves,mostly X-rays)

Mass spectrometry

Excitation of the source:

Primary X-rays Plasma


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Spectrometry

Energy Source AbsorptionDetectorSample

EmissionDetector

Output withabsorption trough

Output withemission peak

Absorbedradiation

Emittedradiation


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X-ray spectrum of an olivine


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Main (modern) devices

XRF (X-ray fluorescence)

Microprobe

The ICP family (Inducively Coupled Plasma):

ICP-AES (Atomic Emission Spectrophotometry)

ICP-MS and LA-ICP-MS

TIMS (Thermo-Ionization Mass Spectrometry)

SHRIMP (High Resolution Ion Microprobe)


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In

situ?

Major Traces Isotopes

XRF Y Some Cheap and robust

Microprobe Y Y Cheap

ICP-AES (difficult) Y Replaced by ICP-MS

ICP-MS (difficult) Y De facto standard

LA-ICP-MS Y (difficult) Y (possible) Increasingly popular;expensive, robust once

set up. Lot of potential

for isotopes

ID-TIMS (possible) Y Basic tool forgeochronology.

Complicated to use(clean chemistry)

SHRIMP Y Y Regarded as stadard forgeochrono, but

extremely expensive

and difficult to use. Will

probably be replaced byLA ICP MS


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SF Laser ablation?

ChemCam instrumentMars Science Laboratory

(Artist rending)


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1.2 Major and traces


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Definitions

Major elements:

Concentration > arbitrary value (0.1 or 1 wt%depending on the authors)

Components of main mineral phases

Trace elements:

Concentration < 0.1 %

Substitue in crystals but do not form phases oftheir own


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Note that...

The above definition means that major andtraces will behave in significantly differentways

Major: control by mineral stability limits (P-Tconditions)

Traces: independant (or partially independant,as will be discussed)

Conceptually, some elements could bemajor in some systems, traces in other (cf.K in the mantle or Zr in crustal magmas)


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Common types of magma


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1.3 Earth structure andgeochemistry


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Composition of Earth shells

Elements wt%Crust Mantle Core

Continental Oceanic Upper Lower Outer Inner

O 41.2 43.7 44.7 43.710--15

Si 28 22 21.1 22.5

Al 14.3 7.5 1.9 1.6

Fe 4.7 8.5 5.6 9.8 80--85 80

Ca 3.9 7.1 1.4 1.7

K 2.3 0.33 0.08 0.11

Na 2.2 1.6 0.15 0.84

Mg 1.9 7.6 24.7 18.8

Ti 0.4 1.1 0.12 0.08

C 0.3

H 0.2

Mn 0.07 0.15 0.07 0.33

Ni 5 20

Cr 0.51


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1.4 Cosmochemistry (how all thisformed?)

Nuclosynthesis in stars

Planetary nebulas

Accretion Differenciation


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Nucleosynthesis

Bethes cycle


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Elements stability


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Elements abundance

Lights > Heavies

Even > Odd

Abundance peak close to Fe (n=56)


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Solar system abundance


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Formation of a planetary nebula-


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Planetary nebulas


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Temperature gradients in the planetary nebula


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Differenciation of planets


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Atmophile

Lithophile

Siderophile


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Elements abundance patterns inEarth are a product of

Nucleosynthesis

Lights > Heavies

Even > OddAbundance peak close to Fe (n=56)

Differenciation

Lithophile mantle (+ crust) Siderophile core


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2. Major elements


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Typical major elements are

Si

Al

Fe

Mg

Ca

Na

K

Ti

Mn

P

Ni

Cr

And O !

Major elementsconcentrations are expressed

as wt % oxydes(SiO2, Al2O3,etc.)

(note the subscripts, by the way)


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2.1 The wt% inheritance

Comes from the days of wet chemistryanalysis

Is sadly inconsistent with both

Trace elements analysis (ppm weight)

Mineral formulas (number of atoms)


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n

mM Molecular weight

Mass (or mass %)

Nb of moles (or of atoms)


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Example 1

What is the wt%analysis of albite? Ofa plagioclase An30?

NaAlSi3O8 CaAl2Si2O8

M(atom) M(oxyde)

Si 28.086 60.09

Al 26.982 101.94

Ca 40.08 56.08

Na 22.989 61.982

O 15.999


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Example 2

What is the atomformula of this rock?

