1. GEOCHRONOLOGY HONOURS 2006 Lecture 2 Interpretation of Radiogenic Isotope Data

2. Two Issues Deciding what to date Deciding what the date means Rock
Mineral phase
Deciding what the date means
Igneous crystallisation
Metamorphism
Deformation
Exhumation
Alteration
Cooling rates

3. Accessory phase growth histories in granulites

4. Folded and sheared ortho- and paragneisses of 1. 86-2
Folded and sheared ortho- and paragneisses of Ga age that were subsequently deformed 50-60Ma ago – Canadian Cordillera

5. Summary of different stages of metamorphism in the Kanadra Granulite and comparison to the Harts Range, Central Australia

6. Geochronological data on rocks from Central Dronning Maud Land, Antarctica

7. Plate Tectonic Reconstructions

9. What can we date? Nearly all minerals in most rock types
Some are better than others at certain P-T conditions because of differences in closure temperatures
The way in which the radioactive and radiogenic isotopes are concentrated in the phase or rock of interest.

10. Mineral isotopic closure temperatures
Parrish, 2001

12. Rb-Sr VS Sm-Nd Isochrons

13. Application of Sm-Nd Similar chemical properties of Sm and Nd
Sm very long half life (106 Byr)
Means that large variations in Sm/Nd ratios in natural rocks are rare
Therefore difficulty in obtaining a wide range of Sm/Nd ratios from a single rock body
Combined with greater technical demands of Nd-isotope work has limited applications

14. Applicability of Sm-Nd
Generally applied to problems where Rb-Sr not appropriate
Very old rocks with likely disturbance of the Rb/Sr ratio
Rocks with very low Rb/Sr ratios, ie achondrites
Mineral pairs that concentrate Sm or Nd

15. Applicability of Sm-Nd
Mineral isochrons for Sm-Nd can often work quite successfully because variations in partition coefficients causes moderately large variations in Sm-Nd ratios unlike whole rock systems
Garnet and Cpx have mirror image partition coefficients which therefore give rise to large variations in Sm/Nd ratios.
Common occurrence of garnet + cpx is in eclogites where Sm-Nd has been used extensively to date the timing of metamorphism
Sm-Nd as REE are relatively immobile and may therefore not fully re-equilibrate during metamorphism

16. Problems with the applicability of Sm-Nd
Put in a photo of an eclogite here

17. Sm-Nd Remobilisation and Re-equilibration

18. Mineral Transformations
Transformation of igneous augite to metamorphic omphacite
Relatively minor cation exchange
(Ca,Mg,Fe,Al)2(Si,Al)2O6 -> (Na,Ca)(Mg,Fe,Al)Si2O6
Monoclinic -> Monoclinic
Often does not completely re-equilibrate
Transformation of plagioclase to garnet
Major chemical exchange and structural re-organisation
CaAl2Si2O8 -> Ca3Al2Si3O12
Triclinic -> Isotropic
Likely to completely reset Sm-Nd systematics and give the metamorphic age

19. Biggest Problem
Deciding what your obtained date means

20. Folded and sheared ortho- and paragneisses of 1. 86-2
Folded and sheared ortho- and paragneisses of Ga age that were subsequently deformed 50-60Ma ago – Canadian Cordillera

21. Reaction textures involving accessory minerals

22. Reaction Textures in Accessory Phases
Major problem of linking the reactions that we see involving the major elements, ie major mineral phases, with the growth histories of accessory phases
Example
PUT IN SOME IMPORTANT REACTIONS HERE

23. Accessory phase growth histories in granulites

24. Reaction Textures
Reaction corona of orthopyroxene (outer rim) and sillimanite (inner rim) separating sapphirine (blue) from quartz in Mg-Al rich quartzites from the Napier Complex, Enderby Land, Antarctica. Corundum occurs as needles at the right hand
end of the sapphirine grain. In these rutile bearing assemblages, the stable coexistinece of sapphirine and quartz implies peak metamorphic temperatures of around 1000°C.

25. Compositional Zoning in Garnet
Ca-zoning in garnet. Purple phase is plagiclase. Change in chemical composition reflects change in the metamorphic P-T conditions

26. Two stage metamorphic history
The grey coloured gneiss contains amphibolite facies assemblages (hornblende-plagioclase), while the green-brown charnockite patches, contain orthopyroxene-bearing granulite facies assemblages.
Sri-Lanka Charnockites

27. Euhedral growth-zoning in zircon
Linking the development of these zircon zones with important reactions or processes occurring in the rock
                                                                                     
Euhedral zircon with growth zones

28. Monazite Dating

29. Getting Good Results Well constrained petrology Multi-isotope approach
Constrain highest T
Constrain lowest T
Build in the in-between parts

30. Parrish, 2001

31. Linking Accessory and Major Phases

32. Linking Accessory and Major Phases

33. Linking Accessory and Major Phases

34. U-Th-Pb in Zircon Has become the preferred method of dating
High temperature range of zircon means that in theory it records evidence for most geological events
However
Expensive
Time-consuming
Sample prep
Analytical work
Very specialised equipment

35. Other Methods Rb-Sr and Sm-Nd mainly doing isotope tracer work
Still used in specific situations
Rb-Sr for looking at lower temperature parts of metamorphic history
Sm-Nd dating of garnet in high-grade metamorphic rocks

36. SHRIMP Resolution

37. Metamorphic Overgrowths on Igneous Zircon

38. Metamorphic Overgrowths on Igneous Zircon
Put in some plots here from Mallee Bore