The pre-atmospheric hydrogen inventory of CM carbonaceous chondrites

Lee, M. R. , Cohen, B. E. , Boyce, A. J. , Hallis, L. J. and Daly, L. (2021) The pre-atmospheric hydrogen inventory of CM carbonaceous chondrites. Geochimica et Cosmochimica Acta, 309, pp. 31-44. (doi: 10.1016/j.gca.2021.06.013)

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Abstract

Understanding the quantity and isotopic composition of water that has been delivered to Earth over its history is crucial for models our planet’s evolution, and predicting habitability across the solar system. Here we have used stepwise pyrolysis to measure the hydrogen inventory of CM carbonaceous chondrites, which are likely to have been a major source of volatiles for the early Earth. Stepwise pyrolysis potentially enables the carriers of pre-terrestrial hydrogen to be identified, and distinguished from hydrogen that may have been added during the meteorite’s time on Earth. Twelve CM meteorites were analysed, and from their bulk hydrogen composition, petrologic type and nature of parent body processing, they can be divided into three subsets. The CMs of subset A have been mildly aqueously altered. Their hydrogen is hosted by isotopically light phyllosilicate, isotopically heavy organic matter, and adsorbed terrestrial water that is comparable to or slightly heavier than phyllosilicate. The subset B meteorites have been heavily aqueously altered and their hydrogen is also in phyllosilicate, organic matter and adsorbed terrestrial water. Their pyrolysis profiles differ from subset A in that the phyllosilicates dehydroxylate at higher temperatures owing to differences in mineralogy and chemical composition. The hydrogen that was evolved from organic matter may also have been isotopically lighter owing to loss of deuterium during aqueous alteration. Subset C meteorites were heated on their parent body after aqueous alteration, leading to loss of hydrogen from phyllosilicates and organic matter such that half of the water that they evolve was added after falling to Earth. Taking the 12 CMs together, an average of 0.20 wt. % H (21 % of total H) is terrestrial, and recalculation of bulk compositions without this component can raise bulk δD of individual meteorites by up to 73 ‰. Carbonaceous chondrites in our collections differ in the abundance and isotopic composition of hydrogen relative to their parent asteroid(s). An accurate understanding of the nature of water that was delivered to early Earth can only come from the analysis of materials that have been isolated from the terrestrial atmosphere, such as those returned from Ryugu and Bennu.

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Daly, Dr Luke and Boyce, Professor Adrian and Hallis, Dr Lydia and Cohen, Dr Benjamin and Lee, Professor Martin
Authors: Lee, M. R., Cohen, B. E., Boyce, A. J., Hallis, L. J., and Daly, L.
College/School:College of Science and Engineering > School of Geographical and Earth Sciences
College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Geochimica et Cosmochimica Acta
Publisher:Elsevier
ISSN:0016-7037
ISSN (Online):1872-9533
Published Online:19 June 2021
Copyright Holders:Copyright © 2021 The Author(s)
First Published:First published in Geochimica et Cosmochimica Acta 309: 31-44
Publisher Policy:Reproduced under a Creative Commons Licence

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
308251UK leadership in extraterrestrial sample returnMartin LeeScience and Technology Facilities Council (STFC)ST/T002328/1P&S - Physics & Astronomy
309276STFC Glasgow Earth 2019 DTPMartin LeeScience and Technology Facilities Council (STFC)ST/T506096/1GES - Geography