Using geochemical data in igneous petrology

1. Using geochemical data in igneous petrology

2. Useful books • Title borrowed from H. Rollinson – “Using geochemical data” (Longman, London, 1993) Chronically out of print; ca. US$60-$100 on www.amazon.com • See also • F. Albarède – “Introduction to geochemical modelling” (quite arduous) & “Geochemistry” • M. Wilson – “Igneous petrology, a global tectonic approach”

3. Week 1: Lectures (± pracs) • 5 lectures, ≈ 10—12 a.m. • Week 2 : Geochemical assignment • No formal lecture time but come and ask if you need help! • Week 3: Students seminars • 7 slots, 1h30—2h: 10—12 am and two afternoons.

4. Some background information • Major elements • Major elements behaviour during magmatic processes (FC, PM, mixing) • Trace elements • Trace elements behaviour during magmatic processes • Geochemical models • Useful software

5. Some background concepts (refreshers!) • Getting geochemical data: the hardware • Major and trace elements • Earth structure and geochemistry • Cosmochemistry and elements abundance • Major elements • Why using wt%? • Norms • Magmatic series • Some diagrams with major elements

6. 1.1 Analytical methods • Spectrometry (electromagnetic waves, mostly X-rays) • Mass spectrometry • Excitation of the source: • Primary X-rays • Plasma

7. Spectrometry

8. X-ray spectrum of an olivine

9. Main (modern) devices • XRF (X-ray fluorescence) • Microprobe • The ICP family (Inducively Coupled Plasma): • ICP-AES (Atomic Emission Spectrophotometry) • ICP-MS and LA-ICP-MS • TIMS (Thermo-Ionization Mass Spectrometry) • SHRIMP (High Resolution Ion Microprobe)

11. SF Laser ablation? « ChemCam » instrumentMars Science Laboratory (Artist rending)

12. 1.2 Major and traces

14. Definitions • Major elements: • Concentration > arbitrary value (0.1 or 1 wt% depending on the authors) • Components of main mineral phases • Trace elements: • Concentration < 0.1 % • Substitue in crystals but do not form phases of their own

15. Note that... • The above definition means that major and traces will behave in significantly different ways • Major: control by mineral stability limits (P-T conditions) • Traces: independant (or partially independant, as will be discussed) • Conceptually, some elements could be major in some systems, traces in other (cf .K in the mantle or Zr in crustal magmas)

17. Common types of magma

20. 1.3 Earth structure and geochemistry

21. Composition of Earth shells

22. 1.4 Cosmochemistry (how all this formed?) • Nuclosynthesis in stars • Planetary nebulas • Accretion • Differenciation

23. Nucleosynthesis « Bethe’s cycle »

24. Elements stability

25. Elements abundance • Lights > Heavies • Even > Odd • Abundance peak close to Fe (n=56)

26. Solar system abundance

28. Formation of a planetary nebula -

29. Planetary nebulas

30. Temperature gradients in the planetary nebula

31. Differenciation of planets

32. Atmophile Lithophile Siderophile

33. 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

34. 2. Major elements

35. Si Al Fe Mg Ca Na K Ti Mn P Ni Cr Typical major elements are Major elements concentrations are expressed as wt % oxydes (SiO2, Al2O3, etc.) (note the subscripts, by the way) And O !

36. 2.1 The wt% inheritance • Comes from the days of wet chemistry analysis • Is sadly inconsistent with both • Trace elements analysis (ppm weight) • Mineral formulas (number of atoms) Weight % oxydes!

37. Mass (or mass %) Molecular weight Nb of moles (or of atoms)

38. Example 1 • What is the wt% analysis of albite? Of a plagioclase An30? • NaAlSi3O8 • CaAl2Si2O8

39. Example 2 • What is the atom formula of this rock? (Darling granite)

40. NaAlSi3O8 CaAl2Si2O8 • In a feldspar, Al = (Na + K + 2Ca) • In this case, Al > Na + K + 2Ca • This rock has « excess » aluminium (it is peraluminous)

41. Figure 18-2. Alumina saturation classes based on the molar proportions of Al2O3/(CaO+Na2O+K2O) (“A/CNK”) after Shand (1927). Common non-quartzo-feldspathic minerals for each type are included. After Clarke (1992). Granitoid Rocks. Chapman Hall.

42. Some useful ratios • A/CNK = Al / (2 Ca + Na + K) • A/NK = Al/ (Na + K)

43. Some other useful (?) ratios • Mg# = Mg/(Mg+Fe) • « an% » = Ca/(Na+Ca) • K/Na Not that all or most use cation numbers … not wt% !! Still, igneous petrologists are very attached to wt% and are used to them. It might make more sense to switch to cation prop altogether, but it is probably not going to happen.

44. 2.2 Norms • Norms are a way to link major elements with mineral proportions • Normative composition (≠ modal) = mineral proportions calculated from chemistry • Norms are a way to compare rocks with different mineralogy • Whether they are more informative than the plain analysis is questionnable… • They were once extremely popular but are getting out of fashion • The most common: CIPW norm (Cross, Iddings, Pearson & Washington)

45. Q: quartz Feldspars: Or: orthoclase Ab: albite An: anorthite Feldspathoids Lc: leucite Ne: nepheline Pyroxenes Ac: acmite (NaFe pyroxene) Di: diopside Hy: hypersthene Wo: wollastonite Ol: olivine C: corundum CIPW normative minerals + minor minerals: apatite Ap, titanite (sphene) Tn (some rare minerals omitted)

46. Some important features • When making norms, feldpars are constructed first (or early) – they are the major component of igneous rocks • Many things are therefore by comparison to the Fsp. • Only anhydrous minerals are used in CIPW– no micas, amphibole

47. Peraluminous and peralkaline • Peraluminous = Corundum normative • Peralkaline = Acmite normative

48. Saturated and undersaturated • If there is not enough silica to build Fsp: undersaturated rocks (≠ saturated) • Orthoyroxene => olivine + qz • Feldspars => feldspathoids + qz • Alkali-rich rocks are commonly undersaturated (not enough SiO2 to accomodate all alkalis in Fsp)

49. Saturation line

50. In norms, rocks are either qz- or ol- normative (saturated or under saturated) • In real life, they can have neither • Note that it has nothing to do with the notion of basic-acid (purely defined as SiO2 %) or felsic-mafic (linked to the amount of light or dark minerals)