K-Th-Ti systematics and new three-component mixing model of HED meteorites: Prospective study for interpretation of gamma-ray and neutron spectra for the Dawn mission

– The Dawn spacecraft carries a gamma-ray and neutron detector (GRaND), which will measure and map the abundances of selected elements on the surface of asteroid 4 Vesta. We compare the variability of moderately volatile/refractory incompatible element ratios (K/Th and K/Ti) in howardite, eucrite, and diogenite (HED) meteorites with those in other achondrite suites that represent asteroidal crusts, because these ratios may be accurately measured by GRaND and likely reflect initial chemical compositions of the HED parent body. The K/Th and K/Ti variations can differentiate HED meteorites from angrites and some unique eucrite-like lithologies. The results suggest that K, Th, and Ti abundances determined from GRaND data could not only confirm that Vesta is the parent body of HED meteorites but might also allow recognition of as-yet unsampled compositional terranes on Vesta. Besides the K-Th-Ti systematics study, we propose a new three-component mixing model for interpretation of GRaND spectra, required because the spatial resolution of GRaND is coarser than the spectral (compositional) heterogeneity of Vesta’s surface. The mixing model uses abundances of K, Ti, Fe, and Mg that will be analyzed more accurately than other prospective GRaND-analyzed elements. We examine propagated errors due to GRaND analytical uncertainties and intrinsic errors that stem from an assumption introduced into the mixing model. The error investigation suggests that the mixing model can adequately estimate not only the diogenite/eucrite mixing ratio but also the abundances of most major and minor elements within the GRaND propagated errors.