(2020) Dy, Er, and Yb Isotope Systematics in Meteorites and their Components
Shollenberger Q & Brennecka G
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01g: Plenary Hall, Thursday 25th June 22:15 - 22:18
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I am little bit confused terms s-process excess used in Murchison leaching and r-process deficits in CAIs, which are same meaning? Both show 160Dy excess? if so, why CC show deficit in 160Dy relative to OC and earth.
I'll start with the CAIs. It is correct that we cannot distinguish between r-process deficits or s-process excesses. However, Mo isotope compositions in coarse-grained CAIs have r-process excesses (and with Mo you can distinguish between r- and s-process variations). As such, that is why we discuss the data in terms of the r-process. But you are correct that the variations in CAIs for Er and Yb could be s-process excesses. At the moment, I have not measured the Dy isotope composition of any CAIs. For the Murchison leachates, we discuss the data in terms of the s-process. Our Dy and Er leaching data are a slightly better fit to s-process mixing lines compared to r-process mixing lines. For Dy and Er in our leachates, the largest s-excess occurs in the final leaching step (which experienced the strongest acid treatment), likely reflecting the dissolution of SiC grains. Our final leaching step has an excess in 160Dy, consistent with an s-process excess (which we interpret to be a SiC signature). For our bulk chondrite measurements. We observe that the CCs may have a deficit in 160Dy consistent with an s-process deficit. The observation that CCs may have formed with a deficit in s-process Dy and Er nuclides is consistent with other elements like Nd in which the CCs have the largest s-deficit relative to Earth. However, we only measured three CCs samples and more work needs to be done.
Do you know where Yb might be/ have been hosted?
At the moment, we do not know where Yb is hosted. Given that Yb is less refractory compared to other rare earth elements, the presolar carrier of Yb may be a presolar silicate, oxide, graphite, or another unidentified presolar phase that is stable below the formation temperature of SiC.
You plot Er, Yb, Hf in CAI as having an r-process deficit. Are these 'pure' r-process deficits or a p+r-process deficit as for Nd and Sm?
We are not able to tell if it is a pure r-process deficit (alternatively s-process excess) or a p+r process deficit. For the CAIs, we only examined the isotopes which have s- and r-process contributions. The pure p-process isotopes for Er, Yb, and Hf have low abundances making them difficult to measure. I think future work that examines all Er, Yb, and Hf isotopes could help us understand what types of variations we are seeing (i.e., r-process deficits/s-process excesses or p+r-process deficits)
Does it make sense that Dy and Er would be incorporated into SiC and Yb would not based on differences in their cosmochemical behaviors?
Yes, it does make sense that Dy and Er are incorporated in SiC and Yb is not. Even though the condensation temperatures are calculated for a gas of solar composition and cannot be directly applied to SiC formation, Dy and Er have similar T50% ~ 1660K that is higher than Yb (T50% ~ 1490K). As such, it makes sense that Dy and Er could condense into SiC and Yb would remain in the gas phase. Also, we know that there is very little Yb in SiC from rare earth element concentrations in Murchison SiC (see Ireland et al. 2018).
Are the ?160Dy and ?164Er deficits in bulk CC compared to NC solely related to s-process deficits or do CAI have an effect with their r-process deficits signature?
Thanks for this question. The Dy and Er isotope compositions that we measure in CCs could also be influenced by CAIs. I have performed mass balance calculations to determine the effects of CAIs on the Er and Yb isotope compositions of the Allende chondrite. (I have not measured the Dy isotope composition of CAIs so I can't say how CAIs influence the Dy isotope composition of CCs.) So I was able to determine "CAI-subtracted Allende" Er and Yb isotope compositions and found they are indistinguishable from our measured Allende Er and Yb isotope compositions within analytical uncertainty. However, the results of our calculations suggest that CAIs drive the measured Allende Er and Yb isotope compositions towards the terrestrial composition and thus, away from the actual Er and Yb composition of the CC source reservoir. Given that we measured 3 CCs, I think analyzing more CC samples and CAIs (especially for Dy) will help us to better understand the influence of CAIs on the Dy, Er, and Yb isotope compositions of chondrites.
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