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Hydrogen in chondrites : Influence of parent body alteration and atmospheric contamination on primordial components

Vacher, L. ; Piani, L. ; Rigaudier, T. ; Thomassin, D. ; Florin, G. ; Piralla, M. ; Marrocchi, Y., GCA

Hydrogen in chondrites : Influence of parent body alteration and atmospheric contamination on primordial components

Vacher, L. ; Piani, L. ; Rigaudier, T. ; Thomassin, D. ; Florin, G. ; Piralla, M. ; Marrocchi, Y.

Geochimica et Cosmochimica Acta, 2020, 281, 53-66

Abstract :

Hydrogen occurs at the near percent level in the most hydrated chondrites (CI and CM) attesting to the presence of waterin the asteroid-forming regions. Their H abundances and isotopic signatures are powerful proxies for deciphering the distri-bution of H in the protoplanetary disk and the origin of Earth’s water. Here, we report H contents and isotopic compositionsfor a set of carbonaceous and ordinary chondrites, including previously analyzed and new samples analyzed after the pow-dered samples were degassed under vacuum at 120°C for 48 hours to remove adsorbed atmospheric water. By comparing ourresults to literature data, we reveal that the H budgets of both H-poor and H-rich carbonaceous chondrites are largely affectedby atmospheric moisture, and that their precise quantification requires a specific pre-degassing procedure to correct for ter-restrial contamination. Our results show that indigenous H contents of CI carbonaceous chondrites usually considered themost hydrated meteorites might be almost a factor of two lower than those previously reported, with uncontaminated D/H ratios differing significantly from that of Earth’s oceans. Without pre-degassing, the H concentrations of H-poor samples(e.g., CVs chondrites) are also affected by terrestrial contamination. After correction for contamination, it appears that theamount of water in chondrites is not controlled by the matrix modal abundance, suggesting that the different chondritic par-ent bodies accreted variable amounts of water-ice grains. Our results also imply that (i) thermal metamorphism play an impor-tant role in determining the H content of both CV and ordinary chondrites but without affecting drastically their H isotopiccomposition since no clear D enrichment is observed with the increase of petrographic type and (ii) the D enrichment of ordi-nary chondrite organics does not result from the loss of isotopically light H2induced by metal oxidation but is rather linked tothe persistence of a thermally resistant D-rich component.

Voir en ligne : https://doi.org/10.1016/j.gca.2020....




publié mardi 26 mai 2020