Introduction to Geochronology Part 1: The basics Geochronology & Tracers Facility NERC Isotope Geosciences Laboratory British Geological Survey

Introduction to Geochronology Pat 1: Geochronology – basic principles – P/D, decay constants, chronometers Part 2: Framing the problem (1) – What and why? – sampling, targeting, minerals and meanings Part 3: Framing the problem (2) - Petrochronology – imaging, trace elements, metamorphic examples Part 4: Rocks into Ratios – mass spectrometry, traceability, dissolution chemistry Part 5: Ratios into Dates – uncertainties, reporting, reading legacy data

Geochronology basics Why geochronology? – calibration Geological timescale Events - mass extinctions Great Oxygenation Event Snowball Earth Age of the Earth and the Solar System Astronomical clock, eustacy…

Geochronology basics Why geochronology? – rates Tectonic – plate to outcrop scales Metamorphism and fluids Evolution of life

Geochronology basics What is geochronology? Wikipedia: Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is provided by tools such as palaeomagnetism and stable isotope ratios.sciencerocksfossilssediments

Geochronology basics What is geochronology? Determining the age at which a particular radioactive decay chain, or part of it, is set/reset/disturbed within a mineral. How?

Geochronology basics The age equation: Assumptions The amount of daughter isotope at the time of formation of the sample is zero (or known independently and can be compensated for). No parent isotope or daughter isotope has entered or left the sample since its time of formation.

Geochronology basics Some parent-daughters used in geochronology: – 238 U – 206 Pb – 235 U – 207 Pb – 232 Th – 208 Pb – 187 Re – 187 Os – 176 Lu Hf – 147 Sm – 143 Nd – 146 Sm – 142 Nd – 87 Rb – 87 Sr – 40 K – 40 Ar

Geochronology basics What makes a good geochronometer? – Up-take of parent daughter during crystallisation – No up-take of daughter during crystallisation – Closed system to parent and daughter since crystallisation – i.e. high initial P/D ratio. Kd lattice-bound? diffusion Kd lattice-bound? diffusion

Geochronology basics Zircon – U-Pb Brewer et al., 2004, Precambrian Research Roberts et al., 2016, Geoscience Frontiers Kd U = ~250

Geochronology basics Most chronometers are not perfect Complications and limitations – Incorporation of daughter (e.g. common lead) – Inheritance of daughter (e.g. radiogenic precursor) – Susceptibility to alteration – Partial open-system behaviour

Geochronology basics Concordia (Wetherill) plot

Geochronology basics Concordia (Tera-Wasserburg) plot

Geochronology basics Incorporation of daughter e.g. common lead in U-Pb dating Many minerals incorporate variable amounts of Pb upon their growth. e.g. apatite, titanite, rutile, allanite calcite, columbite-tantalite, cassiterite

Geochronology basics Incorporation of daughter e.g. common lead in U-Pb dating Rutile (TiO 2 ) ID-TIMS U-Pb data for R10 rutileLA U-Pb data for R10 Braccialli et al., 2013, Chemical Geology Common-lead

Geochronology basics Incorporation of daughter e.g. common lead in U-Pb dating Rutile Braccialli et al., 2013, Chemical Geology

Geochronology basics Incorporation of daughter e.g. common lead in U-Pb dating Titanite (CaTiSiO 5 ) Thomas et al., 2013, Precambrian Research Data from Martin et al., 2015, New Zealand Journal of Geology & Geophsyics radiogenic Pb common Pb

Geochronology basics Inherited radiogenic daughter e.g. radiogenic lead in monazite/allanite Raw LA and ID data for SISS allanite Common-lead corrected ID data for SISS allanite Th-Pb vs U-Pb ID data for SISS allanite Smye et al., 2014, Chemical Geology

Geochronology basics Partially open-system – after metamictisation – above closure temperature – during fluid infiltration

Geochronology basics Partially open-system Thomas et al., 2013, Precambrian Research Lead-loss Thomas et al., 2015, Precambrian Research

Geochronology basics Partially open-system MacDonald et al., 2013, Contributions to Min & Pet.

