Barium and Neodymium Isotope Heterogeneities in Early Solar System Materials: Applications to Planetary Reservoir Models

Details Barium and Neodymium Isotope Heterogeneities in Early Solar System Materials: Applications to Planetary Reservoir Models Barium and Neodymium Isotope Heterogeneities in Early Solar System Materials: Applications to Planetary Reservoir Models

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.v21f..01r&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #V21F-01

Physics

1000 Geochemistry, 1027 Composition Of The Planets, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 6200 Planetary Sciences: Solar System Objects, 6240 Meteorites And Tektites (1028, 3662)

Scientific paper

Heavy element isotopic heterogeneities in early Solar System materials may exist as a result of both incomplete mixing of pre-solar nucleosynthetic components in the Solar Nebula leading to different ratios of p-, r- and s-process isotopes in bulk planetary materials as well as heterogeneities caused by the decay of now extinct nuclides. Boyet and Carlson (2005) reported a difference in 142Nd/144Nd between Earths mantle and chondrites of about 20-30 ppm. Assuming that this difference was due to decay of 146Sm and that the Earth and chondrites formed with identical 146Sm/144Sm they inferred the formation of a deep enriched silicate layer (D'' ?) in the Earth that formed within the first 30 Myr of Solar System history. We have obtained a similar difference in 142Nd/144Nd between Earth and chondrites. However, we are now testing their interpretation with Ba isotope measurements of various chondrites. Barium is an ideal element for testing the origin of small isotopic anomalies because it has two isotopes (134 and 136) derived only from the s-process as well as three isotopes (135,137 and 138) derived from both the r- and s-process with 135Ba possibly having a contribution from the decay of now extinct 135Cs. Six chondrites: Allende (CV3), Peace River (L6), Murchison (CM2), Grady (H3.7), Guarena (H6), and Bruderheim (L6) were measured for Ba isotopic composition with a new generation TIMS instrument (a GV ISOPROBE-T). A terrestrial andesite, AGV-1, was also processed for use as our reference standard. Preliminary results indicate widespread heterogeneity in the fractionation corrected 137Ba/136Ba ratio between different meteorites and our terrestrial standard, as high as 25 ppm. Smaller anomalies are also seen in 134Ba/136Ba. These anomalies are likely caused by slight differences in the mixing proportions of r- and s-process Ba in Earth and chondrites. This calls into question whether or not the differences seen in 142Nd/144Nd are truly caused by early differentiation of the Earth and decay from 146Sm or are they the result of incomplete mixing of r and s components in the solar nebula.

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