SiO2 73.44

Al2O3 14.29

CaO 1.10

MgO 0.58FeO 2.06

K2O 5.39

Na2O 2.60

(Darling granite)


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NaAlSi3O8

CaAl2Si2O8

In a feldspar, Al = (Na + K + 2Ca) In this case, Al > Na + K + 2Ca

This rock has excess aluminium (it is

peraluminous)


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Al2O3 K2O

CaO

Al2O3

K2O

CaO

Al2O3

CaO

biotitemuscovitecordieriteandalusitegarnet

pyroxenehornblendebiotite

aegirineriebeckitearfvedsonite

Peraluminous Metaluminous Peralkaline

m

oles

Na2O Na2O

K2O

Na2O

CaO

Figure 18-2. Alumina saturation classes based on the molarproportions of Al2O3/(CaO+Na2O+K2O) (A/CNK) after

Shand (1927). Common non-quartzo-feldspathic minerals for each type are included. After Clarke (1992). GranitoidRocks. Chapman Hall.


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Metaluminous Peraluminous

Peralkaline

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

0

1

2

3

4

5

6

7

A/CNK

A/NK

Some useful ratios

A/CNK = Al / (2 Ca + Na + K) A/NK = Al/ (Na + K)


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Some other useful (?) ratios

Mg# = Mg/(Mg+Fe)

an% = Ca/(Na+Ca)

K/Na

Not that all or most use cation numbers not wt% !!

Still, igneous petrologists are very attached to wt% and are used to

them. It might make more sense to switch to cation prop altogether, but

it is probably not going to happen.


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2.2 Norms

Norms are a way to link major elements withmineral proportions

Normative composition ( modal) = mineralproportions calculated from chemistry

Norms are a way to compare rocks with differentmineralogy

Whether they are more informative than the plainanalysis is questionnable

They were once extremely popular but are gettingout of fashion

The most common: CIPW norm (Cross, Iddings,Pearson & Washington)


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CIPW normative minerals

Q: quartz

Feldspars:

Or: orthoclase

Ab: albite An: anorthite

Feldspathoids

Lc: leucite

Ne: nepheline

Pyroxenes

Ac: acmite (NaFepyroxene)

Di: diopside Hy: hypersthene

Wo: wollastonite

Ol: olivine

C: corundum

(some rare minerals omitted)

+ minor minerals: apatite Ap, titanite (sphene) Tn


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Some important features

When making norms, feldpars are constructedfirst (or early)they are the major component of

igneous rocks Many things are therefore by comparison to the

Fsp.

Only anhydrous minerals are used inCIPWno micas, amphibole


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Peraluminous and peralkaline

Peraluminous = Corundum normative

Peralkaline = Acmite normative


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Saturated and undersaturated

If there is not enough silica to build Fsp:undersaturatedrocks ( saturated)

Orthoyroxene => olivine + qz

Feldspars => feldspathoids + qz

Alkali-rich rocks are commonlyundersaturated (not enough SiO2 to

accomodate all alkalis in Fsp)


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Saturation line


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In norms, rocks are eitherqz- orol-normative (saturatedor undersaturated)

In real life, they can have neither

Note that it has nothing to do with thenotion of basic-acid (purely defined asSiO2%) or felsic-mafic (linked to theamount of light or dark minerals)

Ol- and foid


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Saturation line

Ol and foidnormative= undersaturated

QuartzNormative= saturated


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In norms, rocks are eitherqz- orol-normative (saturatedor undersaturated)

In real life, they can have neither

Note that it has nothing to do with thenotion of basic-acid (purely defined asSiO2%) or felsic-mafic (linked to theamount of light or dark minerals)


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Basic Acid

Undersaturated

SaturatedMafic

Felsic


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12

10

8

6

4

2

35 40 45 50 55 60 65%SiO2

Alkaline

Subalkaline

2.3 Magmatic series


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Nepheline-Fayalite-SiO2

Not a very good system, asit is a poor equivalent ofmagmatic rocksbut allowsto see nice fetaures.

Thermal di ide separates the silica sat rated


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Ne Ab Q

1070 1060

1713

Ab + Tr

Tr + L

Ab + LNe + L

Liquid

Ab + L

Ne + Ab

Thermal

Divide

Thermal divideseparates the silica-saturated

(subalkaline) from the silica-undersaturated

(alkaline) fields at low pressure

Cannot cross this divide by FX, so cant derive

one series from the other (at least via low-P FX)

Ol

Ne Ab

Opx

Q


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AFM di f h bdi id h b lk li


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AFM diagram:can further subdivide the subalkaline

magma series into a tholeiiticand a calc-alkalineseries

Figure 8-14. AFM diagram showing the distinction

between selected tholeiitic rocks from Iceland, the Mid-

Atlantic Ridge, the Columbia River Basalts, and Hawaii

(solid circles) plus the calc-alkaline rocks of the Cascade

volcanics (open circles). From Irving and Baragar (1971).

