Using geochemical data in igneous petrology

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Using geochemical data in igneous petrology

• Trace elements presenting and interpreting them

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• Trace elements
• Partition coefficients and bulk repartition
  coefficient (Kd and D)
• Representing trace element compositions the use
  of spidergrams
• Main families of trace elements
• The use of ratios
• Some diagrams using trace elements

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Selective affinities
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• Partition coefficient Kd Cs/Cl
• Compatible, incompatible (relative to a mineral)
• Bulk repartition coefficient D S Kdi Xi

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Compatibility depends on minerals and melts
involved. Which are incompatible? Why?
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• Calculate DYb for
• A lherzolite (80 Ol, 10 Opx, 10Cpx)
• A Grt-bearing Lherzolite (70 Ol, 10 Opx-Cpx-Gt)
• Calculate DSr for
• A Cpx-Plag cumulate (50/50)
• A Cpx-Opx cumulate (50/50)
• How will the residual liquid evolve?

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4.2 Spidergrams

• Also (better) known as multi-elements diagram
• Allow to represent the whole composition of a
  sample on a single diagram
• Allow to compare the concentration in elements in
  different ranges
• Allow to get rid of the effects of primordial
  abundances

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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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Solar system abundance
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Concentration of REE in a sample
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Chondrites
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Contrasted REE patterns
Granites
Basalts
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Multi-elements diagrams
Normalized to the PRImitive Mantle (close to
chondrites) (Wood version)
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Various normalizations
To MORB (Mid-Oceanic Ridge Basalts the most
common type of basalt!) Meaningful for basalts
and co.
Look how the elements on the left-hand side
behave in a different way as those on the
right-hand side!
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Various normalizations
To the average continental crust. Meaningful for
granites, sediments, etc.
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4.3 Families of elements
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Commonly used trace elements

• LILE Large Ion Lithophile Elements
• Cs, Rb, K, Ba, Sr, Pb
• Large atoms with a small charge
• Tend to be incompatible to very incompatible
• Some exceptions (Rb in Biotite, Sr in plag)
• Typically fluid mobile (and therefore can be
  subject to weathering)
• Interesting to use but some caution should be
  exercised

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• HFSE High Field Strength Elements
• Sc, Y, Th, U, Pb, Zr, Hf, Ti, Nb, Ta
• Variable behaviours, generally incompatible
  except in some specific phases (Y in Grt, Nb in
  Hbl)
• Normally fluid immobile, insensible to weathering
• Regarded as good petrogenetic indicators

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• HFSE some interesting  pairs  with very
  similar behaviours
• Nb and Ta (Nb/Ta chondritic 15-20, less for
  crustal rocks)
• Zr and Hf (Zr/Hf chondritic 30-35)
• Values largely departing from this call for
  explanation (phases able to fractionnate Nb from
  Ta or Zr from Hf)

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OIB vs. Island-arcs LIL and HFS elements
Figure 16-11a. MORB-normalized spider diagrams
for selected island arc basalts. Using the
normalization and ordering scheme of Pearce
(1983) with LIL on the left and HFS on the right
and compatibility increasing outward from Ba-Th.
Data from BVTP. Composite OIB from Fig 14-3 in
yellow.
Figure 14-3. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
Data from Sun and McDonough (1989) In A. D.
Saunders and M. J. Norry (eds.), Magmatism in the
Ocean Basins. Geol. Soc. London Spec. Publ., 42.
pp. 313-345.
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• REE Rare Earth Elements
• La Ce Pr Nd (Pm) Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
• Technically they are HFS
• Rather incompatible, except in specific phases
• For a given mineral phases, different REE have
  different behaviours
• Nearly insensible to weathering
• Excellent petrogenetic indicators!

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Kds for REE in basaltic liquids
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REE the case of Eu

• REEs are normally 3 (La3, etc.)
• Eu can be Eu3 or Eu2
• Eu2 strongly compatible
• Especially in reducing environments

Reducing (Eu2)
Oxydizing (Eu3)
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• REE ratios
• Eu/Eu is a measure of the size of the Eu anomaly
• La/Yb (or LaN/YbN, also written (La/Yb)N ) is an
  indication of the slope of the REE pattern

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• Transition elements
• Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn
• All compatible, no huge differences
• Low abundances in felsic or intermediate rocks,
  useful for basic or ultrabasic systems, or for
  some mineral deposits (chromite)
• Fluid immobile

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• PGE Platinum Group Elements
• Ru, Rh, Pd, Os, Ir, Pt, Au
• Not that well-known, large uncertainities on Kds
• Low abudances, commonly below detection limit
  (bdl) with usual mehods
• Economic importance, especially in chromitites
  and sulphides
• Marginal petrologic use, could become more
  significant in the future

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4.4 Trace elements ratiosWhy?

