Outgassing history and early atmospheric processes recorded by paleoatmospheric noble gases in Archaean rocks

• Applicants: Dr. Ulrich Ott; Professor Dr. Mario Trieloff
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 276719708

Origin and evolution of planetary atmospheres are key conditions that determine the habitability of a planet. Important atmosphere shaping processes are delivery of volatiles and degassing processes of the solid Earth, i.e. mantle and crust, which is largely governed by planetary geodynamics. Terrestrial noble gas systematics (e.g., 129I-129Xe) implies that a significant fraction of terrestrial volatiles were lost from the planet, and that massive mantle degassing occurred early in Earth´s history. On the other hand, mantle and crustal degassing are ongoing processes. The evolution of the 40Ar/36Ar ratio of the terrestrial atmosphere is a sensitive parameter that can constrain the modes and intensity of mantle and crustal degassing. Our proposed project aims at a) measuring paleoatmospheric argon and xenon trapped in ancient rocks b) constrain the evolution of the atmospheric 40Ar/36Ar ratio and degassing of the solid Earthc) constrain the origin of terrestrial atmospheric xenon, which is unique in composition when compared to other solar system reservoirs (except for the Martian atmosphere)Although recent studies reported interesting new results, they yielded only few values, and their significance depends on data correction for various nuclear effects. We intend to improve the present data base by measuring Archean rocks (e.g., cherts, shales, hydrothermal quartz) from the Kaapvaal craton and Australia. Samples are available from cooperations within SPP1833 and own previous studies. Samples will be measured after neutron irradiation (to correct for in situ 40K decay and 40Ar production) as well as unirradiated, and noble gases will be extracted by both stepwise heating and crushing, in order to differentiate between gases trapped in inclusions and those in mineral matrices (e.g. from radioactive decay). This enables recognition of fluid components (either high saline-high 40Ar/36Ar, or low saline-low 40Ar/36Ar) and their identification via neutron-produced 38Ar from Cl. Stepwise heating and also stepwise combustion extractions will also be possible after crushing without air exposure, and for acid resistant residues, enabling extraction of noble gases form kerogen (from shales and cherts) without mass fractionation. In promising samples, argon and xenon will be extracted simultaneously and subsequently measured by different mass spectrometers, especially for low xenon amounts we will apply a high sensitivity Noblesse mass spectrometer equipped with multi ion counting.

• DFG Programme: Priority Programmes
• Subproject of: SPP 1833: Building a Habitable Earth
• Co-Investigators: Privatdozent Dr. Jens Hopp; Dr. Winfried Schwarz