Sm-Nd systematics of chondrites

We have studied the (SM)-S-147-Nd-143 and Sm-146-Nd-142 isotopic systems in phosphate fractions and chondrules from six ordinary chondrites and one carbonaceous chondrite, previously dated with Pb-Pb method. (SM)-S-147/Nd-144 ratios vary between 0.182 and 0.191 in phosphates, and between 0.179 and 0.243 in chondrules. The Sm-147-Nd-143 isochron regression through all 34 phosphate and chondrule analyses yields a date of 4588 +/- 100 Ma and is in good agreement with more precise Pb-Pb dates of the same chondrites. The initial Nd-143/Nd-144 is 0.50665 +/- 0.00014. The same analyses define a Sm-146-Nd-142 isochron with a slope corresponding to (14)6Sm/Sm-144 = 0.0075 +/- 0.0027. Initial Nd-142/Nd-144 = 1.14160 +/- 0.00011 corresponds to epsilon(142)Nd = -2.62 +/- 0.93. Compilation of the published chondritic whole rock Sm-Nd analyses yields the median Sm-147/Nd-144 = 0.1964 +/- 0.0003/-0.0007, which is our preferred Chondritic Uniform Reservoir (CHUR) value. Using this value and its error limits, we find the present-day CHUR Nd-143/Nd-144 = 0.512637 + 0.000009/ -0.000021 from the chondritic Sm-Nd isochron that includes all available data for whole rocks, chondrules and phosphates. This value is identical within error with the currently accepted number. An estimate of the bulk earth Sm-147/Nd-144 = 0.1941 +/- 0.0059 is obtained from intercept of chondritic Sm-146-Nd-142 isochron with the terrestrial value of Nd-142/Nd-144. This estimate is independent of measured Sm/Nd ratios in chondrites. The same approach was applied to published Sm-146-Nd-142 internal isochrons for differentiated meteorites and yielded similar, although less precise, values. Our data are completely consistent with the currently accepted CHUR parameters and substantiate their use as terrestrial reference values. (C) 2004 Elsevier B.V. All fights reserved.