Coupled W–Os–Pt isotope systematics in IVB iron meteorites: In situ neutron dosimetry for W isotope chronology

Tungsten isotope compositions of magmatic iron meteorites yield ages of differentiation that are within 72 Ma of the formation of CAIs, with the exception of IVB irons that plot to systematically less radiogenic compositions yielding erroneously old ages. Secondary neutron capture due to galactic cosmic ray (GCR) irradiation is known to lower the eW of iron meteorites, adequate correction of which requires a measure of neutron dosage which has not been available, thus far. The W, Os and Pt isotope systematics of 12 of the 13 known IVB iron meteorites were determined by MC-ICP-MS (W, Os, Pt) and TIMS (Os). On the same dissolutions that yield precise eW, stable Os and Pt isotopes were determined as in situ neutron dosimeters for empirical correction of the ubiquitous cosmic-ray induced burn-out of W in iron meteorites. The W isotope data reveal a main cluster with eW of 3.6, but a much larger range than observed in previous studies including irons (Weaver Mountains and Warburton Range) that show essentially no cosmogenic effect on their eW. The IVB data exhibits resolvable negative anomalies in eOs ( 0.6e) and complementary eOs anomalies (þ0.4e) in Tlacotepec due to neutron capture on Os which has approximately the same neutron capture cross section as W, and captures neutrons to produce Os. The least irradiated IVB iron, Warburton Range, has eOs and eOs identical to terrestrial values. Similarly, Pt isotopes, which are presented as ePt, ePt and ePt range from þ4.4e to þ53e, þ1.54e to 0.32e and þ0.73e to 0.20e, respectively, also identify Tlacotepec and Dumont as the most GCR-damaged samples. In W–Os and W–Pt isotope space, the correlated isotope data back-project toward a 0-epsilon value of ePt, eOs and eOs from which a pre-GCR irradiation eW of 3.4270.09 (2s) is derived. This pre-GCR irradiation eW is within uncertainty of the currently accepted CAI initial eW. The Pt and Os isotope correlations in the IVB irons are in good agreement with a nuclear model for spherical irons undergoing GCR spallation, although this model over-predicts the change of eW by 2 , indicating a need for better W neutron capture cross section determinations. A nucleosynthetic effect in eW in these irons of 0.1470.08 is confirmed, consistent with the presence of Mo and Ru isotope anomalies in IVB irons. The lack of a non-GCR Os isotope anomaly in these irons requires more complex explanations for the production of W, Ru and Mo anomalies than nebular heterogeneity in the distribution of s-process to r-process nuclides. & 2012 Elsevier B.V. All rights reserved.