GEOCHRONOLOGY HONOURS 2006 Lecture 2 Interpretation of Radiogenic Isotope Data.
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Transcript of GEOCHRONOLOGY HONOURS 2006 Lecture 2 Interpretation of Radiogenic Isotope Data.
GEOCHRONOLOGY HONOURS 2006
Lecture 2Interpretation 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
Accessory phase growth histories in granulites
Folded and sheared ortho- and paragneisses of 1.86-2.2Ga age that were subsequently deformed 50-60Ma ago – Canadian Cordillera
Summary of different stages of metamorphism in the Kanadra Granulite and comparison to the Harts Range, Central Australia
Geochronological data on rocks from Central Dronning Maud Land, Antarctica
Plate Tectonic Reconstructions
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.
Mineral isotopic closure temperatures
Parrish, 2001
Rb-Sr VS Sm-Nd Isochrons
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
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
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
Problems with the applicability of Sm-Nd
Put in a photo of an eclogite here
Sm-Nd Remobilisation and Re-equilibration
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
Biggest Problem
Deciding what your obtained date means
Folded and sheared ortho- and paragneisses of 1.86-2.2Ga age that were subsequently deformed 50-60Ma ago – Canadian Cordillera
Reaction textures involving accessory minerals
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
Accessory phase growth histories in granulites
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 handend of the
sapphirine grain. In these rutile bearing assemblages, the stable coexistinece of sapphirine and quartz implies peak metamorphic temperatures of around 1000°C.
Compositional Zoning in Garnet
Ca-zoning in garnet. Purple phase is plagiclase. Change in chemical composition reflects change in the metamorphic P-T conditions
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
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
Monazite Dating
Getting Good Results
Well constrained petrology
Multi-isotope approach
– Constrain highest T– Constrain lowest T– Build in the in-between parts
Parrish, 2001
Linking Accessory and Major Phases
Linking Accessory and Major Phases
Linking Accessory and Major Phases
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
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
SHRIMP Resolution
Metamorphic Overgrowths on Igneous Zircon
Metamorphic Overgrowths on Igneous Zircon
Put in some plots here from Mallee Bore