After reading around the subject, I found a lot of literature suggesting that Th is an indication of diagenesis and not a factor of province. Why is this?
West et al. 2013 and Rider 2000 both mention Th in terms of diagenesis but don't seem to go into detail of U and K when talking about XRF and spectral gamma logs. From the mention of Th alone I thought that it maybe an indication but can't understand why? Is it because secondary mineralization is richer in Th than in initial mineralization?
Th is an immobile element during alteration so it is used as a reference for other elements. Hope this helps!
Th is relatively immobile and is associated mostly with the detrital clay fraction of the sediment, as compared to U or K. So it is used as a reference element to study the diagenetic or depositional variation in the sediments. e.g. High U/Th ratio (>1.25) suggests deposition of sediments in anoxic condition, whereas lower values (
As emphasized by C Chauvel and S Acharya, Th-232 is very little mobile. However it may undergo some mobility in forest soils, probably through the influence of organic colloids.
Th-230 and Th-228 may be more easily leached because of the recoil effect.
Dear Shiba Acharya ·
May I ask where you read this? Can you send me some refences about? .
Please writeme to
Thank you
Ricardo
The basis for using Th/U as en environmental fingerprint, is indeed that U is redox-sensitive while Th is almost insoluble.
Actually, U is easily displaced in early diagenesis, particularly in oxic conditions. But one shoud take care that U is not necessarily removed from the formation; it may be just displaced from an oxidised bed (say a sand flushed by meteroric water) towards the nearby (reduced) shale beds where organic matter is preserved.
The second difficulty when using Th/U is that Th is not constant at all in sediments. Th content is generally moderate in distal claystone (say 20 ppm), and often very low in coarse sands, but fine grained sediments (say silts) often have a much higher Th content if they contain detrital monazite (from granitic or high grade metamorphic sources). The main control on Th content is the granulometry of the sediment (if siliciclastic, of course), and it depends also very strongly on the source mineralogy.
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