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Origin and Evolution of Distinct Isotopic Variabilities for Sr, Mo, and Nd within CC and NC Reservoirs

Yokoyama T, Fukai R, Nagai Y & Hirata T

Yokoyama T, Fukai R, Nagai Y & Hirata T (2020) Goldschmidt Abstracts, 2020 3034

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Listed below are questions that have been submitted by the community that the author will try and cover in their presentation. To submit a question, ensure you are signed in to the website. Authors or session conveners approve questions before they are displayed here.

Submitted by Yankun Di on
Thank you Yokoyama san, I have two questions. 1. Do you have an idea why Sr-Nd and Mo have been decoupled in terms of CC-NC dichotomy and within-reservoir variations, i.e., why these elements exhibit different characteristics and requires different mechanisms to explain? For Sr-Nd, within-reservoir variabilities are more limited and seems s-process component controls everything. For Mo, there are so much more variabilities. 2. In the mixing NC-A and NC-B model for Mo, how did you determine the average ISS composition? Why it is not in between NC-A and NC-B, but somewhere off the mixing line?
Thank you for posting questions. 1) Regarding the NC-CC dichotomy, decoupling between the Sr-Nd and Mo could be due the Nd isotopic compositions in CAIs so far obtained. Unlike Sr, the total number of high-precision Nd isotope data for CAIs are limited, so that we don’t know the actual variability of Nd isotope compositions in CAIs. The decoupling may be solved if CAIs possess positive µ150Nd values on average. Additionally, it is important to note that refractory components other than observable CAIs in chondrites could be responsible for the NC-CC dichotomy, yet we can’t fully identify the isotopic compositions of Sr, Nd and Mo in such phases. The relatively large within-reservoir variations for Mo isotopes could be caused by the heterogeneous distribution of metal phases that carry anomalous Mo isotopes, as discussed in Budde et al. (2016; EPSL). Otherwise, parent-body processing under oxidizing conditions could have generated the observed variabilities, because Mo forms a trioxide and becomes more volatile than under reduced conditions. 2) ISS is a hypothetical component that was defined arbitrarily, but it is isotopically identical to the CI chondrite. We considered two models that explain the Mo isotope variabilities within NCs. First, selective removal of r-process-enriched and s-process-enriched components from ISS resulted in the formation of NC-B and NC-A, respectively. In the second model, removal of a refractory component enriched in r-process Mo from ISS resulted in the formation of “initial NC reservoir” defined by Alexander (2019), the component which is plotted on the mixing line between NC-A and NC-B.

Submitted by Larry Nittler on
Great talk, Yoko. Sorry I couldn't be up in the middle of the night for the live Q&A. Where does the idea that early infall is "enriched with supernova materials" come from? Are you associating the r-process with supernovae? Or CAI anomalies generically with supernovae? The r-process is only known to occur in merging neutron stars (but *may* also occur in some supernovae).

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