Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p12b..02m&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P12B-02
Other
[1060] Geochemistry / Planetary Geochemistry, [6200] Planetary Sciences: Solar System Objects
Scientific paper
The degree of isotopic homogeneity in the early Solar System provides information about its environment and dynamic evolution. If the Solar System is found to be isotopically heterogeneous, regional isochrons will have to be developed to obtain reliable age information on cosmochemical processes. Isotopic homogeneity in the Solar System can be investigated using stable isotopes of Ba. This element has seven stable isotopes (130,132,134,135,136,137,138Ba) which sample three key nucleosynthetic processes that could have contributed to the Solar System’s composition. These nucleosynthetic processes are the s-process (134,135,136,137,138Ba), the r-process (135,137,138Ba) and the p-process (130,132Ba). In addition to these nucleosynthetic inputs, the decay of 135Cs to 135Ba (t1/2 = 2.3 Ma [1]) may have contributed to the 135Ba abundance. Barium isotope compositions of two eucrites (Stannern and Juvinas) and four CAIs from Allende (CV3) were determined. CAIs, the oldest solids identified in our Solar System [2], were chosen to be part of this study to estimate the degree of isotopic homogeneity and presence of live 135Cs in the early Solar System. Enrichments of 135,137Ba in CAIs have been previously reported, however, they are often overprinted by r- and s-process anomalies [3]. In this study, Ba was chemically isolated from the major elements and the rare earth elements, using cation exchange chromatography. 132,134,135,136,137,138Ba isotopes were measured (sample size of ~ 400 ng) using a static measuring mode on the Thermal Ionisation Mass Spectrometer. Interferences from 139La and 140Ce were monitored but were found to be negligible in all analyses. Ratios were corrected for mass bias using the s-only process isotopes (134/136Ba = 0.307776 [4]) to avoid any nucleosynthetic bias that may have been incorporated into the mass bias correction by using r- and s-process isotopes as the normalising ratio (e.g. 134/138Ba = 0.033715 [4]). All measurements are 2σ. Two of the measured CAIs possess an excess in 135Ba of ~ ½ ɛ unit, however, no anomalies are resolvable for the other stable Ba isotopes. The remaining CAIs and the eucrites are isotopically indistinguishable from terrestrial samples. The anomaly in 135Ba alone indicates that the excess is due to in situ decay of 135Cs, thus signifying in-growth of 135Ba as the source of the isotopic anomaly. From these data it is clear that eucrites and CAIs are isotopically homogeneous, but for the in-growth of 135Ba from decay of 135Cs seen in two CAIs. This suggests that the bulk material from which the Earth, eucrites and CAIs originated was homogeneous. These 135Ba data are direct evidence for live 135Cs in the early Solar System and could be used to confirm 135/133Csi = (4.8±0.8) x 10-4 reported by Hidaka et al. (2001) [3]. [1] Harper C. L. (1993) J. Phys.G., 19, S81 [2] Krot et al. et al. (2009) GCA, 73, 4963 [3] Hidaka H. et al. (2001) EPSL, 193, 459 [4] Hidaka H. et al. (2003) EPSL, 214, 455
Bermingham K. R.
Mezger Klaus
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