Unusual neon isotopic composition in Neoproterozoic sedimentary rocks: Fluorine bearing mineral contribution or trace of an impact event?
Résumé
Extraterrestrial materials have He and Ne isotopic compositions that are distinct from those of the Earth's surface. In order to track the extraterrestrial material accreted onto Earth during the Ediacaran period, we have analyzed the He and Ne isotopic composition of thirteen sedimentary rocks in the age range similar to 550-600 Ma, coming from the Huqf supergroup in Oman for which carbon and sulfur isotopic data have been characterized previously.He-3/He-4 ratios range between 0.006 +/- 0.003 and 0.27 +/- 0.01 RA, with RA being the atmospheric ratio. He-3/He-4 ratios show a positive relationship with He-3 contents ranging between 0.6 and 31x10(-13) cm(3) STP.g(-1). The He-3 contents are within the literature data for 3 to 480 Myr old samples with evidence of IDP 3He (IDP for interplanetary dust particles), suggesting that extraterrestrial He-3 is still retained in such old samples.Ne-20/Ne-22 ratios are close to or below the modern atmospheric ratio of 9.8 with the minimum value equal to 9.05 +/- 0.03. Ne-21/Ne-22 ratios show a high range of variation, going from 0.0345 +/- 0.0009 to 0.0935 +/- 0.0023. The Ne isotopic compositions follow a nucleogenic trend similar to that of crustal fluids from the literature and predicted continental crust. However, one sample (3404) shows an unusual Ne isotopic composition with a lower Ne-20/Ne-22 at similar Ne-21/Ne-22 compared to the other samples.Two hypotheses can explain this singular Ne isotopic composition. First, it could be the result of a particular nucleogenic trend due to the presence of F-bearing minerals in this sample. SEM-EDS elemental mapping showed that although F-and Ca-rich phases, which could correspond to fluorites, are present in sample 3404. However, their abundance of similar to 0.15% seems too low to explain the unusual Ne isotopic composition. However, due to the high uncertainty of the calculations, we cannot totally rule out this hypothesis. Alternatively, the singular Ne isotopic composition could be due to the presence of a Ne-A component, a component characterizing pre-solar diamonds contained in chondrites. This would indicate that a major object impacted the Earth at the time the sediment was forming, between similar to 600 and similar to 550 Ma, which is coherent with the estimated age range of the Acraman impact in Australia.
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