1. Using geochemical data in igneous petrology
Trace elements: spidergrams, ratios and magical diagrams
(or – presenting and using trace elements data)

2. A slide of a recent presentation by Julian Pearce

3. And therefore…

4. Why is it magical?

5. Trace elements
Representing trace element compositions: the use of spidergrams
Spidergrams and ratios
Main families of trace elements
Some diagrams using trace elements

6. 4.1 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

7. 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

8. Solar system abundance

9. Chondrites

10. Concentration of REE in a sample

14. Building a spidergram (Recipe)
Arrange the elements in given order (generally the more incompatible on the left)
Divide each element’s concentration in the sample by the concentration in a reference material (chondrite, primitive mantle, MORB…)
Plot using a log scale

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. 4.2 Using ratios Example: MORBs and OIB
N-MORBs
OIBs

20. La/Sm
E-MORB
OIB
N-MORB
Gd/Yb

21. « Anomalies » Granites from the Cape Granite Suite
Darling-Vredenburg area

22. Eu anomaly
Eu anomaly is supposed to reflect the implication of plagioclase

23. … because :
Kd’s for REE in basaltic liquids

24. Eu anomaly
Can you invent a « magical number » showing the implication of plagioclase?

25. 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

26. Why can a Eu/Eu. vs. La/Yb diagram be read as a plagioclase vs
Why can a Eu/Eu* vs. La/Yb diagram be read as a plagioclase vs. garnet diagram?

27. 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.

28. What will happen if you plot HFS vs. LIL?
(eg Nb & Rb)
Arcs
Back-arc
Mid-ocean ridges
Within plate

29. Why is it not magical?

30. Ratios (of incompatible elements) Less affected by differentiation
Differences in Nb/Yb reflect (mostly) different primitive magmas; mostly preserved during differentiation

31. Use a diagram showing ratios:
HFS/Reference
LIL/Reference

32. Once you’ve understood the trick, you can build many similar diagrams!

33. Dark arts – Geotectonic diagrams
Wood diagrams (for basalts)

34. Dark arts – Geotectonic diagrams
Another diagram by You-know-Who (Pearce et al. 1984)
(With all due respect for J. Pearce, who is a nice person and one of the best living geochemists !)

35. Why does it work?
Well, this is going to be (part of) Jaco’s seminar – stay tuned.

36. 4.3 Families of elements
Rb follows K & conc. in Ksp, mica, & late melt
Ni follows Mg & conc in olivine

38. 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

39. 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

40. 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)

41. 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.

42. 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!

43. Kd’s for REE in basaltic liquids

44. 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+)

45. 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

46. 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

47. 4.4 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…

48. Diagrams showing different types or groups of rocks
Diagrams showing differentiation and implication of specific minerals (during melting or differentiation)
Diagrams reflecting different sources
Geotectonic diagrams ?

49. Specific minerals
Garnet implication in OIB genesis

50. Different sources
N-, E- and T-MORB

51. Describing different groups
N-MORB
E-MORB
N-MORB
E-MORB

52. Groups of rocks: Potential spurious correlations
Continental arcs
Back-arc
Is this a useful diagram?

53. Our nice diagram just tells us that back-arcs are basalts and cont
Our nice diagram just tells us that back-arcs are basalts and cont. arcs. dacites to rhyolites – we knew that already!

54. Geotectonic Arcs Back-arc Mid-ocean ridges Within plate
WPB
MORB
IAT
IAT
CAB
CAB
Pearce & Cann 1973

55. Geotectonic Arcs Back-arc Mid-ocean ridges Within plate Wood 1980
N-MORB
IAT
E-MORB
WPT
CAB
WPA
Wood 1980

56. Geotectonic diagrams
A specific site = combination of sources + processes (in terms of PT and therefore of minerals)

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

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

61. 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

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