After Irvine and Baragar (1971). Can. J. Earth Sci., 8,

523-548.


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AlkalineCalc-alkalineTholeitic


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Series Alkalicontent

Fe-Mg Al

Alkaline High Fe-rich Metaluminousto peralkaline

Sub-

alkaline

Calc-

alkaline

Low tomoderate

Mg-rich Metaluminousto per-aluminous

Tholeitic Low Fe-rich Metaluminous


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CharacteristicSeries Convergent Divergent Oceanic Continental

Alkaline yes yes yes

Tholeiitic yes yes yes yes

Calc-alkaline yes

Plate Margin Within Plate

A world-wide survey suggests that there may be

some important differences between the three series

After Wilson (1989). Igneous Petrogenesis. Unwin Hyman - Kluwer


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Series and subseries

Alkaline series

Saturated

Undersaturated

Calc-alkaline series

Low K

Med K

High K


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East African rift (Afar)mildly alkaline

Central African Rift Strongly


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Central African Rift Stronglyalkaline


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Series and subseries

Alkaline series

Saturated

Undersaturated

Calc-alkaline series

Low K

Med K

High K


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Figure 16-6. a.K2O-SiO2diagram distinguishing high-K, medium-K and low-K series. Large squares = high-K, stars = med.-K,

diamonds = low-K series from Table 16-2. Smaller symbols are identified in the caption. Differentiation within a series (presumably

dominated by fractional crystallization) is indicated by the arrow. Different primary magmas (to the left) are distinguished by

vertical variations in K2O at low SiO2. After Gill, 1981, Orogenic Andesites and Plate Tectonics. Springer-Verlag.

Classifications based on major elements


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Classification of sub-alkaline lavas

Classifications based on major elements


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At that stage, the notion of magmatic series become to some degree blurredand irrelevant.

As usual, nature does not like pigeonholes and classifications and rocks have tobe studied on a case by case basis


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2.4 Some useful diagrams

They will obviously reflect the fundamentalaspects outlined previously:

Magmatic series

Saturated vs. Undersaturated Peraluminous vs. Peralkaline

Etc.

There is no rule forbiding to plot whatever vs.

anything else But some diagrams tend to give better results


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Harker type diagrams

The most commonly used

X: something related to differenciation(SiO2or MgO)

Y: any other element

22


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12

17

Al2O3

0

5

10

MgO

0

5

10FeO*

0

2

4

6

Na2O

0

5

10

15

CaO

45 50 55 60 65 70 75

0

1

2

3

4

K2O

SiO245 50 55 60 65 70 75

SiO2

Bivariate

(x-y)diagrams

Harker

diagram

for

Crater

Lake


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Harkem problems

Differenciation not always moves to the rightthey can be misleading

When using SiO2, closure effect due to the

overwhelming weight of SiO2 It has been proposed to use oxyde* instead of

oxyde, with e.g.

)100( 2

2*

2SiO

OKOK


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Differenciating between magmatic


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Differenciating between magmaticseries

TAS

Si-K

AFM

Everything with Mg# (thol. vs. CA)

See all previous examples

Showing some fundamental


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Showing some fundamentalfeatures

Diagrams using A/CNK, K/Na, etc. tend towork quite nicely

feldspar triangle (Oconnor)

Generally helpful to differenciate between rocks of

different origins (S vs I type granites, etc)

Classification based on normative


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OConnor diagram for quartz-bearing plutonic rocks

Classification based on normative

composition


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Classifying/naming rocks

Rocks already have perfectly well definednames (IUGS classification)

Therefore, why would you use another

scheme? Strongly weathered

Strongly metamorphosed

Geochem geek

Some people even do it with traces (SiO2vs. Ti/Zr)

Classification based on cationic


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Jensen cationic plot


proportions



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De la Roche et al. R1-R2 diagram


proportions

More creative use of the same diagram


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Batchelor-Bowden interpretation of de la Roches diagram

g

Mantle

Fractionates

Pre-plate

CollisionPost-

collision

UpliftLate-

orogenic

Anorogenic

Syn-collision

Post-

orogenic

-1000 0 1000 2000 3000 4000

0

1000

2000

3000

4000

R1= 4Si - 11(Na + K) - 2(Fe + Ti)

R2=6Ca+

2Mg+Al

OrAb

AnSp

Bt

Ph

En

Fs

Di

Fo

Fa

Hd

Ha

The data Im working on: plutonic rocks of


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The data I m working on: plutonic rocks of

the Abitibi sub province (Canada)

Blue: pre-tectonicGreen and red: syn to

post tectonic

Purple: post tectonic

Note the nice trend of evolutionwith time

hons06 igpet l1 major

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