Using geochemical data in igneous petrology

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Using geochemical data in igneous petrology
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Useful books

• Title borrowed from
• H. Rollinson Using geochemical data (Longman,
  London, 1993)
• Chronically out of print ca. US60-100 on
  www.amazon.com
• See also
• F. Albarède Introduction to geochemical
  modelling (quite arduous) Geochemistry
• M. Wilson Igneous petrology, a global tectonic
  approach

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• Some background information
• Major elements
• Major elements behaviour during magmatic
  processes (FC, PM, mixing)
• Trace elements
• Trace elements behaviour during magmatic
  processes
• Geochemical models
• Useful software

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• Some background concepts
• Getting geochemical data the hardware
• Major and trace elements
• Earth structure and geochemistry
• Cosmochemistry and elements abundance
• Major elements
• Why using wt?
• Norms
• Magmatic series
• 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
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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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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 gt arbitrary value (0.1 or 1 wt
  depending on the authors)
• Components of main mineral phases
• Trace elements
• Concentration lt 0.1
• Substitue in crystals but do not form phases of
  their own

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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)

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Common types of magma
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1.3 Earth structure and geochemistry
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Composition of Earth shells
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1.4 Cosmochemistry (how all this formed?)

• 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 gt Heavies
• Even gt Odd
• Abundance peak close to Fe (n56)

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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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Formation of the solar system
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Differenciation of planets
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Atmophile Lithophile Siderophile
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Elements abundance patterns in Earth are a
product of

• Nucleosynthesis
• Lights gt Heavies
• Even gt Odd
• Abundance peak close to Fe (n56)
• 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

Major elements concentrations are expressed as wt
oxydes (SiO2, Al2O3, etc.)
(note the subscripts, by the way)
And O !
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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)

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Mass (or mass )
Molecular weight
Nb of moles (or of atoms)
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Example 1

• What is the wt analysis of albite? Of a
  plagioclase An30?
• NaAlSi3O8
• CaAl2Si2O8

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Example 2

• What is the atom formula of this rock?

(Darling granite)
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• NaAlSi3O8
• CaAl2Si2O8
• In a feldspar, Al (Na K 2Ca)
• In this case, Al gt Na K 2Ca
• This rock has  excess  aluminium (it is
  peraluminous)

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Figure 18-2. Alumina saturation classes based on
the molar proportions of Al2O3/(CaONa2OK2O)
(A/CNK) after Shand (1927). Common
non-quartzo-feldspathic minerals for each type
are included. After Clarke (1992). Granitoid
Rocks. Chapman Hall.
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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/(MgFe)
•  an  Ca/(NaCa)
• 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 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)

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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

minor minerals apatite Ap, titanite (sphene) Tn
(some rare minerals omitted)
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Some important features

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

• 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.

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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
  undersaturated rocks (? saturated)
• Orthoyroxene gt olivine qz
• Feldspars gt feldspathoids qz
• Alkali-rich rocks are commonly undersaturated
  (not enough SiO2 to accomodate all alkalis in Fsp)

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Saturation line
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• 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)

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Ol- and foid normative undersaturated
Saturation line
Quartz Normative saturated
Fsp foids bearing rocks
Fsp bearing rocks
QzFsp bearing rocks
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• 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)

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Felsic
Undersaturated
Saturated
Mafic
Basic
Acid
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2.3 Magmatic series
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Nepheline-Fayalite-SiO2
Not a very good system, as it is a poor
equivalent of magmatic rocks but allows to see
nice fetaures.
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Thermal divide separates 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)
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AFM diagram can further subdivide the
subalkaline magma series into a tholeiitic and a
calc-alkaline 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.
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Alkaline Calc-alkaline Tholeitic
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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
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Central African Rift Strongly alkaline
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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-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.
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Classifications based on major elements
Classification of sub-alkaline lavas
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• 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

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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

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Harker type diagrams

• The most commonly used
• X something related to differenciation (SiO2 or
  MgO)
• Y any other element

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Bivariate (x-y) diagrams
Harker diagram for Crater Lake
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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.

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Differenciating between magmatic series

• TAS
• Si-K
• AFM
• Everything with Mg (thol. vs. CA)

See all previous examples
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Showing some fundamental features

• Diagrams using A/CNK, K/Na, etc. tend to work
  quite nicely
•  feldspar triangle  (Oconnor)

Generally helpful to differenciate between rocks
of different origins (S vs I type granites, etc)
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Classification based on normative composition
OConnor diagram for quartz-bearing plutonic rocks
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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)

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Classification based on cationic proportions
Jensen cationic plot
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Classification based on cationic proportions
De la Roche et al. R1-R2 diagram
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More creative use of the same diagram
Batchelor-Bowden interpretation of de la Roches
diagram
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The data Im working on plutonic rocks of the
Abitibi sub province (Canada)

• Blue pre-tectonic
• Green and red syn to post tectonic
• Purple post tectonic

Note the nice  trend  of evolution with time