Osmium isotope anomalies in Group IVB irons: Cosmogenic or nucleosynthetic contributions

Introduction: Evidence for isotope anomalies in primitive chondrites, and in iron meteorites, have been reported in Mo [1,2], Ru [3], Ba [4], Nd [5, 6], and Sm [6]. Osmium is a highly refractory element for which isotopic anomalies in bulk chondrites or differentiated meteorites have not been reported, yet. Brandon et al. [7] reported Os isotope anomalies in primitive chondrites, with the largest effects in Tagish Lake, that they interpreted to be the result of partial dissolution of refractory carriers. Magnification of these Os isotope anomalies has been reported [8, 9]. A definitive test of whether the solar nebula had large-scale isotope anomalies [e.g., 1, 10] or not [7] is better achieved by the analysis of differentiated meteorites, since the presolar grain carriers of isotope anomalies are destroyed, and any grain-scale isotope anomalies are eradicated, during melting. It is, therefore, very interesting that endemic Mo [2] and Ru [3], but not Pd [11], isotope anomalies have been reported in iron meteorites, particularly in some members of Group IVB. Since Group IVB irons are enriched in refractory siderophile elements [17], we undertook a search for Os isotope anomalies to determine whether isotope anomalies are found in other refractory elements. It is important to observe that these irons also have some of the longest cosmic-ray exposure ages of any group of meteorites. Cosmogenic neutron capture burn-out of W is a contributor to W isotope effects in these irons, complicating interpretations of HfW ages [12, 13]. We, therefore, analyzed 10 out of the 12 group IVB irons to separate the effects of cosmogenic isotope anomalies from any effect that may be intrinsic to the precursor of IVB irons, a feature that must be shared by all IVB irons.