• Couple of elements with similar behaviour,
  normally not fractionnated and preserved during
  most processes
• Nb and Ta
• Zr and Hf

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• A measure of the importance of an anomaly
• Eu/Eu
• Eu/Sm or Eu/Gd (similar to previous)
• Nb/Th, Nb/Ce (Nb-Ta anomaly)

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• A measure of the shape of a spidergram
• La/Yb, Ce/Yb, La/Lu
• Elements with different behaviours in different
  contexts
• LIL/HFS to differenciate subduction/OIB, e.g.
  Ba/La

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• Fingerprinting the role of a specific mineral
• Ni strongly fractionated ? olivine gt pyroxene
• Cr ? pyroxenes olivine
• Ni/Cr can distinguish the effects of olivine and
  augite in a partial melt or a suite of rocks
  produced by fractional crystallization

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Trace elements ratiosHow?

• Element-Element diagrams with linear scale

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Trace elements ratiosHow?

• Element-ratio diagrams with linear scale

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Trace elements ratiosHow?

• Element-element diagrams with log scale

Nb/Ta15
Nb/Ta20
Nb/Ta50
Nb/Ta10
Nb/Ta5
Nb/Ta1
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Trace elements ratiosBe careful!

• Dividing by a common value yields spurious
  correlations

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4.5 Some trace element diagrams

• In general, far greater diversity than for majors
• You can plot anything against anything else, and
  then start again with ratios
• Its easy to get confused

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Some starting points/suggestions

• Diagrams using rare elements (Ni in a granite, Rb
  in peridotites) will be highly sensitive to
  analytical uncertainities, sampling conditions,
  contamination, etc.
• Diagrams using elements from the same groups are
  likely to give similar results (e.g. Sr and Ba,
  Nb, Ta and Zr ) and are somehow redundant to
  discuss magma evolution

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• Use ratios of similar elements (supposedely not
  fractionnated during common petrogenetical
  processes) to differenciate between different
  groups of otherwise similar rocks

In this case low Nb/Ta vs. High Nb/Ta (and,
well, variable Nb/Ta)
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• Look for correlations ( trends ) or different
  populations (different sources or petrogenetic
  history?)
• Check if trends or grouping are robust in other
  diagrams with similar elements (e.g., replacing
  Rb by Th, Sr by Ba, etc.)

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Some starting points/suggestions

• Differenciation vs. different sources check
  using Harker type plots what is related to
  differenciation!

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• Two populations distinguished with Rb and Sr
  (high Sr, and low Sr) ?

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• A Harker-type diagram reveals that the Sr
  contents whatever the rock type are more or
  less correlated to differenciation. The two
   groups  simply reflect more or less
  differenciated rocks from the same series!

On the other hand, the low Rb, low Sr groups
seems to have an independant existence
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• You will progressively learn, and get used to
  certain elements youll be familiar with
  typical values, behaviours, etc.

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• My personnal favorite subset (NB I work on
  granites!)
• LILE Rb, Sr (used to trace plag, Bt, etc.)
• Th and Cs are too sensible to weathering and
  anyway more difficult to analyse not always
  possible to have data
• HFSE Y (useful for Grt, amp) Nb
• Zr is too affected by zircon Hf and Ta are not
  always analyzed (good to look at Nb/Ta and Zr/Hf,
  though)
• REE La (or Ce), Yb, Eu/Eu
• This carries effectively most of the useful
  information
• No transition elements, no PGEs
• Too low to be meaningful
• Your own choice will be different (especially if
  working on basalts)

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Classical diagrams

• Spidergrams
• Harker type diagrams
• Check the litterature for your type of rocks
  there are some classical diagrams that people are
  used to.
• e.g. TTG and Archaean rocks Sr/Y vs. Y, La/Yb
  vs. Yb (Martin 1987)
• Basalts (MORB) La/Sm, etc.
• Island arcs HFS/LIL (Ba/La) etc.
• Geotectonic diagrams (to be discussed next week)

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Classification based on trace elements
Pearce diagrams (for granites)
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Classification based on trace elements
Wood diagrams (for basalts)