using geochemical data in igneous petrology
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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