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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 a surface expression of the pre-eruptive volatile loss, from 12 oceanic island and continental rift volcanoes. We find that 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 carbon in driving melt formation in the upper mantle.

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Fig. 1: CO2 content in mafic melts from plume-related and continental rift volcanoes.
Fig. 2: Time-averaged volcanic gas CO2/ST ratios versus mean WR Sr/Sm and Sr/Nd ratios.
Fig. 3: C content in source mantles of plume-related (OIB and Iceland) and continental rift volcanoes.
Fig. 4: Composition of natural carbonatites and kimberlites.
Fig. 5: Modelling the gas–rock association.
Fig. 6: Schematic representation of vertical distribution of carbon in upper mantle.

Data availability

All data generated or analysed during this study are included in this published article (Extended Data Tables 14). The dataset is also publicly available in the EarthChem data repository106 (https://www.earthchem.org/ecl/). Source data are provided with this paper.

Code availability

The code that supports the findings of this study and that was used to generate Figs. 1 and 5 and Extended Data Fig. 6 is available from G.T. (giancarlo.tamburello@ingv.it) upon request.

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Acknowledgements

This work received funding from the Deep Carbon Observatory (subcontract no. 10759-1238; A.A.) and from the Italian Ministero Istruzione Università e Ricerca (Miur, Grant N. 2017LMNLAW; A.A.). The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. The authors thank A. Rohrbach for useful comments on an earlier version of the manuscript.

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A.A. devised the study concept. A.A., F.C., M.C., V.S. and G.T. contributed to refinement of the initial concept, and to data analysis and interpretation. A.A. drafted the original version of the manuscript with contributions from all coauthors.

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Correspondence to Alessandro Aiuppa.

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Extended data

Extended Data Fig. 1 Global map illustrating the location of the 12 hot-spot and continental rift volcanoes.

The base map is a relief and bathymetry Raster called “Natural Earth II with Shaded Relief and Water” file #NE2_HR_LC_SR_W.tiff (Made with Natural Earth. Free vector and raster map data @ naturalearthdata.com). The shaded relief is the CleanTOPO2 layer, a modified SRTM30 Plus World Elevation Data also edited by Tom Patterson, US National Park Service (original data source, ref. 62).

Extended Data Fig. 2 Whole-rock compositions.

(a) Total Alkali Silica (TAS) diagram for the 12 volcanoes. The most primitive magmas erupted from Kilauea, Piton de la Fournaise, Erta Ale, Ardoukoba, Surtsey and Holuhraun (Bardabunga) are subalkaline, with Na2O + K2O contents not exceeding 3.5 wt%, at SiO2 comprised between 43 and 50 wt%. They all plot in the basaltic field, except for the one belonging to Piton de la Fournaise, which is a picrite. The most primitive magmas erupted from Etna and Nyamuragira are mildly alkaline (alkali basalts; Na2O + K2O around 4.0 wt%; SiO2 = 45-46 wt%), whereas those belonging to Erebus, Pico do Fogo, Nyiragongo and Ol Doinyo Lengai are alkaline to highly alkaline, with alkali and silica contents up to 7.4 wt% and down to 35 wt%, respectively. They range in composition from tephrites/basanites (Erebus and Pico do Fogo) to melilitites/nephelinites (Nyiragongo and Ol Doinyo Lengai). (b) Chondrite-normalised spider diagram84. The chondrite-normalized incompatible element patterns demonstrate the discrimination between subalkaline and alkaline rocks. The Bardarbunga basalt has the most depleted pattern, with a flat REE profile [(La/Yb)N = 1.7]. Primitive basalts/picrites from Kilauea, Surtsey, Piton de la Fournaise, and Ardoukoba are progressively enriched in LILE and Nb-Ta and are all characterized by Ti negative anomalies, except for Surtsey. They are characterized by variable HREE abundances at comparable LREE contents, resulting in moderately to significantly steep REE profiles. (La/Yb)N vary from 3.6 (Ardoukoba) to 5.0-6.3 (Kilauea and Piton de la Fournaise), testifying for the increasingly important role played by garnet in their mantle sources. With respect to the other subalkaline rocks, Erta Ale basalt is enriched in Th-U, Nb-Ta, and is typified by steeper REE profiles [(La/Yb)N = 6.5]. Etna and Nyamuragira alkali basalts are enriched in Rb, Ba, Th, U and have analogous REE profiles [(La/Yb)N = 15.3-22.1], at HREE abundances comparable to those of Piton de la Fournaise picrite. With respect to Nyamuragira, however, Etna has lower Nb-Ta and Ti concentrations. Erebus basanite and Pico do Fogo tephrites have Rb-Ba enrichments, Nb-Ta positive anomalies and steep REE profiles, at HREE concentrations comparable to Etna, Nyamuragira and Piton de la Fournaise basalts/alkali basalts [(La/Yb)N = 10.1-19.1]. The Nyiragongo and Ol Doinyo Lengai melilitites/nephelinites exhibit the most significant enrichments in incompatible elements, accompanied by Ti negative anomalies and very steep REE patterns, with (La/Yb)N up to 41.9-50.9 at YbN values comprised between 17 and 23.

