Part 9- Zircon Geochronology Flashcards

0
Q

Zircons have a very high mu value. What does this mean?

A

They have a very high U/Pb ratio.

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1
Q

Give 4 reasons why zircon is great for geochronology.

A
  1. Resistant to mechanical weathering.
  2. Resistant to chemical weathering.
  3. Concentrates U but excludes Pb.
  4. Reasonably common accessory phase.
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2
Q

What special diagram does zircon produce?

A

The Concordia diagram.

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3
Q

What is the Concordia diagram a plot of?

A

206Pb/238U vs. 207Pb/235U

Pb* = radiogenic lead only.

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4
Q

What does the Concordia diagram show?

A

A locus of points for which the 238U-206Pb age is equal to the 235U-207Pb age.

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5
Q

How does then Concordia curve evolve with time?

A

It “grows” out of the origin of the plot.

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6
Q

Why does the curve get steeper and steeper with time?

A

Loss/use of 235U.

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7
Q

If zircon has experienced completely closed system behaviour, where would it plot on a graph?

A

Exactly on the Concordia curve.

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8
Q

What is the most common form of open system behaviour that zircons experience?

A

Pb-loss.

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9
Q

In what proportions would 206Pb and 207Pb be lost and why?

A

The same proportions because they are chemically identical.

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10
Q

Which is most common, complete or partial loss of Pb?

A

Partial loss.

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11
Q

What is the discordia?

A

A straight line that plots under the Concordia from the current age to the origin if the graph showing zircons with loss of Pb.

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12
Q

Why is the discordia so useful?

A

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.

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13
Q

Which part of the zircon grain typically yields the oldest age?

A

The core.

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14
Q

From oxygen isotope studies of the oldest zircons on earth, what has been deduced about the earth at 4404 Ma?

A

Had liquid water, therefore oceans even that long ago!

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