SM-ND SYSTEMATICS OF CHONDRITES

Abstract

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 ε142Nd=−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.