1. Using rock compositions to understand their origin and evolution
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 Unstable isotopes See GEOL214
Used largely for surface processes, some use as magmatic tracers
Unstable isotopes
Parent -> daughter
Used for dating
Tracers of sources

5. Isotopic systems Used for dating Used as tracers 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
See G.214

8. The U-Pb-Th System
Concordia = Simultaneous co-evolution of 206Pb and 207Pb via:
238U  234U  206Pb
235U  207Pb
206Pb* means radiogenic 206Pb
The example shows 2.5 Ga of development of an isotopic system
Figure 9-16a. After Faure (1986). Principles of Isotope Geology. 2nd, ed. John Wiley & Sons. New York.

9. The U-Pb-Th System Discordia = loss of both 206Pb and 207Pb
Discordia: loss of both 206Pb and 207Pb due to some thermal event (metamorphism)
All -> origin but not same amount
Suppose this occurs 2.5 Ga after original crystallization
Figure 9-16a. After Faure (1986). Principles of Isotope Geology. 2nd, ed. John Wiley & Sons. New York.

10. The U-Pb-Th System Concordia diagram after 3.5 Ga total evolution
3.5 Ga = age of igneous crystallization
1.0 Ga = age of metamorphism
We will leave the details of this system until Chapter 14 when we will use it to distinguish crustal contamination of mantle magmas
Figure 9-16b. After Faure (1986). Principles of Isotope Geology. 2nd, ed. John Wiley & Sons. NY

11. Isotopic tracers

15. Trace elements
Substitutions and Kd

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

17. Rb follows K & conc. in Ksp, mica, & late melt
Ni follows Mg & conc in olivine

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

19. Which are incompatible? Why?
Compatibility depends on minerals and melts involved.
Which are incompatible? Why?
Not exact, since D varies with the composition of mins & melt

20. How will the residual liquid evolve?
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?

21. 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
In all of the above cases using ratios, the idea is to find a mineral with a unique pair of elements for which it alone has a relatively high value of D for one element and a relatively low value of D for the other. The ratio of these elements is then sensitive only to liquid/crystal fractionation associated with that particular mineral

22. Identifying magmatic processes
… using geochemistry

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

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

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

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

27. 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

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

29. Biggest difference for compatible elements

30. Comp. Vs. Incomp diagrams