NEW Nd EVIDENCE FOR A NON-CHONDRITIC COMPOSITION OF THE MOON

Introduction: The coupled Sm-Nd systematics of lunar samples has been extensively studied for estimating the timescale of lunar differentiation [1-3]. The published datasets yield consistent ages for Nd isotopic closure within the lunar mantle of ~200 Myr after CAI formation. Although this time constraint is consistent with estimates derived from Hf-W chronometry of the Moon (>60 Myr after CAI formation, [4]), there is debate as to whether this age has chronological significance. Furthermore, there are discrepancies regarding the Nd isotope composition of the bulk Moon. Rankenburg et al. [2] obtained a εNd vs. Sm/Nd correlation for lunar samples passing though the chondritic reference value (Sm/Nd = 0.1967, εNd = -0.21 [5]), suggesting that the Moon has a chondritic bulk composition. In contrast, the other datasets [1,3] define a correlation line that passes ~10-20 ppm above, suggesting that the Moon has a superchondritic Sm/Nd (∼0.206, [6]), close to that of the early depleted Earth (EDM). We present new Sm-Nd data for a high-Ti mare basalt (70135), two low-Ti mare basalt (LAP 02205 and MIL 05035) and a KREEPy low-Ti mare basalt (NWA 2977). These data are used to evaluate the significance of the Sm-Nd systematics for constraining the timescale of lunar differentiation and the bulk Nd isotope composition of the Moon. Methods: The Nd measurements were performed in dynamic mode with amplifier rotation using the Thermo-Finnigan Triton TIMS at ETH Zurich. Each run was performed with a signal intensity of ~8 V for Nd and at least 500 ratios were measured, resulting in within-run statistics of 2-5 ppm. Sm and Ce isobaric interferences were monitored but for all samples the interference corrections were insignificant. All isotope ratios were normalized to Nd/Nd=0.7219 using the exponential law. Neodymium isotope compositions of the samples were determined relative to repeated measurements of the Nd standard JNdi-1 over one year that yield a Nd/Nd of 1.141837 ± 0.000006 (n=50, 2σ SD) and a Nd/Nd of 0.512035 ± 0.000012 (n=50, 2σ SD, static mode). The accuracy of our measurements was monitored by repeated measurements of two gravimetrically prepared spike-standard mixtures having Nd anomalies of 23 and 84 ppm. Sm and Nd contents were measured by isotope dilution. Results: Sm-Nd data obtained here are shown in Fig. 1a and 1b. For each sample, the Sm/Nd source ratio is calculated using a three-stage model to reach its measured Nd/Nd. The source first evolved with a chondritic Sm/Nd ratio from 4.568 Ga ago until crystallization of the LMO at 218 Ma after CAI formation, then with the calculated Sm/Nd ratio until the eruption age and finally with the measured Sm/Nd ratio to its present-day value. The KREEPy meteorites have the lowest (Sm/Nd)source (∼0.16) and least radiogenic εNd (∼-20 ppm). In contrast, the high-Ti mare basalt 70135 has the highest (Sm/Nd)source (∼0.29) and εNd (∼+19 ppm). Low-Ti mare basalts have (Sm/Nd)source and εNd intermediate between KREEP and high-Ti mare basalts.