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

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 (ε ¹⁸⁰W_i = 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.