Hf-W Systematics of Cumulate Eucrites and the Chronology of the Eucrite Parent Body

Introduction: Key issues regarding the early evolution of planetary bodies are the timescales of accretion, core formation, and silicate differentiation. The extinct Hf-W decay system has proven particularly useful as a chronometer for these early planetary processes because Hf-W fractionations occurred during both core formation and melting processes in the silicate mantle. The latter is mainly related to the presence of clinopyroxene and ilmenite that have very high Hf/W ratios. For instance, the crystallization of these minerals in the lunar magma ocean produced large Hf/W variations in the lunar mantle, such that Hf-W chronometry could provide a precise age for the crystallization of the lunar magma ocean [1]. The combined Sm-Nd and Lu-Hf isotope systematics of cumulate eucrites indicate that ilmenite was present in their source regions [2]. Therefore, cumulate eucrites may exhibit highly variable Hf/W ratios and the formation of their source region can potentially be dated using the Hf-W system. Here we present new Hf-W data for five cumulate eucrites (Serra de Mage, Moore County, Talampaya, Moama, and Binda) and use these data to constrain the chronology of core formation and mantle differentiation in the eucrite parent body. Methods: Samples (~1 g) were finely crushed in an agate mortar and ~400 mg of the whole rock powder were dissolved in Savillex beakers using HFHNO3 at 120°C for 72 hours. After drying, the samples are re-dissolved several times in HNO3-H2O2 and finally in 6 M HCl-0.06 M HF. At this stage, complete dissolution is achieved and a ~5% aliquot is spiked with a mixed Hf-W tracer for concentration determination by isotope dilution. The remaining ~95% were first loaded onto cation exchange columns to remove most of the sample matrix. Tungsten together with other high field strength elements (HFSEs) was eluted from this column using 1 M HCl-0.1 M HF. Separation of W from the other HFSEs was achieved using our previously established anion exchange techniques [3]. The anion exchange chemistry was repeated once to ensure complete Ti removal. All measurments were performed using a Nu Plasma MCICPMS at ETH Zurich. Tungsten isotope compositions of the samples were determined relative to the W/W obtained for two bracketing measurements of the W standard. In each run 40 ratios were measured and, depending on the W content of the samples, the measurements were performed with ~0.6-1.5V for W, resulting in within-run statistics of 0.2-0.4 ε units. The external reproducibility of these measurements is ±0.4-0.8 ε (2σ) for the W/W ratio. Results: In spite of largely different Hf/W ratios, Moama, Serra de Mage and Talampaya have similar εW values of 18.0±0.8, 17.2±0.4, 18.2±0.4, respectively (Fig. 1). Moore County has the highest Hf/W of the cumulate eucrites investigated here but has slightly lower εW value of 14.9±0.4. In contrast to the radiogenic εW values and low W contents (~4-14 ppb) of these four cumulate eucrites, Binda has much lower εW=2.6±0.3 and higher W content (∼70 ppb). Compared to basaltic eucrites, the range in Hf/W among the cumulate eucrites is much larger and their εW values are lower. Quitté et al. [4] obtained εW~0 and ~525 ppb W in their analyses for Serra de Magé, clearly inconsistent with the results presented here.