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Author Topic: Why do older continents contain lower 144Nd/143Nd ratios?  (Read 4151 times)

Offline ConfusedEar

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I've read several articles that state that neodymium isotopes in sediment are a reflection of the neodymium content of the surrounding continental rocks. This seems straightforward and is used to deduce ancient ocean circulations by analysing sediment cores. However, I cannot find out why exactly older rocks would contain lower 144Nd/143Nd ratios. 144Nd is unstable and decays with time, but surely this has been happening since the Solar System came together and would be constant for all rocks. Can anybody explain why older rocks/continental shelves have lower 143Nd/144Nd ratios?

Thanks in advance.


 

Offline frethack

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Why do older continents contain lower 144Nd/143Nd ratios?
« Reply #1 on: 18/02/2010 22:27:14 »
Only a minor detail, but 144Nd is the stable isotope and 143Nd is the radiogenic daughter of 147Sm.

My guess would be that it has something to do with partial melting.  Both samarium and neodymium are incompatible (neodymium far more so) and very easily move from solid phase and into the melt when magmas are forming.  As a magma evolves and becomes more silicic, it also becomes enriched in trace elements such as Sm and Nd, as they would prefer to remain in the melt rather than crystallize.  Early continental crusts were more basic than the modern crust, and as a result, had less concentrated incompatibles.

This of course is an educated guess.  Bass or JimBob would be MUCH more helpful.
 

Offline Bass

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Why do older continents contain lower 144Nd/143Nd ratios?
« Reply #2 on: 19/02/2010 05:26:09 »
Only a minor detail, but 144Nd is the stable isotope and 143Nd is the radiogenic daughter of 147Sm.

My guess would be that it has something to do with partial melting.  Bass or JimBoob would be MUCH more helpful.

Frethack is correct on 144Nd being the stable isotope and 143Nd being the radiogenic daughter of 147Sm.  So the ratio measured is actually 143Nd/144Nd.

Since 147Sm continues to decay (albeit slowly), the abundance of 143Nd increases as the earth ages.  More 143Nd means a higher 143Nd/144Nd in younger rocks.  Compared to other isotopes, Sm and Nd are little affected by weathering or metamorphism- which makes them more reliable for dating. (I have yet to meet the isochron that Id ask on a date) [:X].

Nd and Sm behave differently during fractionation of igneous melts.  Nd prefers silicic minerals (continental) and Sm prefers mafic minerals (oceanic).  This results in lower 143Nd/144Nd in continental rocks, and a higher ratio in oceanic rocks.
 

Offline ConfusedEar

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Why do older continents contain lower 144Nd/143Nd ratios?
« Reply #3 on: 19/02/2010 11:46:26 »
Hi,
Thank you very much Frethack and Bass :), that makes a lot more sense! I was fairly sure that 144Nd decays into Cerium (140Ce) though?
 

Offline JimBob

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Why do older continents contain lower 144Nd/143Nd ratios?
« Reply #4 on: 20/02/2010 02:22:06 »
See you didn't need a sedimentologist to answer this one.

By the way what is the difference between silicic and mafic?

(Just kidding  ;D )
 

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Why do older continents contain lower 144Nd/143Nd ratios?
« Reply #4 on: 20/02/2010 02:22:06 »

 

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