Excess 180W in IIAB iron meteorites: Identification of cosmogenic, radiogenic, and nucleosynthetic components

David L. Cook, Thomas Smith, Ingo Leya, Connor D. Hilton, Richard J. Walker, Maria Schönbächler

Earth and Planetary Science Letters
Volume 503, 1 December 2018, Pages 29-36



• We present MC-ICPMS measurements of 180W/184W ratios for IIAB iron meteorites.
• Galactic cosmic rays affect the abundance of the minor isotope 180W.
• Decay of 184Os to 180W is confirmed, with a half-life of ≈ 3 × 1013 years.
• IIAB irons show evidence for p-process heterogeneity in the early Solar System.”

“The origin of 180W excesses in iron meteorites has been a recently debated topic. Here, a suite of IIAB iron meteorites was studied in order to accurately determine the contribution from galactic cosmic rays (GCR) and from potential decay of 184Os to measured excesses in the minor isotope 180W. In addition to W isotopes, trace element concentrations (Re, Os, Ir, Pt, W) were determined on the same samples, as well as their cosmic ray exposure ages, using 36Cl–36Ar systematics. These data were used in combination with an improved model of GCR effects on W isotopes to correct effects resulting from neutron capture and spallation reactions. After these corrections, the residual 180W excesses correlate with Os/W ratios and indicate a clear contribution from 184Os decay. A newly derived decay constant is equivalent to a half-life for 184Os of (3.38 ± 2.13) × 1013 a. Furthermore, when the data are plotted on an Os–W isochron diagram, the intercept (Wi = 0.63 ± 0.35) reveals that the IIAB parent body was characterized by a small initial nucleosynthetic excess in 180W upon which radiogenic and GCR effects were superimposed. This is the first cogent evidence for p-process variability in W isotopes in early Solar System material.”