Carbon concentration increases with depth of melting in Earth’s upper mantle

Abstract

Carbon in the upper mantle controls incipient melting of carbonated peridotite and so acts as a critical driver of plate tectonics. The carbon-rich melts that form control the rate of volatile outflux from the Earth's interior, contributing to climate evolution over geological times. However, attempts to constrain the carbon concentrations of the mantle source beneath Oceanic Islands and Continental Rifts is complicated by pre-eruptive volatile loss from magmas. Here, we compile literature data on magmatic gases, as surface expression of the pre-eruptive volatile loss, from 12 Oceanic Island and continental Rift volcanoes. We find the levels of carbon enrichment in magmatic gases correlate with the trace element signatures of the corresponding volcanic rocks, implying a mantle source control. We use this global association to estimate that the mean carbon concentration in the upper mantle, down to 200 km depth, is approximately 350 ppm (range, 117-669 ppm). We interpret carbon mantle heterogeneities to reflect variable extents of mantle metasomatism from carbonated silicate melts. Finally, we find that the extent of carbon enrichment in the upper mantle positively correlates with the depth at which melting starts. Our results imply a major role of C in driving melt formation in the upper mantle.

Machine Learning Investigation of Clinopyroxene Compositions to Evaluate and Predict Mantle Metasomatism Worldwide

“…The results show that no correlation exists between the probability of metasomatism and rock type, including clinopyroxenite, dunite, harzburgite, lherzolite, peridotite, pyroxenite, and wehrlite (Figure S6 in Supporting Information S1). Indeed, it has been suggested that metasomatism may occur in various tectonic settings (Aiuppa et al., 2021; Dawson, 1984; Liu et al., 2021; Menzies & Murthy, 1980; Roden & Murthy, 1985; Wang et al., 2022). In particular, carbon and water lower the melting temperatures of peridotites, and carbonated and hydrous silicate melts have been suggested as effective metasomatic agents (Dasgupta & Hirschmann, 2006; Hirschmann, 2000; Sarafian et al., 2017; Sun & Dasgupta, 2019; Thomson et al., 2016).…”

“…Metasomatism modifies the mineralogy and composition of pre‐existing rocks through reaction with melt/fluid at high temperature. This important process produces geochemical and isotopic heterogeneities within Earth's mantle (Aiuppa et al., 2021; Roden & Murthy, 1985; Wang et al., 2022; Zhang et al., 2009), which in turn affect chemical differentiation in the mantle, craton stability, and the physical properties of the lithosphere (Araújo et al., 2009; Dawson, 1984; Liu et al., 2021; Lloyd & Bailey, 1975; Menzies & Murthy, 1980; O’Reilly & Griffin, 2013; Pearson et al., 2021; Peng et al., 2021; Rudnick et al., 1993). Therefore, evaluating mantle metasomatism at the global scale is essential to understanding mantle heterogeneity.…”

Gas Leakage From Shallow Ponding Magma and Trapdoor Faulting at Sierra Negra Volcano (Isabela Island, Galápagos)

“…The dashed gray lines, referred as "H 2 S/SO 2 re-equilibration," describe the evolution of the uprising magmatic gas while its SO 2 /H 2 S ratio re-equilibrates (at constant CO 2 /S T and redox at ΔNNO = −0.75) during decompression from 50 to 60 MPa (magmatic sill) down to 0.1 MPa. Intraplate-continental rift magmatic gases (white circles) range from C-rich to C-poor, depending on the C-enriched or C-depleted signature of their mantle source (Aiuppa et al, 2021).…”

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Magmatic gas delivered by intraplate, hot-spot related volcanism offers important insight into the abundance and distribution of volatiles in the Earth's upper mantle (Aiuppa et al, 2021) and hence into the rates and mechanisms of volatile exchange in and out our planet (Dasgupta & Hirschmann, 2010). Hot-spot magmatic gases have long been recognized (Gerlach, 1982;Symonds et al, 1994) to exbibit CO 2 -richer (and H 2 O-poorer) compositions relative to arc magmatic gases (Fischer, 2008;Fischer & Chiodini, 2015;Oppenheimer et al, 2014;Taran & Zelenski, 2015), attesting for the presence of a carbon-rich mantle reservoir (Aiuppa et al, 2021, and references therein) at depths higher than the shallow (<50 km) Depleted Mantle (DM) sampled by MORBs (Mid-Ocean Ridge Basalts;Hauri et al, 2019).

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“…Combined with the specific trace element and radiogenic isotope signatures of hot-spot volcanic rocks (Hoffman, 2003;Zindler & Hart, 1986), the composition of intraplate magmatic gases may thus provide unique information on volatiles' heterogeneities in mantle plumes. Unfortunately, however, the existing data set for hot-spot magmatic gases is still limited (Aiuppa et al, 2021), detailed information being available for only the…”