Carbon, the fourth most abundant element by mass in the universe, is a key element for life. To and from the Earth's interior, its recirculation regulates CO2 levels in the atmosphere, thus playing a key role in making our planet habitable. Carbon is a unique element because it can be stored deep in the Earth in various forms: in fluids, as a component of mineral phases, or dissolved in magmas. It is also believed that carbon plays a crucial role in Earth's geodynamics, as this element can control the melting processes in the upper mantle. Because of its tendency to be incorporated into magmas produced by melting peridotite rocks in the upper mantle, carbon is easily transported to the Earth's surface, where it is released as CO2 in gaseous emissions from active or dormant volcanoes. Therefore, magmas and mantle-derived gases are the most effective means of transporting carbon to the hydrosphere and atmosphere, where it plays a major role in controlling climate change on a geological scale.

But how much carbon is stored inside the Earth? This question has inspired research in several areas of the geosciences, using a variety of empirical approaches, such as the study of gases emitted in volcanic regions, the CO2 content of lavas erupted along mid-ocean ridges and/or magma inclusions within crystals, fluid inclusions in mantle xenoliths brought to the surface by magmas, and experimental measurements developed to understand the maximum amount of CO2 that can be dissolved in magmas at pressures and temperatures typical of the Earth's interior.

Unfortunately, these approaches have often led to conflicting conclusions, to the extent that estimates of the carbon content of the mantle (as well as the Earth as a whole) diverge by more than an order of magnitude. Melt inclusions, which are small droplets of silicate melt trapped in crystals as they form in magmas, can be unique sources of information for quantifying the carbon content of the mantle from which magmas themselves are segregated. However, the massive release of gases (outgassing), including CO2, to which magmas are subjected during their ascent to the surface (before emplacement and eruption), has been a limiting factor in understanding changes in carbon concentration in the mantle.

In a new study, recently published in the journal Nature Geoscience, Vincenzo Stagno from the Department of Earth Sciences at Sapienza University of Rome, together with researchers from the Universities of Palermo and Ferrara and the National Institute of Geophysics and Volcanology (INGV), have reviewed and catalogued data on the CO2 (and sulphur) content of volcanic gases emitted by 12 hot-spot and continental rifting volcanoes, whose magmas are generated by deeper mantle sources than those of the depleted mantle from which the magmas of mid-ocean ridges derive. The results showed that the upper mantle (50-250 km depth), which feeds volcanism in continental rifting and hot-spot areas, contains an average of 350 parts per million (ppm) of carbon (range of 100 to 700 ppm C). This wide range confirms the view of a highly heterogeneous upper mantle, whose composition has been variably modified over geological times by the infiltration of carbonate-silicate melts generated at depth.

New estimates obtained by the researchers indicate that the upper mantle has a total carbon capacity of about ~1.2-1023 g. It is possible that the Earth, in its inner portions, can hold even more carbon, as suggested by diamonds from sub-lithospheric depths (up to 700 km), which show evidence of the existence of minerals and melts containing significant amounts of this element.

In addition, the research team estimated that carbon content increases with partial melting depth in the mantle. This finding validates experimental data suggesting that carbon plays a role in determining the rate and depth of partial melting in mantle sources that feed volcanoes in continental rift and hot-spot areas.

By showing that carbon-rich portions of the mantle melt deeper than carbon-poor portions, the results confirm the major role played by carbon in driving geodynamic cycles.

The existence of a carbon-rich mantle has profound implications for how primordial carbon is stored in the mantle and recycled through time and space. The results obtained in this study are also important for understanding possible variations in the geological carbon cycle caused by large magnitude volcanic events, such as the establishment of Large Igneous Provinces (LIP). If magmas produced by mantle plumes are rich in carbon, as this study suggests, then the release of carbon from large igneous provinces in the Phanerozoic may have contributed to mass extinctions, traces of which are preserved in sedimentary records around the world.

The research was funded by the Deep Carbon Observatory (https://deepcarbon.net/) and Miur, Project PRIN2017 Connect4Carbon.

References:

Carbon concentration increases with depth of melting in Earth’s upper mantle - Alessandro Aiuppa, Federico Casetta, Massimo Coltorti, Vincenzo Stagno and Giancarlo Tamburello - Nature Geoscience (2021) https://doi.org/10.1038/s41561-021-00797-y

Further Information  

Vincenzo Stagno
Department of Earth Sciences
vincenzo.stagno@uniroma1.it