Cosmogenic Stable Isotope Effects in Carbo

Introduction: The 182 Hf– 182 W decay system (t 1/2 =9 My) is useful to date early Solar System processes, notably metal-silicate differentiation [1-10]. Hf-W systematics of refractory inclusions indicate that these objects formed with an initial 182 Hf/ 180 Hf ratio of ~1×10-4 and ε 182 W of –3.47 ± 0.20 [6]. Recent high precision studies revealed negative ε 182 W values relative to CAIs for some iron meteorites [3, 5-10]. Taken at face value, this would indicate that core formation in the parent bodies of these meteorites predated CAI formation. Markowski et al. [9] measured the W isotopic compositions along two depth profiles in the Grant (IIIB) and Carbo (IID) iron meteorites. They showed that these iron meteorites were affected by cosmogenic effects corresponding to a decrease of ε 182 W by capture on W isotopes of secondary thermal neutrons. In a previous study, we analyzed pieces of Grant from various locations and found a similar trend [10]. However, the measurements were not very precise due to the low W concentration of this meteorite. One major challenge in establishing the 182 Hf-182 W system as a reliable chronometer is to find a way to accurately correct for these cosmogenic effects [11, 12]. Knowledge of exposure ages alone is not sufficient because the cosmic irradiation is also modulated by the depth of burial in the parent-body. A robust way of estimating cosmic-ray effects would be to identify correlated cosmogenic effects in other isotopes which are also produced by neutron capture processes. A recent study found Os isotopic variations in IVB meteorites [13]. The correlation between the various Os isotopes is consistent with model predictions of neutron-capture. The magnitude of the cosmogenic effect is largest in Tlacotepec because of its long exposure age. Thus, Os isotope compositions can potentially be used to monitor neutron capture effects in iron meteorites. In this study we revisit W isotope variations in Carbo. Methods: Chemical separation. We recently developed an ion exchange procedure coupling one cation-exchange column and several anion-exchange columns to remove matrix and interfering elements in iron meteorites [14,15]. The same column chemistry was used in this study. The final W-cut was evaporated to dryness in aqua regia more than 5 times, followed by several drying down in mixtures of HNO 3 and H 2 O 2. This was done to remove Os and organic matter from the resin. HClO 4 was …