Physics
Scientific paper
May 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusm.v32a..02r&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #V32A-02
Physics
3662 Meteorite Mineralogy And Petrology (1028, 6240), 1000 Geochemistry, 1028 Composition Of Meteorites (3662, 6240)
Scientific paper
A persistent view among geochemists is that the Solar System formed as a hot, well mixed disk of dust and gas that had the same initial isotopic composition of heavy elements throughout. Thus, all meteorite parent bodies and terrestrial planets would have identical stable isotopic ratios of refractory, heavy elements. In order to explain the initial isotope ratios of various extinct nuclides, both injection of supernova and AGB star material to the proto-solar molecular cloud or disk within a few million years of formation has been proposed. A common view is that these r-process and s-process contributions were homogenized throughout the disk. The existence of pre-solar grains in meteorites shows that grains with extremely large isotopic differences have been preserved. Some rare, refractory inclusions in carbonaceous chondrites have relatively large r-process excesses while a large portion of pre-solar grains found in chondrites are almost pure s-process material. The survival of these grains means that the nebula could not be as homogenized as previously thought. Thus, it is plausible for various planets and planetesimals to have slightly different inventories of r- and s-process nuclides. Small isotopic differences between bulk meteorites and the Earth have been measured for both Mo and Zr (Yin et al. 2002). Boyet and Carlson (2005) measured a 20 ppm difference in 142Nd between the Earth and chondrites which could either be due to decay of 146Sm or an r-process enrichment in chondritic meteorites compared to the Earth, consistent with the Mo and Zr data. We have measured Ba isotopes in various chondrites to further investigate heavy element isotope heterogeneities in bulk planetary bodies. Barium is an ideal element to study because it has many isotopes, some of which are formed only in the s-process. All chondrites show r-process excesses of up to 40 ppm in 138Ba compared to the Earth. This is likely an r-process excess due to incomplete mixing of r- and s-process nucleosynthetic components in the early Solar System. The fact that the solar nebular disk was not well mixed needs to be taken into account when using small variations in ratios such as 142Nd/144Nd to infer processes and timescales of early planetary evolution.
Jacobsen Stein B.
Ranen Michael C.
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