GEOCHRONOLOGY HONOURS 2006 Lecture 2 Interpretation of Radiogenic Isotope Data

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

2. Two Issues • Deciding what to date • 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 1.86-2.2Ga 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 1.86-2.2Ga 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