GY305 Geophysics. Radioactive Decay

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1 GY305 Geophysics Radioactive Decay

2 Periodic Table of Elements Periodic table of the elements by atomic number

3 Atomic Particles and Isotopes Protons: mass = 1; charge = +1 Neutron: mass = 1; charge = 0 Electron: mass = 0; charge = 1 Isotope: an isotope of an element has a specific number of protons that identifies the element; but may have varying numbers of neutrons Atomic Number: sum of the protons in the nucleus of the atom Atomic Mass: sum of the protons and neutrons in the nucleus of the isotope Example: 39Ar is the isotope of the element argon that has an atomic mass of 39 (14 Protons + 25 Neutrons) Elemental Weight: weighted average of all of the known isotopes of an element Example: K = (most K isotopes are 39K but there are a small percentage of 40K that moves the average mass to 39.1)

4 Radioactive Decay Systematics Certain isotopes spontaneously decay by the release of radiation Alpha Decay: ejection from nucleus of 2 protons and 2 neutrons Beta decay: ejection from a neutron of an electron Gamma decay: release of energy in the form of gamma radiation via electron capture (converts a proton to a neutron

5 Examples of Decay 147Sm > 143Nd (1 alpha particle) Atomic mass lowered by 4 (2 protons + 2 neutrons lost) Atomic number lowered by 2 (2 protons lost) (Sm=62; Nd=60) 87Rb > 87Sr (1 beta particle) Atomic mass unchanged by loss of electron Atomic number increased by one (Rb=37 ; Sr=38) 40K > 40Ar (gamma decay via electron capture) Atomic mass unchanged by gain of electron Atomic number decreased by one (K=19; Ar=18) Quantum mechanics states that it is impossible to predict whether or not a specific isotope atom will decay, however, the number of parent isotope atoms that decay to daughter isotopes over large intervals of time is a constant (i.e. radioactive decay constant)

6 Decay Systems

7 Abundances of Radioactive Parent/Daughter Isotopes used in Radiometric Dating Note that all but Nd/Sm are concentrated in felsic to intermediate rocks

8 Closure Temperature for Radiometric Systems At temperatures above the closure threshold the system is open and daughter products are lost The radiometric clock does not start until the temperature is below the closure threshold Note that closure T is within the realm of metamorphism so rocks do not have to be melted to re set the radiometric clock

9 Isochron Diagrams Isochron diagram calculates the age of the geological sample from isotopic analysis The slope ( t) of the best fit line is proportional to the age The Y intercept of the isochron (87Sr/86Sr) provides information about the origin of the material 86Sr is not part of a decay sequence it is used for convenience in measuring isotopic abundance

10 Isochron Diagram for Rb/Sr Isochron is a statistical best fit (linear regression) 86Sr is nonradiogenic Y intercept is 87Sr/86Sr initial ratio at time of last homogenization

11 Practical Measurement of Age from Isochron The slope of the isochron is calculated from the slope coefficient of the linear regression equation (y = mx+b ; m=slope) Slope = (e t 1) Ln(slope) = t Ln(1) Ln(slope + 1)/ = t {where t = age of sample; = decay constant}

12 Example Layout of a Isochron Spreadsheet

13 87Sr/86Sr Growth Curves Mantle growth curve for 87Sr/86Sr generates values <= Crustal growth curves generate values >= 0.708

14 U/Pb Concordia Equation Uses the 235U>207Pb and 238U > 206Pb systems λ 235 = 9.85e 10 λ 238 = 1.55e 10

15 Concordia Diagram for 235U/238U Ratios 2 systems: 235U>207Pb and 238U>206Pb Because 238U decays slower than 235U the Concordia curve is concave down Discordant samples indicate 2 separate events

16 Example Concordia Spreadsheet Layout

17 Limitations of Radiometric Dating All radiometric systems require a starting homogenization event: Melting Metamorphism Hydrothermal alteration Mineral isochrons date the last homogenization event Whole rock isochrons date the genetic event Sedimentary rocks are rarely dated by isotopic methods: Cement Chemical/Biochemical sediments Detrital zircons may yield minimum age of source rock Most systems are specific to intermediate or felsic igneous rocks (U, Th, Rb, K) Mafic/Ultramafic rocks: Sm/Nd Most valuable dates come from volcanics because they obey the law of superposition

18 Problems with Radiometric Dates Contamination Above ground nuclear weapons testing produces Pb isotopic fallout U/Th/Pb samples must be processed in clean rooms K/Ar: daughter product is a gas and may be easily lost U/Pb is contained in refractory minerals Ion probe can overcome isotopic zonation in zircon/sphene Rb/Sr are susceptible to leaching by hydrothermal fluids