GEOCHRONOLOGY HONOURS 2006 Lecture 2 Interpretation of Radiogenic Isotope Data

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GEOCHRONOLOGY HONOURS 2006Lecture
2Interpretation of Radiogenic Isotope Data
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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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Accessory phase growth histories in granulites
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Folded and sheared ortho- and paragneisses of
1.86-2.2Ga age that were subsequently deformed
50-60Ma ago Canadian Cordillera
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Summary of different stages of metamorphism in
the Kanadra Granulite and comparison to the Harts
Range, Central Australia
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Geochronological data on rocks from Central
Dronning Maud Land, Antarctica
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Plate Tectonic Reconstructions
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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.

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Mineral isotopic closure temperatures
Parrish, 2001
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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

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Rb-Sr VS Sm-Nd Isochrons
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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

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Biggest Problem

• Deciding what your obtained date means

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Folded and sheared ortho- and paragneisses of
1.86-2.2Ga age that were subsequently deformed
50-60Ma ago Canadian Cordillera
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Intepreting Geochronological Data
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Reaction textures involving accessory minerals
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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

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Linking Accessory and Major Minerals
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Linking Accessory and Major Phases
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Linking Accessory and Major Mineral Growth
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Linking Accessory and Major Mineral Growth
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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 1000C.

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Compositional Zoning in Garnet
Ca-zoning in garnet. Purple phase is plagiclase.
Change in chemical composition reflects change in
the metamorphic P-T conditions
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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
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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
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Monazite Dating
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Getting Good Results

• Well constrained petrology
• Multi-isotope approach
• Constrain highest T
• Constrain lowest T
• Build in the in-between parts

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Parrish, 2001
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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

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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

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SHRIMP Resolution
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Metamorphic Overgrowths on Igneous Zircon
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Resolution of Accessory Phase Dates in Garnet