Radiometric Dating Flashcards
What is radiometric dating?
This is a form of absolute dating where you get a number at the end. Based on radioactive decay and calculating age off known half live times.
Age = number of units that have changed since instability / rate of activity.
Assumptions of radiometric dating
- Half lives are constant through time.
- No contamination of sample (no daughter or intermediate elements have been introduced or leached from material.
- We can determine how much daughter element was there to begin with.
What radioactive elements do we use and what are their half lives?
14C to 14N: 5730 years.
238U to 206Pb: 4.5 Bn yrs.
40K to 40Ar: 1.3 Bn yrs.
What evidence sources can we find each radioactive element in that we want to date?
Carbon: shells, bones, plants.
Uranium: Coral, speleothems.
Potassium: Igneous and metamorphic rocks.
What does luminescence dating do?
It is based on the accumulation of radiation energy in the crystal lattice of rock grains. We can date the last exposure to sunlight that empties the stored energy out of the lattice (80% of trapped charge lost in 1s of sunlight exposure). So we date when the system was buried.
Note: There is a subtle different between age of deposition of sediment and its burial. This could be 1 week - 1 year, but that doesn’t matter when dating something over 1000 years old.
Age limit of radiometric dating
After 5 half lives there is hardly any parent material left, so the temporal limit of each technique is 5* the half life length, e.g. for carbon is around 40,000 years.
Assumptions of radiocarbon dating
- C14 in atmosphere is constant through time. We know this to vary with solar cycles, with less C14 in cold phases.
- C14 ages are not real years and drift through time, we need recalibration using ice core gas samples.
- Assumes every organism are absorbing radiocarbon in equilibrium with the environmental state. In part this isn’t the case due to fractionation where rocks absorb more than plants, however we can represent the error in the age value.
- Assumes different reservoirs of carbon have the same concentrations.
- Assumes no contamination.
How do we calculate age with luminescence dating?
We measure the luminescence given off in the laboratory.
Age = accumulated radiation dose (Gy) / Radiation dose rate (Gy/yr).
What is the dating range of luminescence dating?
Between 100 and 100,000 years, with error limits of 3-10%. It is also able to date things that lack carbon.
Quartz and feldspar rich sands are most suitable.
What are the assumptions of luminescence dating?
- Reprecipitation and or disequilibrium and water content changes. (water can absorb radiation)
- We know what event is being dated, has the sediment been moved, mixed, reworked?
- We assume full resetting prior to burial, removing all of the signal.
