1. Using geochemical data in igneous petrology
Trace elements: presenting and interpreting them

2. 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

3. Selective affinities Fe2+ Mg2+ Ni2+ Au3+ Ag3+ Compatible
(right size & charge)
Fe2+
Mg2+
Incompatible
(size/charge does
not match)

4. Partition coefficient Kd = Cs/Cl
Compatible, incompatible (relative to a mineral)
Bulk repartition coefficient D = S Kdi Xi

5. Which are incompatible? Why?
Compatibility depends on minerals and melts involved.
Which are incompatible? Why?
Not exact, since D varies with the composition of mins & melt

6. How will the residual liquid evolve?
Calculate DYb for…
A lherzolite (80% Ol, 10% Opx, 10%Cpx)
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?

7. 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

8. 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

9. Solar system abundance

10. Concentration of REE in a sample

14. Chondrites

15. Contrasted REE patterns
Granites
Basalts

16. Multi-elements diagrams
Normalized to the PRImitive Mantle (close to chondrites) (Wood version)

17. 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!

18. Various normalizations:
To the average continental crust.
Meaningful for granites, sediments, etc.

19. 4.3 Families of elements
Rb follows K & conc. in Ksp, mica, & late melt
Ni follows Mg & conc in olivine

21. 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

22. 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

23. 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)

24. OIB vs. Island-arcs: LIL and HFS elements
Figure 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
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.

25. 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!

26. Kd’s for REE in basaltic liquids

27. 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+)

28. 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

29. 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

30. PGE= Platinum Group Elements
Ru, Rh, Pd, Os, Ir, Pt, Au
Not that well-known, large uncertainities on Kd’s
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

31. 4.4 Trace elements ratios Why?
Couple of elements with similar behaviour, normally not fractionnated and preserved during most processes
Nb and Ta
Zr and Hf

32. 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)

33. 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

34. Fingerprinting the role of a specific mineral
Ni strongly fractionated  olivine > 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
In all of the above cases using ratios, the idea is to find a mineral with a unique pair of elements for which it alone has a relatively high value of D for one element and a relatively low value of D for the other. The ratio of these elements is then sensitive only to liquid/crystal fractionation associated with that particular mineral

35. Trace elements ratios How?
Element-Element diagrams with linear scale

36. Trace elements ratios How?
Element-ratio diagrams with linear scale

37. Trace elements ratios How?
Element-element diagrams with log scale
Nb/Ta=15
Nb/Ta=20
Nb/Ta=50
Nb/Ta=10
Nb/Ta=5
Nb/Ta=1

38. Trace elements ratios Be careful!
Dividing by a common value yields spurious correlations…

39. 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
It’s easy to get confused…

40. 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

41. 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…)

42. 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.)

43. Some starting points/suggestions
Differenciation vs. different sources: check using Harker type plots what is related to differenciation!

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

45. 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…

46. You will progressively learn, and get used to certain elements – you’ll be familiar with typical values, behaviours, etc.

47. 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…)

48. 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)

49. “Classification” based on trace elements
Pearce diagrams (for granites)

50. “Classification” based on trace elements
Wood diagrams (for basalts)