What are the effects of grain size and/or sedimentary processes on Sr-Nd isotopic compositions in provenance analysis?
Dear Imran,
Large grain sizes like pebbles, cobbles and boulders would allow more accurate inferences on the provenance (the source of detritus), particularly based on the Rb-Sr system. Large grain sizes suggest that, (a) limited physical break down of the source has taken place and the chemical weathering is at a minimum, thus, fractionation of Rb/Sr during the weathering and the sedimentary process is at a minimum. (b) the sediments are lying more closer the source area. Thus, one could expect that a pebbly, cobbly or bouldery sediment to preserve the Rb-Sr isotopic signature of the source to a larger extent. As chemical weathering is having extremely limited influence on the Sm-Nd system, inferences about the source (provenance) based on that system in coarse sediments is very accurate as it is almost unaffected. Contrary, the small grain sizes (sand, silt and clay) mean that the physical break down is at a maximum promoting chemical weathering, thus fractionation of elements during sedimentary processes (particularly Rb/Sr) is very heavy, and the source is far away from the sediment and there was plenty of time available for chemical modification. Thus, isotopic signature of the provenance has been significantly altered. One has to remember, however, that even the coarse sediments are not fully sorted, thus, there are plenty of fine to medium-grained material admixed with it allowing some deviations from the true provenance age. Hope this will be of help to you.
Best Wishes!
LRK
Dear Imran Khan,
Study on grain size is not sufficient and you need to consider for petrology and mineralogy aspects for identifying the provenance analysis using REE species. And the usually the finer grain size will increase the concentration of Nd but decrease the concentration of Sr. However become wondering how to measure Nd isotopic composition in sediment as we known in dissolves realms. This very interesting!. Kindly please refer to paper enclosed;
regard's
Zolhizir bin daud
Hello Imran,
The answer to your question depends on two factors: 1) the provenance and antiquity of your source(s); 2) whether the isotope system is affected by weathering or alteration processes. In short, a very small amount of ancient sediment input into a depositional/weathering system can significantly alter bulk isotopic composition - especially if the system is otherwise 'juvenile'. In effect a small amount of a very different composition can greatly skew the signature of the host.
We looked at Nd and Sr isotopes in weathering profiles in India. Both isotope systems are significantly affected by the input of aeolian(?) dust (loess- possibly) derived from the surrounding ancient (Archean-Proterozoic basement). Babechuk et al. 2014 and 2015. (See my Researchgate page). To my knowledge, there is no known way of significantly fractionating either Sr or Nd isotopes during parent rock breakdown by weathering process alone.
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