V31A-4720:
Early Earth evolution: new insight from Sm and Nd isotopes in meteoritic inclusions

Wednesday, 17 December 2014
Audrey Bouvier, University of Western Ontario, Earth Sciences, London, ON, Canada and Maud Boyet, Laboratoire Magmas et Volcans, Clermont-Ferrand Cedex, France
Abstract:
The interpretation of Sm-Nd systematics for the early Earth relies on knowing the composition of the silicate Earth and the 146Sm decay constant. We have measured both 146Sm-142Nd and 147Sm-143Nd internal systematics of four individual Calcium, Aluminum-rich Inclusions (CAIs), the first solids formed in the Solar System [1], from 3 different carbonaceous chondrites from the CV3 group: Allende, Northwest Africa (NWA) 2364 and NWA 6991. Results obtained on NWA 6991 plot on a well-defined mineral and bulk isochron with a Solar System initial 146Sm/144Sm ratio of 0.0070 ±0.0024. This ratio is more consistent with the ratio defined from internal isochrons of differentiated meteorites using the half-life of 103 Ma for 146Sm [2], instead of the value obtained considering the half-life of 68 Ma [3].

On the basis of nucleosynthethic anomalies in Sm and Nd isotopes [4], the ordinary (O) and enstatite (E) chondrites remain potential candidates for the Earth’s building blocks. OC have an average deficit of -18±3 ppm relative to modern terrestrial 142Nd/144Nd, whereas EC range from the OC to the terrestrial values [4-6]. Sm stable isotope compositions of the analyzed CAIs indicate that galactic cosmic rays did not affect the 142Nd/144Nd compositions, but deficits are found in the pure p-process 144Sm nuclide (-240 to -290 ppm/ standard). These deficits may translate to 142Nd deficits of a few ppm. NWA 6991 CAI 146Sm-142Nd internal isochron passes through a 142Nd/144Nd ratio of -6 ±6 ppm relative to the terrestrial standard at a chondritic 147Sm/144Nd of 0.1960. We note that this value is identical to the enstatite chondrite average and the 142Nd/144Nd ratio of the lunar mantle, as defined recently by [7] using a chondritic Sm/Nd and Lu/Hf for the bulk Moon. While the determination of the Sm-Nd reference parameters for the bulk Earth is still contentious, the difference in 142Nd/144Nd between modern terrestrial rocks and meteorites analyzed so far is <10ppm.

[1] Bouvier and Wadhwa (2010) Nat. Geosc. 3, 637. [2] Boyet et al. (2010) Earth Planet. Sci. Lett. 291, 172. [3] Kinoshita et al. (2012) Science 335, 1614. [4] Carlson et al. (2007) Science 316, 1175. [5] Boyet and Carlson (2005) Science 309, 576. [6] Gannoun et al. (2011) Proc. Nat. Acad. Sc. 108, 7693. [7] Sprung et al. (2013) Earth Planet. Sci. Lett. 380, 77.