Lecture 15 // Age Dating Flashcards
Isotopes
Atoms of the same element that have different mass numbers because they differ in the numbers of neutrons in the nucleus.
• Eg. Carbon-12 and Carbon-14
Radioisotopes
Some isotopes are unstable and will spontaneously convert (decay) to more stable isotopes.
Radioisotopic decay
The release of atomic particles and radiation (energy + heat).
- Eg. Carbon-14 decays by releasing a beta particle. (a proton is converted to a neutron and an electron in the nucleus, and the electron is ejected)
- The Carbon-14 thus loses a proton and becomes Nitrogen-14. (the atomic number is increased by one but the mass number remains the same)
Parents & daughters isotopes
Radioisotopes are termed parents (eg. U238), the stable isotopes they convert into are termed daughters (eg. Pb206).
Mass spectrometer.
The proportions of parents and daughters within a sample of a mineral grain can be determined using a mass spectrometer.
Isotopic decay
Isotopic decay occurs at a uniform, fixed rate (the decay rate goes unchanged over geologic time). If you know the rate and the proportion of parents and daughters, then you can determine the age.
Half life
- The time required for half of the parents in a mineral grain to decay.
- Let’s say that the proportion of parents to daughters is 1:31. That means that for every 32 atoms, there are 1/32 parents and 31/32 daughters.
- Some radioisotopes have short half lives (eg. C-14 is 5730 years) but most are very long (eg. U-238 is 4.5 billion years, eg. K-40 is 1.25 billion years).
- C-14 is used to date organic matter up to 50,000 years old. The other methods are used to date the age of igneous and metamorphic rocks of any age.
Determining the radioisotopic age of minerals within rock…
- Using a mass spectrometer to determine the ratio of radioactive parent isotopes and their stable daughter isotopes within a single mineral grain.
- Knowing the half-life of the radioactive parent isotope.
- Plugging in that information into a known formula.
What do you find out if you
radioisotopic-age date…
Igneous rocks:
• The age of crystallization of that mineral from magma or lava.
Metamorphic rocks:
• The age or recrystallization (due to heat, pressure, and/or hot fluid activity).
Sedimentary rocks:
• The age of the source of that mineral grain (original source would have been either igneous rock or metamorphic rock)…NOT the age of deposition of the sediments!
Determining a sequence of
geologic events.
Geologic events: 1. Deposition of strata 2. Deformation (faulting / folding / tilting) 3. Igneous intrusion (plutons such as dikes and batholiths) 4. Metamorphism 5. Erosion
Strata = any material laid down on the Earth’s surface as layers. This includes all sediments as well as volcanic ash and lava flows.
Geologists use seven basic stratigraphic principles in relative age dating.
- Superposition
- Original Horizontality
- Original Lateral Continuity
- Cross Cutting Relationships
- Inclusions
- Fossil Succession
- Unconformities
Superposition
Within most sequences of strata, the oldest layer is at the bottom, and the layers get progressively younger as you move upwards. However, this principle does not hold true in all instances.
Original Horizontality
Strata is typically deposited in horizontal layers. If horizontally laid down strata is found otherwise, then deformation must have occurred following deposition.
Original Lateral Continuity
Strata continues in all directions until it either thins out, grades into another sediment type, or comes up against a barrier. This principle is used when correlating beds from one location to another.
Cross Cutting Relationships
A unit (or feature) that cuts across another must be younger than the rock(s) it cuts across.
• They must have already been there to get cut!
• Applies to intrusions
• Applies to faults
