Using geochemical data in igneous petrology

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

12. 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

13. Contrasted REE patterns Granites Basalts

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

15. 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!

16. 4.2 Using ratiosExample: MORBs and OIB N-MORBs OIBs

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

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

19. Eu anomaly • Eu anomaly is supposed to reflect the implication of plagioclase

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

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

22. 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

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

24. OIB vs. Island-arcs: LIL and HFS elements 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. 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. Arcs Back-arc Mid-ocean ridges Within plate What will happen if you plot HFS vs. LIL? (eg Nb & Rb)

26. Why is it not magical?

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

28. Use a diagram showing ratios: • HFS/Reference • LIL/Reference

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

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

31. 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 !)

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

33. 4.3 Families of elements

35. 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

36. 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

37. 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)

38. OIB vs. Island-arcs: LIL and HFS elements 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. 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.

39. 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!

40. 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+)

41. 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

42. 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

43. 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…

44. 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 ?

45. Specific minerals • Garnet implication in OIB genesis

46. Different sources • N-, E- and T-MORB

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

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

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

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