Sm-Nd systematics of chondrites

We have studied the ¹⁴⁷Sm-¹⁴³Nd and ¹⁴⁶Sm-¹⁴²Nd isotopic systems in phosphate fractions and chondrules from six ordinary chondrites and one carbonaceous chondrite, previously dated with Pb-Pb method. ¹⁴⁷Sm/¹⁴⁴Nd ratios vary between 0.182 and 0.191 in phosphates, and between 0.179 and 0.243 in chondrules. The ¹⁴⁷Sm-¹⁴³Nd 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 ¹⁴³Nd/¹⁴⁴Nd is 0.50665±0.00014. The same analyses define a ¹⁴⁶Sm-¹⁴²Nd isochron with a slope corresponding to ¹⁴⁶Sm/¹⁴⁴Sm=0.0075±0.0027. Initial ¹⁴²Nd/¹⁴⁴Nd=1.14160±0.00011 corresponds to ε ¹⁴²Nd=-2.62±0.93. Compilation of the published chondritic whole rock Sm-Nd analyses yields the median ¹⁴⁷Sm/¹⁴⁴Nd=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 ¹⁴³Nd/¹⁴⁴Nd=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 ¹⁴⁷Sm/¹⁴⁴Nd=0.1941±0.0059 is obtained from intercept of chondritic ¹⁴⁶Sm-¹⁴²Nd isochron with the terrestrial value of ¹⁴²Nd/¹⁴⁴Nd. This estimate is independent of measured Sm/Nd ratios in chondrites. The same approach was applied to published ¹⁴⁶Sm-¹⁴²Nd 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.