Dear Damera
- Chondritic uniform reservoir (CHUR) is a scientific model in astrophysics and geochemistry for the mean chemical composition of the part of the Solar Nebula from which, during the formation of the Solar System, chondrites formed. This hypothetical chemical reservoir is thought to have been similar in composition to the current photosphere of the Sun.
- Bulk silicate earth (BSE) refers to the original chemical composition of the silicate part of the Earth after the accretion and separation of a core but prior to differentiation of the first crust.
BSE= primitive mantle
for more info:
https://link.springer.com/content/pdf/10.1007/978-3-642-27833-4_5237-1.pdf
https://www.revolvy.com/main/index.php?s=Chondritic+uniform+reservoir
Regards
See Rollinson 1993 book about 'CHUR' and 'BSE'.
Basically, they are the base models used for comparing geochemical composition of depleted or enriched mantle, oceanic crust, upper and lower continental crust, various rocks, etc.
I would add that the BSE is mainly used for comparisons of abundances of trace elements and incompatible elements in rocks (e.g., in spider diagrams).
CHUR was introduced by DePaolo and Wasserburg in 1976 (DePaolo, D. J. and Wasserburg, G. J. (1976) Nd isotopic variations and petrogenetic models. Geophysical Research Letters, 3 (5). pp. 249-252 ) as the ratios 143Nd/144Nd and 147Sm/144Nd in a chondrite-like early Earth. After a melt is extracted on Earth, the same ratios for the mantle and the crust are compared to the CHUR ratios (variable with time because of the decay of 147Sm into 143Nd). For convenience they developed the expression εNd, supposed to represent the degree of Nd depletion or enrichment, and to allow the dating of rocks. The concept of CHUR has since been used for other isotope pairs such as Lu and Hf.