The pre-atmospheric hydrogen inventory of CM carbonaceous chondrites

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.