Using geochemical data in igneous petrology

Useful books

• Title borrowed from H. Rollinson – “Using geochemical data”

(Longman, London, 1993)Chronically out of print; ca. US$60-$100 on

www.amazon.com

• See also– F. Albarède – “Introduction to geochemical

modelling” (quite arduous) & “Geochemistry”– M. Wilson – “Igneous petrology, a global

tectonic approach”

• Week 1: Lectures (± pracs)– 5 lectures, ≈ 10—12 a.m.

• Week 2 : Geochemical assignment– No formal lecture time but come and ask if

you need help!

• Week 3: Students seminars– 7 slots, 1h30—2h: 10—12 am and two

afternoons.

1. Some background information

2. Major elements

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

4. Trace elements

5. Trace elements behaviour during magmatic processes

6. Geochemical models

7. Useful software

1. Some background concepts (refreshers!)1. Getting geochemical data: the hardware

2. Major and trace elements

3. Earth structure and geochemistry

4. Cosmochemistry and elements abundance

2. Major elements1. Why using wt%?

2. Norms

3. Magmatic series

4. Some diagrams with major elements

1.1 Analytical methods

• Spectrometry (electromagnetic waves, mostly X-rays)

• Mass spectrometry

• Excitation of the source:– Primary X-rays– Plasma

Spectrometry

Energy Source AbsorptionDetectorSample

EmissionDetector

Output withabsorption trough

Output withemission peak

Absorbedradiation

Emittedradiation

X-ray spectrum of an olivine

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)

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 for geochronology. Complicated to use (clean chemistry)

SHRIMP Y Y Regarded as stadard for geochrono, but extremely expensive and difficult to use. Will probably be replaced by LA ICP MS

SF Laser ablation?

« ChemCam » instrumentMars Science Laboratory

(Artist rending)

1.2 Major and traces

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

of their own

Note that...

• The above definition means that major and traces will behave in significantly different ways– Major: control by mineral stability limits (P-T

conditions)– Traces: independant (or partially independant,

as will be discussed)

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

Common types of magma

1.3 Earth structure and geochemistry

Composition of Earth shellsElements 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    

1.4 Cosmochemistry (how all this formed?)

• Nuclosynthesis in stars

• Planetary nebulas

• Accretion

• Differenciation

Nucleosynthesis

« Bethe’s cycle »

Elements stability

Elements abundance

• Lights > Heavies

• Even > Odd

• Abundance peak close to Fe (n=56)

Solar system abundance

Formation of a planetary nebula-

Planetary nebulas

Temperature gradients in the planetary nebula

Differenciation of planets

Atmophile

Lithophile

Siderophile

Elements abundance patterns in Earth are a product of

• Nucleosynthesis– Lights > Heavies– Even > Odd– Abundance peak close to Fe (n=56)

• Differenciation– Lithophile mantle (+ crust)– Siderophile core

2. Major elements

Typical major elements are

• Si• Al• Fe• Mg• Ca• Na• K

• Ti• Mn• P• Ni• Cr

And O !

Major elements concentrations are expressed as wt % oxydes (SiO2, Al2O3, etc.)

(note the subscripts, by the way)

2.1 The wt% inheritance

• Comes from the days of wet chemistry analysis

• Is sadly inconsistent with both– Trace elements analysis (ppm weight)– Mineral formulas (number of atoms)

Weight % oxydes!

n

mM Molecular weight

Mass (or mass %)

Nb of moles (or of atoms)

Example 1• What is the wt%

analysis of albite? Of a 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

Example 2

• What is the atom formula of this rock?

SiO2 73.44

Al2O3 14.29

CaO 1.10

MgO 0.58

FeO 2.06

K2O 5.39

Na2O 2.60

(Darling granite)

NaAlSi3O8

CaAl2Si2O8

• In a feldspar, Al = (Na + K + 2Ca)

• In this case, Al > Na + K + 2Ca

• This rock has « excess » aluminium (it is peraluminous)

Al2O3K2O

CaO

Al2O3

K2O

CaO

Al2O3

CaO

biotitemuscovitecordieriteandalusitegarnet

pyroxenehornblendebiotite

aegirineriebeckitearfvedsonite

Peraluminous Metaluminous Peralkaline

mol

es

Na2ONa2O

K2O

Na2O

CaO

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

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

Metaluminous Peraluminous

Peralkaline

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

01

23

45

67

A/CNK

A/N

K

Some useful ratios• A/CNK = Al / (2 Ca + Na + K)

• A/NK = Al/ (Na + K)

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.

2.2 Norms• Norms are a way to link major elements with

mineral proportions• Normative composition (≠ modal) = mineral

proportions calculated from chemistry• Norms are a way to compare rocks with different

mineralogy• Whether they are more informative than the plain

analysis is questionnable…• They were once extremely popular but are getting

out of fashion• The most common: CIPW norm (Cross, Iddings,

Pearson & Washington)

CIPW normative minerals

• Q: quartz• Feldspars:

– Or: orthoclase– Ab: albite– An: anorthite

• Feldspathoids– Lc: leucite– Ne: nepheline

• Pyroxenes– Ac: acmite (NaFe

pyroxene)– Di: diopside– Hy: hypersthene– Wo: wollastonite

• Ol: olivine• C: corundum

(some rare minerals omitted)

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

Some important features

• When making norms, feldpars are constructed first (or early) – they are the major component of igneous rocks

• Many things are therefore by comparison to the Fsp.

