Precise determination of initial ?Nd from Sm-Nd isochron data

Current methods for evaluating Sm-Nd isochron data determine initial ?_Nd (?_Ndⁱ) values by extrapolation to ${}^{147}\text{Sm}{}^{144}\text{Nd = 0}$, resulting in unnecessarily large uncertainties. It is shown that values of ?_Ndⁱ can be optimised by determining the present ${}^{143}\text{Nd}{}^{144}\text{Nd}$ composition of a hypothetical sample having chrondritic $\text{Sm}\text{Nd}$. This method of determination requires only a simple data translation for its implementation using standard line-fitting techniques, and is particularly advantageous in providing precise ?_Ndⁱ values for sample suites of limited compositional range close to chondritic $\text{Sm}\text{Nd}$.A method of constructing error (uncertainty) polygons appropriate to the optimised (?_Ndⁱ, T) values is discussed, and optimum?_Ndⁱ values are tabulated for all published Precambrian Sm-Nd isochrons.