The Solar Initial Abundance of Hafnium-176 Revealed by Eucrite Zircon

Introduction: The Lu-Hf decay system can be used to resolve planetary differentiation timescales, but this application requires knowledge of Hf isotopic evolution for bulk planets. Because Lu and Hf are refractory and lithophile, the isotopic evolution of bulk planets and their silicate portions can be reconstructed by back-calculation from the present-day Hf/Hf and Lu/Hf in chondrites [1–3]. However, these ratios in chondrites are variable due to re-distribution of Lu and Hf during metamorphism [4], making it difficult to unambiguously define the “chondritic uniform reservoir (CHUR)”. Thus, the validity of the proposed Lu-Hf parameters for CHUR [1–3] needs to be substantiated with using a more direct approach. The initial Hf abundance of the solar system is the cornerstone for understanding Lu-Hf systematics of CHUR and terrestrial planets, but it remains poorly constrained. The back-calculation using the proposed Lu-Hf CHUR parameters [1–3] and λLu of 1.867 x 10 a [5–7] yields solar initial Hf/Hf of 0.279794–0.279820. Notably, these values are clearly lower than the initial Hf/Hf defined by a Lu-Hf internal isochron for the angrite SAH99555 [8] and whole-rock isochrons for eucrites [9] and combined eucrites and chondrites [10]. In addition, these meteorite isochrons give apparent old dates (~4850 Ma), while terrestrial rocks and meteorite phosphates yield Lu-Hf internal isochron dates consistent with their UPb ages up to 4557 Ma [5–7]. Some have interpreted the apparent old dates and lower initial Hf/Hf to reflect Hf excesses in the meteorites resulting from an accelerated decay of Lu via photoexcitation on the parent planetesimals [8,11,12]. However, this interpretation is inconsistent with the lack of Lu depletions in the meteorites that should accompany the accelerated Lu decay [13]. Alternatively, they may reflect metamorphic re-distribution of Lu and Hf. In this study, we determine the solar initial Hf abundance using eucrite zircon. Zircon has high Hf contents with low Lu/Hf so that it records near-initial Hf/Hf at the time of crystallization. Moreover, the crystallization age can be determined by U-Pb isotopes and the U-Pb data can validate whether it remained closed U-Pb and Lu-Hf systems. Hence, zircon is suitable for Lu-Hf isotopic study and, in particular, meteorite zircon is ideal to define the solar initial value [14]. Nevertheless, the use of meteorite zircon for Lu-Hf isotopic study has been hampered by its rarity and small size (<20 μm). Recently we found exceptionally large zircon grains (~80 μm) from the eucrite Agoult [15]. These grains allow us to conduct the first highprecision Lu-Hf isotopic analysis of meteorite zircon. Sample and Methods: Non-cumulate eucrites represent basaltic crust that experienced metamorphic and impact events. Agoult is a non-cumulate unbrecciated eucrite showing granulite textures [16]. The zircon in Agoult typically occurs in association with ilmenite, spinel and/or tridymite in the mesostasis (Fig. 1), whereas ilmenite grains often contain baddeleyite needles. The Ti-in-zircon thermometer showed that the zircon grains crystallized at sub-solidus temperatures of ~900 ̊C [15]. The mineral assemblage and subsolidus crystallization temperatures indicate that the Agoult zircon formed by baddeleyite exsolution from ilmenite followed by its reaction with surrounding tridymite during high-temperature metamorphism. We measured Lu-Hf isotopic ratios in eight zircon grains by solution MC-ICPMS. All eight grains displayed no zoning structure under back-scattered electron and cathodoluminescence imaging, and yielded concordant ID-TIMS U-Pb ages with a Pb/Pb age of 4554.5 ± 2.0 Ma [15]. Thus, we consider that their U-Pb and Lu-Hf systems remain intact and that the Pb/Pb age reflects the timing of the zircon crystallization during metamorphism, which is ~10 Ma later than the crystallization age of basaltic eucrites [17].