Tomohiro Usui^1,2,*, Hikaru Iwamori¹

¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan
²Lunar and Planetary Institute, Universities Space Research Association, Houston, Texas, USA

Dawn has recently revealed that the surface of Vesta is heterogeneously covered by polymictic regoliths represented by mixtures of howardite, eucrite, and diogenite (HED) meteorites. Mixing relations of the HED suite are examined here using a new computational statistical approach of independent component analysis (ICA). We performed eight-component ICA (Si, Ti, Al, Cr, Fe, Mn, Mg, and Ca) for 209 HED bulk-rock compositions. The ICA results indicate that the HED bulk-rock compositions can be reduced into three independent components (IC) and these IC vectors can reasonably explain compositional variation, petrographic observations, and the mixing relations of the HED suite. The IC-1 vector represents a eucrite variation that extends from cumulate eucrite toward main-group (MG) and incompatible-element enriched eucrites. The IC-2 vector represents a compositional variation of howardites that extends from diogenites to MG-eucrites, indicating the well-known two-component mixing trend of diogenite and eucrite. The IC-3 vector represents a compositional variation defined by diogenites and olivine-bearing diogenites, suggesting mixing of olivine and orthopyroxene. Among the three ICs, the diogenite-eucrite mixing trend IC-2 is most statistically robust and dominates the compositional variations of the HED suite. Our ICA study further indicates that the combination of only three elements (Mg, Si, and Fe) approximates the eight-component ICA model, and that the limited number of resolvable γ-ray spectra obtained by the Dawn mission possibly discriminates olivine lithologies from the olivine-free regolith breccias on the surface of Vesta.

Reference
Usui T and Iwamori H (in press) Mixing relations of the howardite-eucrite-diogenite suite: A new statistical approach of independent component analysis for the Dawn mission. Meteoritics & Planetary Science
[doi:10.1111/maps.12205]
Published by arrangement with John Wiley & Sons

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M. L. Grange^1,*, A. A. Nemchin^1,2, R. T. Pidgeon¹

¹Department of Applied Geology, Western Australian School of Mines, Curtin University, Perth, Australia
²Swedish Museum of Natural History, Stockholm, Sweden

Zircon and phosphate grains from matrix and quartz-monzodiorite (QMD) clasts in two thin sections of Apollo 15 impact melt breccia 15405 were investigated using optical microscopy, scanning electron microscopy, Raman spectroscopy, and ion microprobe U-Pb analyses. U-Pb results for zircon grains with well-defined cathodoluminescence zoning define the primary (i.e., magmatic) crystallization age as 4330 ± 6 Ma (2σ). One zircon consists of a preserved inner part surrounded by a porous polycrystalline (“granular”) mixture of zircon and baddeleyite, indicating incomplete reaction of the zircon with melt. Previous work showed that this microstructure could form at pressures above 60 GPa and a temperature close to ~1700°C and is evidence of an impact-related melting event. The U-Pb system of this grain indicates a resetting event at 1940 ± 10 Ma, interpreted as the age of this impact (impact #1). Other zircon and phosphate grains also have disturbed U-Pb systems, showing an even younger reset event (impact #2) at 1407 ± 57 Ma. Evidence of impact is supported by microstructures of zircon and baddeleyite such as secondary rims. These impacts are tentatively identified as those having formed Autolycus and Aristillius craters.

Reference
Grange ML, Nemchin AA and Pidgeon RT (in press) The effect of 1.9 and 1.4 Ga impact events on 4.3 Ga zircon and phosphate from an Apollo 15 melt breccia. Journal of Geophysical Research – Planets
[doi:10.1002/jgre.20167]
Published by arrangement with John Wiley & Sons

Link to Article