Tracking the fate of subducted carbonate platforms in the sub-arc mantle using field observations and laboratory experiments: implications for deep carbon cycling

Subduction has pushed carbonate platforms, like those in the Tethyan ocean, into the Earth. This caused a lot of carbon to move from the surface to the interior. The fate of those carbonate platforms in the Earth’s upper mantle is key. It helps in understanding the amount of carbon returned to the Earth’s surface by volcanic outgassing. It also helps in understanding the carbon that has stayed in the Earth’s mantle. It is expected that carbonate in platforms will rise from the slab to the Earth's crust when it is subducted, as the researchers found in their previous study. This likely results in a large fraction of the carbon returning to the Earth’s surface and would impact the paleoclimate. This study will test if subducted carbon ends up in the atmosphere from volcanoes. It will analyze the geochemical fingerprints of subducted marbles and the volcanic products in the rocks of the Carpathian region. Lab experiments will model what happens to the carbon that is subducted into the Earth to understand how the marble's geochemical signatures may change. This will happen when they are subducted and mix with the rocks in the Earth's mantle. The researchers will search for the changed chemical fingerprint in the Carpathian volcanic rocks. This will show whether subducted marbles were at the source of the volcanoes. It will also show how carbon returned to the Earth's surface after subduction. The study aims to improve the understanding of the behavior of carbon and focuses on its movement between the surface and interior of the Earth due to plate tectonics through Earth’s history. The researchers will make a mini documentary which will cover carbon's journey from the Earth's surface to its interior and back. It will have sections on regional geology, will cover the usefulness of high pressure-temperature experiments, and how they are useful for solving key geological problems. The documentary will be shared on YouTube and will be promoted through social media and local organizations, such as high schools, the Tucson Gem and Mineral Society, and others. A major sink of the global carbon cycle is subduction into the mantle. Some of that carbon returns to the surface or the crust via magmatism, although the quantification of those budgets is difficult and so far, uncertain. Subduction margins and convergent tectonics sensu lato are major players in the overall budget of carbon circulation at geologic time scales. Carbonate can be present in the oceanic lithosphere as (1) veins and relatively small volume layers in the crust and lithospheric mantle but also (2) as larger carbonate platforms under certain subduction margins. Whereas the first scenario leads to sizable carbon recycling over time, subduction of large platforms have the potential to return an order of magnitude more carbonate material than in the first case. The metamorphic changes and possible melting reactions of minor carbonate masses into larger silicate sedimentary cover or basement have been investigated to a fair extent. However, little is known when large carbonate masses as platforms are being subducted, a scenario that may have been rather common in the geologic history during the evolution of the various branches of the Tethyan ocean. This study will provide new key constraints on the fate of carbonate as it subducts in the form of carbonate platforms using the Carpathian region in the Alpine Tethys as the study location. Impure marbles, arc volcanics, peridotite and pyroxenite xenoliths will be analyzed for major and trace elements as well as Sr, Nd, Pb and Ca isotopes. The goal is to (a) obtain a range of geochemical and isotopic compositions of subducted marbles, (b) identify geochemical fingerprints of a sedimentary carbonate component in the volcanics and lithospheric samples. Phase equilibria experiments will be performed to investigate the partial melting behavior of impure marbles under sub-arc conditions and assess the fate of reactive infiltration of these partial melts through the sub-arc lithosphere. The thermal conditions at which partial melting likely takes place upon subduction of carbonate platforms will be noted, and the melt and residue compositions (and mineralogy) will be compared with the natural data. The goal is to assess whether subduction of carbonate platforms and consequent diapirism (as predicted by the researcher's previous study) govern the evolution of the lithosphere and magmatism in the Carpathian region and if the products from this study can be used to place constraints on carbon budgets due to subduction-lithosphere interaction. A mini documentary on deep carbon cycling with sections focusing on regional geology and the usefulness of high pressure-temperature experiments in tackling first order geological problems will be developed. The documentary will be disseminated to the public through YouTube and publicized through social media and local organizations such as high schools, the Tucson Gem and Mineral Society and others. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.