Geochronology basics Alteration monazite Seydaux-Guillaume et al., 2012, Chemical Geology

Geochronology basics Alteration monazite Seydaux-Guillaume et al., 2012, Chemical Geology

Geochronology basics Most chronometers are not perfect Complications and limitations – Incorporation of daughter (e.g. common lead) – Inheritance of daughter (e.g. radiogenic precursor) – Susceptibility to alteration – Partial open-system behaviour

Geochronology basics Most chronometers are not perfect Complications and limitations – Incorporation of daughter (e.g. common lead) – Inheritance of daughter (e.g. radiogenic precursor) – Susceptibility to alteration – Partial open-system behaviour Can we use any of this to our advantage?

Geochronology basics Closure Temperature and Thermochronology – In ideal terms – the closure temperature is the temperature at the time given by the measured date of a mineral – The temperature at which a given parent-daughter decay chain is set, because no diffusion of the parent or daughter isotopes takes place at lower temperatures – The reality – closure temperature varies with grain size, and the rate of cooling.

T c = R/ [E a ln(AτD 0 /a 2 )] Dodson closure temperature (1973) Gas constant Activation energy Diffusion parameter (Experimental) Shape parameter radius Cooling rate Important: Closure temperature dependent on cooling rate! This is normally what we want to find!!

Assumptions implicit in linking thermochronology ages to Dodson T c 1.Thermally-activated volume diffusion 2.No initial daughter product in grain 3.Infinite grain boundary reservoir (open system) K K K K K K K K K K K K K K K K K K K K K K K K K Andy Smye 1 2 3

White mica Harrison et al 2009 diffusion parameters

Geochronology basics Closure Temperature and Thermochronology – In ideal terms – the closure temperature is the temperature at the time given by the measured date of a mineral – The temperature at which a given parent-daughter decay chain is set, because no diffusion of the parent or daughter isotopes takes place at lower temperatures – The reality – closure temperature varies with grain size, and the rate of cooling. Reiners, 2005, Reviews in Min & Geochem

Geochronology basics Mineral closure temperatures Chew & Spikings, 2015, Elements Closure temperature GeochronometersThermochronometers

Geochronology basics Pressure Temperature muscovite biotite rutile titanite monazite zircon allanite garnet Crystallisation chronometers: Sm-Nd, Lu-Hf garnet U-Pb allanite, zircon, monazite Thermochronometers: Ar/Ar micas U-Th-He apatite, zircon Fission track apatite, zircon

Geochronology basics Parrish, 2001, Geol Soc Spec Pub 184

Geochronology basics Roberts et al., 2016, Geoscience Frontiers

More on thermochronology: Analytical Evolution Images: Catherine Mottram; Chris McDonald Laser probe Multi-grain; Single grain Single spot

Dispersion in 40 Ar/ 39 Ar data Ages, Ma White micaBiotite Single grain fusion, 1mm diameter grains Caledonides: ~400 Ma. Argon ‘ages’ from Ma Ar: Chris McDonald PhD data; Zir: Hacker et al.; Titanite: Kylander-Clark et al. zircon titanite

Within-grain variability Chris McDonald PhD data 15 μm diameter spots  4-6 Ma >450 Ma <370 Ma Same spread within grains as between grains

What do the ‘ages’ mean? Crystallisation? Cooling? Contamination? Effect of geologic process(es)? Combination? Pressure Temperature

Oceanic sediments Indian continent Himalayan sediments Metamorphosed and melted Indian margin sediments Main Central Thrust

Mottram et al 2015

Geochronology basics What haven’t we covered: Intermediate daughter isotopes Initial disequilibrium Isotopic model ages Isochrons Fission track plus much more…

Take home messages Geochronometers – Initial P/D ratios – Closed vs open system behaviour – Which mechanisms for alteration, disturbance, ‘scatter’ in data (outside of analytical) may be expected? – Closure (and formation) temperature