• Comprised of protons, neutrons and electrons:
• Protons: positive charge (+)
• Neutrons: no charge
• Electrons: negative charge (-)
• Protons and neutrons make up the nucleus while electrons orbit nucleus

• Atomic Number: sum of the number of protons in an element
• Atomic Weight: sum of the number of protons and neutrons in an element

• Isotopes of a given element have the same number of protons but differ in the number of neutrons
• Isotopes of an element therefore have same atomic number…
• but differ in atomic weight

• Gives absolute age
• Uses radioactive isotopes

• Radioactive element has unstable nucleus
• Radioactive decay: spontaneous break-up of nucleus to form a new element

• Parent: the original unstable radioactive isotope
• Daughter: the new isotope formed as a result of radioactive decay of parent.

-- unstable daughters along decay path

-- at end stable daughter

• Loss of 2 protons and two neutrons due to emission of Alpha particle.
• Atomic number decreased by -2.
• Atomic weight decreased by -4.

• Neutron changes to proton due to emission of Beta particle (electron)
• Atomic number increased by +1
• No change in atomic weight

• Proton captures electron and is changed into a neutron
• Atomic number decreases by -1
• No change in the atomic weight

• Time required for half the number of certain radioactive isotopes to decay to daughter isotopes

• If the half life of an isotope is known…
• and the parent/daughter ratio is determined…
• then the age of the sample can be calculated

-- a parent/daughter ratio of 1:1 means 1 half life has passed

--the age of the sample equals 1 half life

• Use elements with longer half lives to date older rocks
• K-40 (half life is 1.3 billion years) dates the oldest rocks
• C-14 (half life is 5,730 years)

-- dates archeological finds

-- recent geologic formations

• Meteorites: 4,300 - 4,600 million years
• Moon Rocks: 3,500 - 4,200 million years
• Pb dating Earth: 4,600 - 4,700 million years
• Oldest crustral rocks: 3,900 - 4,200 million years
• Beginning Phanerozoic Eon: 600 - 700 million years
• Paleozoic-Mesozoic boundary: 250 million years
• Mesozoic-Cenozoic boundary: 66 million years

• The isotopic clock can be reset by metamorphism when temperatures exceed the blocking temperature of the minerals containing daughter isotopes

• Mineral age is actually the time when mineral cooled below its blocking temperature
• Blocking Temperature:
• temperature below which daughter isotopes begin to accumulate in mineral
• temperature below which mineral becomes a closed chemical system for radioactive decay

• Some ²³⁸U parent isotopes spontaneously split to form two energetic particles
• Particles speed apart to produce fission tracks in the mineral
• Density of fission tracks counted under microscope
• Also measure amount of ²³⁸U still present in mineral using conventional instruments
• Age determined by comparing number of fission tracks relative to amount ²³⁸ U still remaining
• Half-life ²³⁸U fission about 10¹⁶ years
• Useful for dating samples less than 100 million years old

• Cosmic rays in atmosphere shatter gas atoms, release neutrons
• Some neutrons absorbed by nitrogen atoms, releasing protons
• 14N in atmosphere converted to ¹⁴C
• Living matter absorbs carbon dioxide containing ¹⁴C
• During life, the ¹²C / ¹⁴C ratio in living matter is constant
• After death, ¹⁴C in organism decays to ¹⁴N (half-life 5,730 years)
• Good for dating carbon-bearing material younger than 80,000 years

• Igneous rocks can provide an approximate crystallization age using whole rock analysis. Minerals also provide a crystallization age provided the rock has not experienced subsequent metamorphism.
• A metamorphic rock can provide an age for the last metamorphic event. If the rock experienced more than one episode of metamorphism, then usually only the most recent event can be dated.
• In general, clastic sedimentary rocks do not give meaningful ages because the minerals contained in these rocks were derived from other sources. At best, clastic minerals can only provide the age(s) of their parent source rocks.
• Isotopic dating of a sedimentary rock is only possible if it contains an authigenic mineral (e.g. glauconite or K-feldspar) that crystallized in the sedimentary environment of deposition.
• The ages of sedimentary rock packages can be bracketed by dating underlying and overlying igneous and metamorphic rocks, interlayered volcanic ash deposits and cross-cutting igneous dikes.