Part 7- Lu-Hf and Re-Os Decay Systems

Question 0

What does 176Hf stand for?

Answer 0

Hafnium with a mass number of 176.

Question 1

What does 167Lu stand for?

Answer 1

Lutetium with a mass of 176.

Question 2

What decay does the Lu-Hf system undergo?

Answer 2

Beta/Negatron.

Question 3

What group of elements does Lu belong to?

Answer 3

Heavy REE.

Question 4

What is the compatibility of Lutetium?

Answer 4

Moderately incompatible but the most compatible of the REEs.

Question 5

What are the important carrier phases of Lu?

Answer 5

Garnet and Zircon.

Question 6

What group of elements does Hafnium belong to?

What are the characteristics of this group?

Answer 6

High Field Strength Elements (HFSE).

High charges and low atomic radii.

Question 7

What is the relationship between Zirconium and Hafnium?

Answer 7

They are geochemical twins-

Have the same charge (4+) and almost identical radii.

Question 8

Which is the most important mineral phase for Hf?

Answer 8

Zircon- mineral can’t tell the difference between Zr and Hf.

Question 9

Which rocks is the Lu-Hf system most useful for dating and in particular what type of rock?

Answer 9

Mafic rocks especially eclogites.

Question 10

Which decay system is the Lu-Hf system very similar to?

Answer 10

The Sm-Nd system.

Question 11

Is the the Lu-Hf system easily fluid-mobilised?

Answer 11

Not at all

Question 12

Of Lu and Hf which is more incompatible?

Answer 12

Hf the radiogenic daughter.

Question 13

What classifications do both belong to?

Answer 13

Refractory and lithophile.

Question 14

What Lu/Hf ratios do the BSE and BE have?

Answer 14

Similar to CI carbonaceous chondrites.

Question 15

What abundances of Lu and Hf are in small degree mantle melts (OIBs and CC) and why?

Answer 15

Relatively high abundances because they are incompatible.

Question 16

Why do OIBs and the CC have low Lu/Hf ratios?

Answer 16

Cpx and garnet are present in the magma source and they preferentially retain Lu (more compatible).

Question 17

What kind of Lu/Hf ratio does the depleted MORB mantle have and why?

Answer 17

High.

Hf is more incompatible than Lu in MORB melting.

Question 18

Why do MORBs have low Lu abundances?

Answer 18

The melting of them is too shallow for garnets to form so the Lu is not taken up. P

Question 19

What is the half life of the Lu-Hf decay system?

Answer 19

36 Gyr

Question 20

What does 187Re stand for?

Answer 20

Rhenium with a mass number of 187.

Question 21

What does 187Os mean?

Answer 21

Osmium with a mass number of 187.

Question 22

What decay does the Re-Os system undergo?

Answer 22

Beta/Negatron.

Question 23

Which of Goldschmidt’s classifications do Re and Os belong to?
What tendencies do they also have?

Answer 23

They are siderophile but also have chalcophile tendencies.

Question 24

Where are Re and Os concentrated in the Earth?

Answer 24

In the core and magmatic sulphides.

Question 25

What compatibility does Re have?

Answer 25

Mild incompatibility similar to HREE.

Question 26

What compatibility does Os have?

Answer 26

Strong incompatibility. (D = 10 vs. D = 0.1 for Re).

Question 27

What process can generate very strong Re/Os ratio fractionations?

Answer 27

Partial (mantle) melting.

Question 28

Give two uses of the Re-Os decay system?

Answer 28

Dating of iron meteorites, precious metal ores and some UM rocks.

Question 29

Which classifications do both Re and Os belong to?

Answer 29

Refractory and highly siderophile (HSE).

Question 30

What Re/Os ratio does the mantle have? Why is this strange?

Answer 30

Almost chondritic. They are siderophile therefore partitioned into the core.

Question 31

What is the theory to explain an almost chondritic Re/Os ratio in the mantle?

Answer 31

Core formation was completed before the accretion of the Earth was finished.