Part 7- Lu-Hf and Re-Os Decay Systems Flashcards
What does 176Hf stand for?
Hafnium with a mass number of 176.
What does 167Lu stand for?
Lutetium with a mass of 176.
What decay does the Lu-Hf system undergo?
Beta/Negatron.
What group of elements does Lu belong to?
Heavy REE.
What is the compatibility of Lutetium?
Moderately incompatible but the most compatible of the REEs.
What are the important carrier phases of Lu?
Garnet and Zircon.
What group of elements does Hafnium belong to?
What are the characteristics of this group?
High Field Strength Elements (HFSE).
High charges and low atomic radii.
What is the relationship between Zirconium and Hafnium?
They are geochemical twins-
Have the same charge (4+) and almost identical radii.
Which is the most important mineral phase for Hf?
Zircon- mineral can’t tell the difference between Zr and Hf.
Which rocks is the Lu-Hf system most useful for dating and in particular what type of rock?
Mafic rocks especially eclogites.
Which decay system is the Lu-Hf system very similar to?
The Sm-Nd system.
Is the the Lu-Hf system easily fluid-mobilised?
Not at all
Of Lu and Hf which is more incompatible?
Hf the radiogenic daughter.
What classifications do both belong to?
Refractory and lithophile.
What Lu/Hf ratios do the BSE and BE have?
Similar to CI carbonaceous chondrites.
What abundances of Lu and Hf are in small degree mantle melts (OIBs and CC) and why?
Relatively high abundances because they are incompatible.
Why do OIBs and the CC have low Lu/Hf ratios?
Cpx and garnet are present in the magma source and they preferentially retain Lu (more compatible).
What kind of Lu/Hf ratio does the depleted MORB mantle have and why?
High.
Hf is more incompatible than Lu in MORB melting.
Why do MORBs have low Lu abundances?
The melting of them is too shallow for garnets to form so the Lu is not taken up. P
What is the half life of the Lu-Hf decay system?
36 Gyr
What does 187Re stand for?
Rhenium with a mass number of 187.
What does 187Os mean?
Osmium with a mass number of 187.
What decay does the Re-Os system undergo?
Beta/Negatron.
Which of Goldschmidt’s classifications do Re and Os belong to?
What tendencies do they also have?
They are siderophile but also have chalcophile tendencies.
Where are Re and Os concentrated in the Earth?
In the core and magmatic sulphides.
What compatibility does Re have?
Mild incompatibility similar to HREE.
What compatibility does Os have?
Strong incompatibility. (D = 10 vs. D = 0.1 for Re).
What process can generate very strong Re/Os ratio fractionations?
Partial (mantle) melting.
Give two uses of the Re-Os decay system?
Dating of iron meteorites, precious metal ores and some UM rocks.
Which classifications do both Re and Os belong to?
Refractory and highly siderophile (HSE).
What Re/Os ratio does the mantle have? Why is this strange?
Almost chondritic. They are siderophile therefore partitioned into the core.
What is the theory to explain an almost chondritic Re/Os ratio in the mantle?
Core formation was completed before the accretion of the Earth was finished.
