Part 9- Zircon Geochronology Flashcards
Zircons have a very high mu value. What does this mean?
They have a very high U/Pb ratio.
Give 4 reasons why zircon is great for geochronology.
- Resistant to mechanical weathering.
- Resistant to chemical weathering.
- Concentrates U but excludes Pb.
- Reasonably common accessory phase.
What special diagram does zircon produce?
The Concordia diagram.
What is the Concordia diagram a plot of?
206Pb/238U vs. 207Pb/235U
Pb* = radiogenic lead only.
What does the Concordia diagram show?
A locus of points for which the 238U-206Pb age is equal to the 235U-207Pb age.
How does then Concordia curve evolve with time?
It “grows” out of the origin of the plot.
Why does the curve get steeper and steeper with time?
Loss/use of 235U.
If zircon has experienced completely closed system behaviour, where would it plot on a graph?
Exactly on the Concordia curve.
What is the most common form of open system behaviour that zircons experience?
Pb-loss.
In what proportions would 206Pb and 207Pb be lost and why?
The same proportions because they are chemically identical.
Which is most common, complete or partial loss of Pb?
Partial loss.
What is the discordia?
A straight line that plots under the Concordia from the current age to the origin if the graph showing zircons with loss of Pb.
Why is the discordia so useful?
As time goes on the discordia will remain the line between the age of zircon formation and the age of the metamorphic event. With one measurement we can date both.
Which part of the zircon grain typically yields the oldest age?
The core.
From oxygen isotope studies of the oldest zircons on earth, what has been deduced about the earth at 4404 Ma?
Had liquid water, therefore oceans even that long ago!
