Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010apj...720..150y&link_type=abstract
The Astrophysical Journal, Volume 720, Issue 1, pp. 150-154 (2010).
Astronomy and Astrophysics
Astronomy
6
Astrochemistry, Meteorites, Meteors, Meteoroids, Minor Planets, Asteroids: General, Nuclear Reactions, Nucleosynthesis, Abundances
Scientific paper
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 ~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 53Mn-53Cr 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 54Cr isotopic data for this and seven additional bulk ureilites show homogeneous ɛ54Cr of ~-0.9, a value distinct from other achondrites and chondrites. Using the ɛ54Cr signatures of Earth, Mars, and Vesta (HED), we noticed a linear decrease in the ɛ54Cr 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 ɛ54Cr signatures of monomict ureilites will place the position of the ureilite parent body at ~2.8 AU. These observations imply that the differentiation of achondrite parent bodies began nearly simultaneously at ~4565 Ma in different regions of the inner solar system. The distinct ɛ54Cr value between ureilite and carbonaceous chondrite also implies that a genetic link commonly proposed between the two is unlikely.
Makishima Akio
Nakamura Eizo
Yamakawa Akane
Yamashita Katsuyuki
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