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

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.

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