• Only anhydrous minerals are used in CIPW– no micas, amphibole

Peraluminous and peralkaline

• Peraluminous = Corundum normative

• Peralkaline = Acmite normative

Saturated and undersaturated

• If there is not enough silica to build Fsp: undersaturated rocks (≠ saturated)– Orthoyroxene => olivine + qz– Feldspars => feldspathoids + qz

• Alkali-rich rocks are commonly undersaturated (not enough SiO2 to accomodate all alkalis in Fsp)

Saturation line

• In norms, rocks are either qz- or ol- normative (saturated or under saturated)

• In real life, they can have neither

• Note that it has nothing to do with the notion of basic-acid (purely defined as SiO2 %) or felsic-mafic (linked to the amount of light or dark minerals)

Saturation lineOl- and foidnormative= undersaturated

QuartzNormative= saturated

Qz+

Fsp

bear

ing

rock

s

Fsp b

earin

g rock

s

Fsp

+ fo

ids

bear

ing

rock

s

• In norms, rocks are either qz- or ol- normative (saturated or under saturated)

• In real life, they can have neither

• Note that it has nothing to do with the notion of basic-acid (purely defined as SiO2 %) or felsic-mafic (linked to the amount of light or dark minerals)

Basic Acid

Undersaturated

SaturatedMafic

Felsic

12

10

8

6

4

2

35 40 45 50 55 60 65

%SiO2

%N

a2O

+ K

2O

Alkaline

Subalkaline

2.3 Magmatic series

Nepheline-Fayalite-SiO2

Not a very good system, asit is a poor equivalent of magmatic rocks –but allowsto see nice fetaures.

Ne Ab Q

1070 1060

1713

Ab + Tr

Tr + L

Ab + LNe + L

Liquid

Ab + L

Ne + Ab

ThermalDivide

Thermal divideThermal divide separates the silica-saturated separates the silica-saturated (subalkaline) from the silica-undersaturated (subalkaline) from the silica-undersaturated (alkaline) fields at low pressure(alkaline) fields at low pressure

Cannot cross this divide by FX, so can’t derive Cannot cross this divide by FX, so can’t derive one series from the other (at least via low-P FX)one series from the other (at least via low-P FX)

Ol

Ne Ab

Opx

Q

Alkalin

e fie

ld

Subalkaline field

Dividing line

F

A M

Calc-alkaline

T

ho leiitic

AFM diagram:AFM diagram: can further subdivide the subalkaline can further subdivide the subalkaline magma series into a magma series into a tholeiitictholeiitic and a and a calc-alkalinecalc-alkaline series series

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.

AlkalineCalc-alkalineTholeitic

Series Alkali content

Fe-Mg Al

Alkaline High Fe-rich Metaluminous to peralkaline

Sub-alkaline

Calc-alkaline

Low to moderate

Mg-rich Metaluminous to per-aluminous

Tholeitic Low Fe-rich Metaluminous

CharacteristicSeries Convergent Divergent Oceanic ContinentalAlkaline yes yes yesTholeiitic yes yes yes yesCalc-alkaline yes

Plate Margin Within Plate

A world-wide survey suggests that there may be A world-wide survey suggests that there may be some important differences between the three seriessome important differences between the three series

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

Series and subseries

• Alkaline series– Saturated– Undersaturated

• Calc-alkaline series– Low K– Med K– High K

East African rift (Afar) – mildly alkaline

Central African Rift – Strongly alkaline

Series and subseries

• Alkaline series– Saturated– Undersaturated

• Calc-alkaline series– Low K– Med K– High K

Figure 16-6. a. K2O-SiO2 diagram 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.

Classification of sub-alkaline lavas

Classifications based on major elements

• At that stage, the notion of magmatic « series » become to some degree blurred and irrelevant.

• As usual, nature does not like pigeon holes and classifications and rocks have to be studied on a case by case basis

2.4 Some useful diagrams

• They will obviously reflect the fundamental aspects 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…

Harker type diagrams

• The most commonly used

• X: something related to differenciation (SiO2 or MgO)

• Y: any other element

12

17

22

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

SiO2

45 50 55 60 65 70 75

SiO2

Bivariate Bivariate (x-y) (x-y)

diagramsdiagrams

HarkerHarkerdiagram diagram

forforCraterCraterLakeLake

Harkem problems

• Differenciation not always moves to the right – they 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*2 SiO

OKOK

Differenciating between magmatic series

• TAS

• Si-K

• AFM

• Everything with Mg# (thol. vs. CA)

See all previous examples

Showing some fundamental features

• Diagrams using A/CNK, K/Na, etc. tend to work quite nicely

• « feldspar triangle » (O’connor)

Generally helpful to differenciate between rocks of different origins (S vs I type granites, etc)

O’Connor diagram for quartz-bearing plutonic rocks

Classification based on normative composition

Classifying/naming rocks

• Rocks already have perfectly well defined names (IUGS classification)

• Therefore, why would you use another scheme?– Strongly weathered– Strongly metamorphosed– Geochem geek

• Some people even do it with traces (SiO2 vs. Ti/Zr)

Jensen cationic plot

Classification based on cationic proportions

De la Roche et al. R1-R2 diagram

Classification based on cationic proportions

Batchelor-Bowden interpretation of de la Roche’s diagram

More creative use of the same diagram

MantleFractionates

Pre-plateCollisionPost-

collisionUplift

Late-orogenic

AnorogenicSyn-collision

Post-orogenic

-1000 0 1000 2000 3000 4000

01000

2000

3000

4000

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

R2=

6C

a +

2M

g +

Al

OrAb

AnSp

Bt

Ph

En

Fs

Di

Fo

Fa

Hd

Ha

Example: plutonic rocks of the Abitibi sub province (Canada)

Blue: pre-tectonic

Green and red: syn to post tectonic

Purple: post tectonic

Note the nice (hem) « trend » of evolution with time