CHROMIUM ISOTOPE SYSTEMATICS OF ACHONDRITES: CHRONOLOGY AND ISOTOPIC HETEROGENEITY OF THE INNER SOLAR SYSTEM BODIES

The standard planetary formation models assume that primitive materials, such as carbonaceous chondrites, are the precursor materials of evolved planetesimals. Past chronological studies have revealed that planetesimals of several hundred kilometers in size, such as the Howardite-Eucrite-Diogenite (HED) parent body (Vesta) and angrite parent body, began their differentiation as early as similar to 3 million years of the solar system formation, and continued for at least several million years. However, the timescale of planetesimal formation in distinct regions of the inner solar system, as well as the isotopic characteristics of the reservoirs from which they evolved, remains unclear. Here we present the first report for the precise (53)Mn-(53)Cr ages of monomict ureilites. Chemically separated phases from one monomict ureilite (NWA 766) yielded the Mn-Cr age of 4564.60 +/- 0.67 Ma, identical within error to the oldest age preserved in other achondrites, such as angrites and eucrites. The (54)Cr isotopic data for this and seven additional bulk ureilites show homogeneous epsilon(54)Cr of similar to-0.9, a value distinct from other achondrites and chondrites. Using the epsilon(54)Cr signatures of Earth, Mars, and Vesta (HED), we noticed a linear decrease in the epsilon(54)Cr value with the heliocentric distance in the inner region of the solar system. If this trend can be extrapolated into the outer asteroid belt, the epsilon(54)Cr signatures of monomict ureilites will place the position of the ureilite parent body at similar to 2.8 AU. These observations imply that the differentiation of achondrite parent bodies began nearly simultaneously at similar to 4565 Ma in different regions of the inner solar system. The distinct epsilon(54)Cr value between ureilite and carbonaceous chondrite also implies that a genetic link commonly proposed between the two is unlikely.