Naked Science Forum

Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: ConfusedEar on 18/02/2010 11:12:25

Title: Why do older continents contain lower 144Nd/143Nd ratios?
Post by: ConfusedEar on 18/02/2010 11:12:25

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.

Title: Why do older continents contain lower 144Nd/143Nd ratios?
Post by: frethack 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.

Title: Why do older continents contain lower 144Nd/143Nd ratios?
Post by: Bass on 19/02/2010 05:26:09

Quote from: frethack 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.  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 I’d 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.

Title: Why do older continents contain lower 144Nd/143Nd ratios?
Post by: ConfusedEar 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?

Title: Why do older continents contain lower 144Nd/143Nd ratios?
Post by: JimBob 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] )