ANOMALOUS Cr AND Cr AND NEARLY NORMAL Ni IN DIFFERENTIAL DISSOLUTION STEPS OF MURCHISON

Introduction: We report the Cr isotope compositions of differential dissolution steps (leachates) of Murchison, as a companion report to the results on the partial dissolution residues of Orgueil and Murchison in [1, 2]. We also present preliminary results on Ni measurements on the first two Murchison leachates. There has been extensive Cr isotope work on leachates of carbonaceous chondrites, starting with the pioneering work of Rotaru et al. [3]. These workers also reported briefly that they investigated the neutron-rich isotopes of other Fe-peak elements (Ti, Fe, Ni, Zn) for the leachates with the higher Cr effects but found only a hint of very small excesses for Zn. The Cr work has been extended by additional measurements by other groups [4, 5]. This is the first attempt to follow up for Ni, on the brief report and negative findings in [3]. The leachates are the supernates of differential dissolution steps, starting with weak acids and then proceeding to stronger acids, which attack more aggressively most phases except possibly large refractory oxides and spinels. The partial dissolution techniques used in different studies have been similar, but with some important differences. The dissolution steps (see Table 1) for samples in this work were described in Reisberg et al. [6], in connection with Os isotope measurements. Results: The dissolution steps in this study and in [6] are listed in Table 1. The leaching techniques used by [3] and [5], for Murchison, which are very similar to each other, are listed in Table 2. Cr and Ni results are listed respectively in Tables 3 and 4. The εCr and εCr results are plotted in Fig. 1 and Fig. 2 (top portions) for our work and in Fig. 1 and 2 (bottom, for ease in comparing), for the work on Murchison from [3, 5]. The general patterns for εCr include deficits for Cr in the acetic acid and nitric acid leachates, with significant excesses in Cr appearing with the first use of HCl and then HCl+HF, and gradually decreasing as the acids used are heated. The effects for εCr are significantly smaller and range from +0.4 εu to -1.0 εu for our work and from +1.0 εu to -0.5 εu for the work by [3] and [5, 7]. The observed range of effects for εCr may reflect a combination of general