Download Radiometric Dating

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 = (et – 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