(2020) Deciphering Nd and Sm Isotope Composition in Chondrites
Frossard P, Boyet M, Bouvier A, Bonnand P & Auclair D
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01g: Room 1, Tuesday 23rd June 08:33 - 08:36
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EC residues show large isotope anomaly, in contrast to OC. Is it related to acid soluble sulfide in EC that contain a significant amount of REE? How REE concentrations in leachates are compared between EC and OC?
The striking difference between the large anomalies in EC and small in OC can be explained by two processes. Either the carriers of the anomalies were destroyed during parent body processing or the carriers signatures are diluted the non-anomalous material. In EC, the first leachate corresponds to oldhamite, a CaS that is very easily leached as you mentioned and carries up to 80% of REE. Therefore, anomalous Nd carriers such as SiC are less diluted. OC are very different because REE are more evenly distributed, most leachates contain 100 to 300 ng of Nd, which will inevitably dilute the extreme signatures. However, leachates obtained by Qin et al. (GCA, 2011) on a ordinary chondrite show large anomalies so I would suggest that we observe a mix of these processes, namely a dilution of the anomalous signatures and that this OC (NWA 8007) experienced significant thermal processing on the parent body.
When compared to bulk OC, EC, CC, in your page 2 and 3, the Earth has an endmember mu142Nd, seems higher than bulk chondrites. However on page 11 the composition of Earth seems lower than the chondrite s-process mixing line (lower u142Nd), from the intercept of the line. Can you comment on what might cause this?
The value of the intercept (µ145Nd-µ142Nd) is actually within error of the terrestrial standard (6±4) for µ142Nd. Bulk chondrites plot on this regression line in µ145Nd-µ142Nd space (not represented). This intercept means that Earth’s composition plots on the nucleosynthetic processes mixing line as sampled in chondrites leachates and Earth's µ142Nd is nucleosynthetic in origin. This confirms the studies of Bouvier and Boyet (Nature, 2016) and Burkhardt et al. (Nature, 2016), with more reliable nucleosynthetic processes mixing lines than astrophysical calculations and the very few data on presolar SiC.
If EL3 is the main contributor to the Earth's building block, how does this affect the major element compositions of bulk Earth as well as those of the modern terrestrial mantle?
There are very little, if at all, whole-rock major element analysis on EL3 chondrites, but I would expect it to be similar to EH3 chondrites. Javoy et al. (EPSL, 2010) proposed that EH3 are the main building blocks for the Earth, therefore the difference with an EL3-Earth should be minor, and implications similar.
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