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From the origin of Earth's volatiles to atmospheric oxygenation

Periodic Reporting for period 3 - O2RIGIN (From the origin of Earth's volatiles to atmospheric oxygenation)

Reporting period: 2018-03-01 to 2019-08-31

Aim of this project is to understand the connection between endogenic and exogenic processes of our planet that led to the redox contrast between Earth’s surface and interior. For this purpose the time constraints on atmospheric oxygenation can be refined and for the first time linked with a new approach to Earth’s endogenic processes like plate tectonics, mantle melting, volcanism, continent formation and subduction-related sediment- and crust recycling. These objectives will be achieved by using the unique geochemical capabilities of various stable isotope systems including the Selenium (Se) isotope system to unlock the geological record of changing oxygen fugacities in the mantle-crust-atmosphere reservoirs. The power of the Se isotope system lies in its redox sensitivity and in the volatile and highly siderophile/chalcophile character of elemental Se. This links Se to the evolution of other volatiles during key geological processes from Earth formation ca. 4.5 Ga ago until today. The occurrence and behavior of Se is fully controlled by accessory micrometric sulfide minerals in the silicate Earth, which may conserve their original Se isotopic signatures over large geological timescales and can be dated via the 187Re-187Os geochronometer. This offers high resolutions in time and space that are groundbreaking for research on Earth System Oxygenation. Covering Earth geologic history, new high-precision Se isotope data of the sedimentary and representative mantle-derived magmatic rock record from all major plate tectonic settings will be combined with the mineral-scale record of robust and global “time capsules” such as diamond inclusions. Once the evolution into todays dynamic Earth’s Redox System is understood, the investigation will be pushed back in time to Earth’s formation. This involves a reconciliation of the meteoritic and Archean rock and mineral-scale Se isotope record to constrain the origin of volatiles essential for the oceans, generation of an atmosphere and development of life on our planet.
During this period the basis for the entire project was successfully established and first significant results obtained and published. This includes the development and setup of routine Se isotope and elemental Se-Te analyses of various geological materials (Kurrawa et al. 2017; Yierpan et al., 2018), implementation of micro-seperation techniques for extraction of single grain sulfides from different rocks and the possibility to combine Re-Os geochronological with Se isotope investigation on the same sample. Among the first results are coupled Se isotope and Se-Te elemental data of mid ocean ridge basalts, mantle peridotites, ocean island and subduction zone basalts, that altogether contribute to the ongoing characterisation of various terrestrial reservoirs. Characterisation of these reservoirs is a prerequisite for further investigation of secular variations within the Earth's mantle and links to atmospheric evolution that may be recorded in Se isotope variations in bulk rocks and/or single sulfide phases through geological history. The investigation of the origin of Earth's volatiles started with Se isotope analyses of different chondritic meteorites (Labidi et al., 2018). These results will be combined with Se isotope signatures of the bulk silicate Earth as derived from characterisation of the different reservoirs. Further investigation and first results are obtained for understanding the Se isotope system, often regarded as similar to that of Sulfur, during processes at the Earth's surface (Banning et al., 2018) such as degassing upon volcanic eruption in fumarole volatile escape (in preparation) and during incorporation into surface minerals such as hydrothermal pyrite. This is an important basis for the future application of the Se isotope palaeo-redox proxy in deep Earth time (König et al., 2019).
The chemical separation (Yierpan et al., 2018) and instrumental measurement techniques (Kurrawa et al., 2017) established in this project allow for unprecedented precision of Se isotope and Se-Te elemental analysis of geological materials such as mafic rocks (Yierpan et al., 2019), chondritic meteorites (Labidi et al., 2018) and single sulfides (König et al., 2019) with as low as 3 ng total Se. In contrast to all previously published studies, our technique allows for the first time to resolve Se isotope variations between different chondritic meteorites (Labidi et al., 2018), with cosmochemical implications for the understanding of sulfide formation in the early solar nebula and potential for discriminating between meteorites as building materials for Earth and the origin of terrestrial volatiles.
Together with the possibility to analyse Os isotopes of the same dissolved sample material, it is expected that, for the first time, robust Re-Os minimum ages can be obtained for a given Se isotope signature. This will push the limits in exploring the terrestrial volatile and redox evolution from the perspectives of bulk rocks and single sulfides through Earth's geological history.