Lecture 22
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What is the first assumption of radiometric dating?
the rate of decay is known and constant
What is assumption 2 of radiometric dating?
The daughter nuclides are all produced by the radioactive decay process (238U produces 206Pb)
What do you need for radiometric dating?
A material that traps radioactive isotopes. parent isotopes
In uranium-lead dating, what does volcanic ash do?
The ash made up of tiny shards of galss that live in the air until they eventually fall down with rain form the ash bed layer
Where is Zircon found?
In the ash bed layer
What does Zircon do?
It is a mineral that traps information that allows us to determine the age of the ash bed by accepting Uranium into the crystal, but rejecting Pb when it is formed.
How does Zircon allow us to date the ash bed?
Any Pb within the crystal was formed by the Uranium inside of it
What are the characteristics of Zircon?
It is highly refractory, common in Earth’s crust, and at the time of crystallization is when the radioactive clock starts ticking
What is the fieldwork portion of U-Pb dating?
collecting volcanic ash and crushed rocks and finding zircons, it takes over 2 weeks
What is the lab work of U-Pb dating?
In about 1.5 weeks, you chemically separate isotopes of U and Pb. Then you measure the ratios of U and Pb isotopes in a mass spectrometer for about 4 hours
What is a mass spectrometer?
The Thermal Ionization Mass Spectrometer at LDEO, has a source, a giant magnet, and a detector
The ions produced from the source do what?
They separated on the basis of their mass/charge ratios where a light isotope deflects a larger amount then a heavier isotope which deflects by a smaller amount
What is the age equation?
206Pb/238U (experimentally determined) = (e^(half-life *238(known) *time) -1)