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GEOCHRONOLOGY HONOURS 2006 Lecture 2 Interpretation of Radiogenic Isotope Data

GEOCHRONOLOGY HONOURS 2006 Lecture 2 Interpretation of Radiogenic Isotope Data. Two Issues. Deciding what to date Rock Mineral phase Deciding what the date means Igneous crystallisation Metamorphism Deformation Exhumation Alteration Cooling rates.

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GEOCHRONOLOGY HONOURS 2006 Lecture 2 Interpretation of Radiogenic Isotope Data

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  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

  8. 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.

  9. Mineral isotopic closure temperatures Parrish, 2001

  10. 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

  11. Rb-Sr VS Sm-Nd Isochrons

  12. 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

  13. Biggest Problem • Deciding what your obtained date means

  14. Folded and sheared ortho- and paragneisses of 1.86-2.2Ga age that were subsequently deformed 50-60Ma ago – Canadian Cordillera

  15. Intepreting Geochronological Data

  16. Reaction textures involving accessory minerals

  17. 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

  18. Linking Accessory and Major Minerals

  19. Linking Accessory and Major Phases

  20. Linking Accessory and Major Mineral Growth

  21. Linking Accessory and Major Mineral Growth

  22. 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.

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

  24. 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

  25. 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

  26. Monazite Dating

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

  28. Parrish, 2001

  29. 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

  30. 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 • Lu-Hf very similar to Sm-Nd, widely applied to dating of metamorphic rocks but again also very applicable for isotope tracer work and calculating model ages • Re-Os generally very low concentrations, in the range ppt, so need rocks or minerals that concentrate these elements • Os very low concentrations in most silicate minerals • Re-Os good for dating sulphide minerals of Mo and Cu and for Os bearing minerals such as osmiridium and laurite

  31. SHRIMP Resolution

  32. Metamorphic Overgrowths on Igneous Zircon

  33. Resolution of Accessory Phase Dates in Garnet

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