What is Chondritic uniform reservoir?

The chondritic uniform reservoir (CHUR) is a model reservoir with the samarium–neodymium composition of the chondrites, closed since the formation of the Solar System. Its present ratios are $(^{143}\mathrm{Nd}/^{144}\mathrm{Nd})_{\mathrm{CHUR}} = 0.512638$ and $(^{147}\mathrm{Sm}/^{144}\mathrm{Nd})_{\mathrm{CHUR}} = 0.1966$.