Geochemical Journal, Vol. 51 (No. 1), pp. 17-29, 2017
Yusuke Fukami1* and Tetsuya Yokoyama2
1Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan
2Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama, Tokyo 152-8851, Japan
(Received January 22, 2016; Accepted September 1, 2016)
We present the tellurium (Te) isotope compositions of the acid leachates and residues from three carbonaceous chondrites, namely, Allende, Murchison, and Tagish Lake. Most of the Te isotope compositions in the acid leachates and residues were indistinguishable from that of the terrestrial standard within analytical uncertainties, indicating a homogeneous distribution of Te isotopes in the solar nebula. Previous studies have reported nucleosynthetic isotope anomalies for Sr, Mo, W, and Os in the leachates and residues from the same meteorites. This suggests that the anomalous Te isotope signatures within the carbonaceous chondrites, including presolar phases, have presumably been nearly completely erased via a temperature-controlled nebular processes that acted on the relatively volatile elements before the onset of parent body formation. In contrast, the final residue of the Allende chrondrite displays a small but resolvable Te isotope anomaly. We performed mixing calculations to reproduce the observed Te isotopic pattern for the Allende final residue, which can be explained by the depletion of a theoretical r-process component. This result suggests that our Allende final residue was depleted in presolar nanodiamonds, which were enriched in the theoretical r-process component, because nanodiamonds are strongly acid resistant and can survive the leaching steps used in this study. The presolar SiC is not responsible for the observed r-process depletion. The discrepancy might instead be attributed to the existence of another presolar phase, including glassy carbon, in the final Allende residue.
Key words: tellurium isotopes, acid leaching, N-TIMS, carbonaceous chondrites, isotope anomaly