In a new one studio, published in the journal Nature Geoscience (https://www.nature.com/ngeo/), a team of Italian researchers led by Alessandro Aiuppa (University of Palermo) and whose co-authors are Federico Casetta (University of Ferrara), Massimo Coltorti (University of Ferrara), Vincenzo Stagno (Sapienza University of Rome) and Giancarlo Tamburello (National Institute of Geophysics and Volcanology, Bologna Section), has developed a new approach to reconstruct the amount of carbon stored in the upper mantle of the earth, from whose melting magmas are segregated.
Carbon, the fourth most abundant element by mass in the universe, is a key element for life. Its recirculation, to and from the Earth's interior, regulates CO2 levels in the atmosphere, thus playing a fundamental role in making our planet habitable. Carbon is a unique element, because it can be stored deep inside the Earth in various forms: inside fluids, as a component of mineral phases, or dissolved in magmas. Furthermore, it is believed that Carbon plays a key role in the Earth's geodynamics, as this element is able to control the fusion processes that occur in the upper mantle. Given its tendency to be incorporated in magmas produced by fusion of peridotite rocks in the upper mantle, Carbon is easily transported to the earth's surface, where it is then released as CO2 in the gaseous emissions of active or dormant volcanoes. Magmas and gases derived from the mantle are, therefore, the most effective means of transport for carrying carbon towards the hydrosphere and the atmosphere, where it plays a primary role in controlling climate change on a geological scale.
But how much carbon is stored inside the earth?
This question has inspired research in various fields of geosciences, which have made use of multiple empirical approaches, such as the study of gases emitted in volcanic areas, of the CO2 content in lava erupted along mid-ocean ridges and/or in magma inclusions within the crystals, fluid inclusions in mantle xenoliths brought to the surface by magmas, and experimental measurements developed with the aim of understanding 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 point that estimates of the carbon content of the mantle (as well as the entire Earth) diverge by more than an order of magnitude. The "melt inclusions", or magma inclusions, i.e. small droplets of silicate melt trapped in the crystals at the time of their formation in the magmas, can be unique sources of information to quantify the carbon content of the mantle from which the 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 their emplacement and eruption) has been a limiting factor in understanding the concentration variations of Carbon in the mantle.
In their study, Aiuppa and co-authors reviewed and cataloged data on CO2 (and sulfur) content in volcanic gases emitted by 12 hot-spot and continental rifting volcanoes, whose magmas are generated from deeper mantle sources compared to those of the impoverished mantle from which the magmas of the mid-ocean ridges derive.
The results obtained allowed us to understand that the upper mantle (50-250 km deep) that feeds volcanism in continental rifting and hot-spot areas contains an average of 350 parts per million (ppm) of Carbon (range between 100 and 700 ppm of C). This broad 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 in depth. The new estimates obtained by Aiuppa and co-authors indicate that the upper mantle has a total carbon capacity of about ~1.2 1023 g. It is possible that the Earth, in its internal portions, is able to contain 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 that contain significant amounts of C.
In addition, the team of researchers estimated that the carbon content increases with the depth of partial melting in the mantle. This discovery allows us to validate the experimental data, which suggest that carbon plays a role in determining the percentage and depth of partial melting in the mantle sources that feed volcanoes in continental rift areas and hot-spots. The results obtained, indicating that the carbon-rich portions of the mantle melt more deeply than the carbon-poor portions, confirming the role of primary importance played by this element in guiding geodynamic cycles.
The existence of a carbon-rich mantle, highlighted by Aiuppa and co-authors, has profound implications for the ways in which primordial carbon is stored in the mantle, and for its recycling in time and space. The results obtained with this study are also important for understanding the possible variations in the geological carbon cycle caused by large-magnitude volcanic events, such as the emplacement of "Large Igneous Provinces (LIP)", or large igneous provinces. If the magmas produced by mantle plumes are carbon-rich, as this study suggests, then the release of carbon from large igneous provinces in the Phanerozoic may have contributed to the mass extinctions, traces of which are preserved in sedimentary records around the world.
QUOTE
Alessandro Aiuppa, Federico Casetta, Massimo Coltorti, Vincenzo Stagno and Giancarlo Tamburello (2021), Carbon concentration increases with depth of melting in Earth's upper mantle, Nature Geoscience, https://doi.org/10.1038/s41561-021-00797-y
The research was funded by the Deep Carbon Observatory (https://deepcarbon.net/) and from the Miur, Project PRIN2017 Connect4Carbon (https://prin2017.wixsite.com/connectforcarbon)
Photo - CO2-rich magmatic gases released from the outgassing of the open vent lava lake at Nyiragongo volcano, Democratic Republic of Congo (photo by Sergio Calabrese, University of Palermo).
Picture 1 - Increase in carbon concentration with melting depth in the Earth's upper mantle. The magmas produced in Oceanic Islands and Continental Rift contexts are fed by mantle sources richer in Carbon than the portions of the "Depleted MORB Mantle (DMM)", i.e. depleted mantle from which the "Mid-Ocean Ridge Basalts" are produced (MORB)”, or mid-ocean ridge basalts.
picture 2 - Schematic section from the Atlantic Ocean to the Indian Ocean (passing through the African craton), showing the variations in Carbon concentrations reconstructed in the mantle sources from which the Oceanic Islands and Continental Rifts magmas are produced.