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GY305Geophysics Radioactive Decay Periodic Table of Elements • Periodic table of the elements by atomic number 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 = 39.10 (most K isotopes are 39K but there are a small percentage of 40K that moves the average mass to 39.1) • • • • 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 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) Decay Systems Abundances of Radioactive Parent/Daughter Isotopes used in Radiometric Dating • Note that all but Nd/Sm are concentrated in felsic to intermediate rocks 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 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 Isochron Diagram for Rb/Sr • Isochron is a statistical best‐fit (linear regression) • 86Sr is non‐ radiogenic • Y‐intercept is 87Sr/86Sr initial ratio at time of last homogenization 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 = (et – 1) • Ln(slope) = t ‐ Ln(1) • Ln(slope + 1)/ = t {where t = age of sample; = decay constant} Example Layout of a Isochron Spreadsheet 87Sr/86Sr Growth Curves • Mantle growth curve for 87Sr/86Sr generates values <= 0.704 • Crustal growth curves generate values >= 0.708 U/Pb Concordia Equation • Uses the 235U>207Pb and 238U > 206Pb systems • λ235 = 9.85e‐10 • λ238 = 1.55e‐10 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 Example Concordia Spreadsheet Layout 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 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