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Decoding Rock Evolution with Geochemistry: Isotopes and Tracers

Explore how geochemistry using rock compositions reveals the origin and evolution through isotopes, tracers like K-Ar and Rb-Sr, and major elements. Learn about stable and unstable isotopes, isotopic systems like U-Pb and Lu-Hf, and more. Understand identifying magmatic processes, major element trends, isotopic mixing, and key equations for trace elements.

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Decoding Rock Evolution with Geochemistry: Isotopes and Tracers

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  1. Geochemistry Using rock compositions to understand their origin and evolution

  2. Major elements (not very useful, same information as mineralogy!) • Trace elements • Isotopes

  3. Isotopes • Same number of protons, different number of neutrons (same atomic number, different atomic masses) • Same chemical properties… • … but different masses • Chemical etc. processes fraction only slightly isotopes (mostly the lights)

  4. Stable isotopes • See GEOL214 • Used largely for surface processes, some use as magmatic tracers • Unstable isotopes • Parent -> daughter • Used for dating • Tracers of sources

  5. Used for dating K-Ar Rb-Sr Sm-Nd U-Pb (and Pb-Pb, and more) Used as tracers Sm-Nd Rb-Sr Pb-Pb Lu-Hf Re-Os Isotopic systems See G.214

  6. The U-Pb-Th System Concordia = Simultaneous co-evolution of 206Pb and 207Pb via: 238U 234U 206Pb 235U 207Pb Figure 9-16a. After Faure (1986). Principles of Isotope Geology. 2nd, ed. John Wiley & Sons. New York.

  7. The U-Pb-Th System Discordia = loss of both 206Pb and 207Pb Figure 9-16a. After Faure (1986). Principles of Isotope Geology. 2nd, ed. John Wiley & Sons. New York.

  8. The U-Pb-Th System Concordia diagram after 3.5 Ga total evolution Figure 9-16b. After Faure (1986). Principles of Isotope Geology. 2nd, ed. John Wiley & Sons. NY

  9. Isotopic tracers

  10. Trace elements • Substitutions and Kd

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

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

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

  14. 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?

  15. Fingerprinting specific minerals: • Ni strongly fractionated olivine > pyroxene • Cr and Scpyroxenes » olivine • Ni/Cr or Ni/Sc can distinguish the effects of olivine and augite in a partial melt or a suite of rocks produced by fractional crystallization

  16. Identifying magmatic processes … using geochemistry

  17. What is the problem? • Several processes can generate magma diversity (FC, PM, mixing…) • They correspond to very similar (if not identical) reactions!

  18. FC: L1 => L2+Ol+Cpx • PM: L1+Cpx+Ol => L2 • Mixing: L1+(contaminant) => L2

  19. Major elements « trends » • Mass balance applies in all case: major elements do not allow to make the difference!

  20. Isotopes and mixing • Isotopes do allow to trace mixing (between isotopically contrasted sources)

  21. FC and PM: what difference? • PM: generally batch melting (an equilibrium process). Liquid stays in equilibrium with the solids. • FC: a continuous process; crystals removed as they are formed, allowing for quick depletion in compatible elements

  22. Basic equations for trace elements Mixing Partial melting Frac. Cryst.

  23. Biggest difference for compatible elements

  24. Comp. Vs. Incomp diagrams

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