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Using geochemical data in igneous petrology

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

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Using geochemical data in igneous petrology

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

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

  5. Compatibility depends on minerals and melts involved. Which are incompatible? Why?

  6. 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

  11. Chondrites

  12. Contrasted REE patterns Granites Basalts

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

  14. 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!

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

  16. 4.3 Families of elements

  17. 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

  18. 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

  19. 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)

  20. 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.

  21. 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!

  22. Kd’s for REE in basaltic liquids

  23. 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+)

  24. 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

  25. 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

  26. 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

  27. 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

  28. 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)

  29. 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

  30. 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

  31. Trace elements ratiosHow? • Element-Element diagrams with linear scale

  32. Trace elements ratiosHow? • Element-ratio diagrams with linear scale

  33. Trace elements ratiosHow? • Element-element diagrams with log scale Nb/Ta=15 Nb/Ta=20 Nb/Ta=50 Nb/Ta=10 Nb/Ta=5 Nb/Ta=1

  34. Trace elements ratiosBe careful! • Dividing by a common value yields spurious correlations…

  35. 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…

  36. 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

  37. 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…)

  38. 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.)

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

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

  41. 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…

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

  43. 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…)

  44. 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)

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

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

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