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

Accessory phase growth histories in granulites

Folded and sheared ortho- and paragneisses of Ga 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

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

Rb-Sr VS Sm-Nd Isochrons

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

Biggest Problem  Deciding what your obtained date means

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

Intepreting Geochronological Data

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

Linking Accessory and Major Minerals

Linking Accessory and Major Phases

Linking Accessory and Major Mineral Growth

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.

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

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  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

SHRIMP Resolution

Metamorphic Overgrowths on Igneous Zircon

Resolution of Accessory Phase Dates in Garnet