Testing Models for the Origin of the Earth-Moon System with 142Nd/144Nd Measurements

Abstract:

The Sm-Nd system is widely used for tracking the differentiation and evolution of planetary silicate reservoirs, due to the well understood, strong Sm-Nd fractionation between melt and mantle minerals. The short-lived ¹⁴⁶Sm-¹⁴²Nd system with a half-life of 103 Ma or 68 Ma has been used to constrain early planetary differentiation events based on early Archean terrestrial rocks, lunar rocks and meteorites. Early Archean terrestrial rocks show significant variations in ¹⁴²Nd/¹⁴⁴Nd of about 30 ppm, demonstrating very early differentiation of the Earth’s mantle and crust. In contrast, present day ¹⁴²Nd/¹⁴⁴Nd ratios of mantle-derived ocean island basalts and MORBs show almost no variation at the reported analytical precision level (2σ = ± 6 ppm), suggesting that such early variations have been erased with time due to crustal recycling and mantle mixing. The ¹⁴²Nd/¹⁴⁴Nd ratio of the lunar mantle has been reported to be offset from terrestrial standards by about –5 ppm, barely resolvable with the reported analytical uncertainties. Differences in the ¹⁴²Nd/¹⁴⁴Nd ratios between the bulk Earth and Moon may suggest early large scale silicate differentiation events on the Earth that predate the Giant Moon forming impact. To address this problem, we carry out new ¹⁴²Nd/¹⁴⁴Nd measurements of terrestrial rocks, and lunar rocks and meteorites with a TIMS (Isoprobe T) equipped with new Xact Faraday amplifiers provided by Isotopx. We find that the Xact amplifiers provide lower noise than the earlier generation preamplifiers and operate close to the theoretical thermodynamic noise limit calculated from the Johnson equation. So far we have been able to improve multidynamic measurements to be reproducible to within ± 2 ppm at the 2σ level, and with this precision we find no variations in a few young terrestrial rocks. Our next step is measurements of lunar rocks and E-chondrites. If these turn out to be identical to the modern Earth, then the Nd isotope system may tell the same story as isotope ratios of many other elements: they are identical in the Earth, Moon and E-chondrites, and different from most other Solar System objects. There would thus be no need to postulate hidden reservoirs in the Earth’s mantle to explain the ¹⁴²Nd/¹⁴⁴Nd ratios of the modern Earth’s mantle.