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