Extended Data Fig. 3 Gas CO2/ST ratios vs. whole-rock trace element ratios.

Examples of scatter plots contrasting, for the 12 volcanoes, the time-averaged gas CO2/ST ratios with trace-element compositions of the corresponding whole-rocks (trace-element ratios use element Sr as the common reference). The best-fit regression lines (with equations and regression coefficients) are shown in red. The averaged composition of the Depleted MORB Mantle (DMM) is calculated by combining data in ref. 1,20 (C), ref. 3 (S) and ref. 18,85 (trace elements).

Extended Data Fig. 4 Comparison between Melt Inclusion (MIs) and whole-rock compositions for Kilauea volcano.

Sr/Nd and Sr/Sm ratios (upper panels) are overlapping for the two datasets, and show no dependence on magma differentiation degree (SiO2, in wt. %). The dashed vertical line indicate 52 wt. % SiO2, the threshold below which data are considered for calculating the “mean” compositions. The bottom panels compare the mean Ba (left) and Nb (right) contents (from averaging of all < 52 wt. SiO2 data; red triangle) with the inferred parental melt contents (from averaging the most primitive products only; yellow triangle). Both data types are listed in Extended Data Table 2.

Extended Data Fig. 5 The composition of natural carbonatites and kimberlites, with the composition of experimental melts formed by incipient melting of mantle peridotites/eclogites.

These are used to constrain the C, S and Sr composition of the mCm(s) (Cb1-2 and, Kb1-3). (a) CO2 vs. Sr global distribution of natural carbonatitic and kimberlitic rocks (source GeoRoc; http://georoc.mpch-mainz.gwdg.de/georoc/). These define a compositional array overlapping (but extending to more enriched compositions) the compositional array exhibited by our parental CO2 (inferred, Fig. 1 and Extended Table 3) vs. Sr trend for the 12 hot-spot/rift volcanoes (symbols are as in Fig. 1); (b) The derived Sr/Sm ratios are similar in the 5 mCm model scenarios; (c) Global datasets (GeoRoc) demonstrate that Ca and Sr are globally correlated in natural carbonatitic and kimberlitic rocks, suggesting that calcic (dolomitic) incipient mantle melts are very likely to be Sr-rich, too; (d and e) Global correlations of CO2 vs. SiO2 and CO2 vs. S in natural carbonatitic and kimberlitic rocks, compared with the composition of experimentally derived mantle melts4,105. From data in plot (e), we infer a characteristic (GeoRoc mean) S content of 1000-2000 ppm in carbonatitic to carbonated silicate melts in the mantle105.

Extended Data Fig. 6 The procedure used to model our gas (CO2/ST) - trace element volcano relationships.

The original dataset (red symbols) is back-processed using the Batch melting equations to calculate the corresponding ratios in the source mantle, using F values of 0.2, 0.1, 0.05 and 0.025. Each of the 4 newly obtained datasets (1 for each F value) is best-fitted with a mixing equations (eq. 5), in which the mixing end-members are the DMM (with (CO2/ST)DMM and (Sr/Sm)DMM from ref. 1,3,18,20,85) and a C-rich metasomatizing melt (mCm). The C, S and Sr concentration of the mCm are fixed based on results for natural and experimental carbonatitic/kimberlitic melts (Extended Data Table 4), and the Sm concentration is derived from data regression. The same operation, repeated for couples of trace-element ratios, allows the entire trace-element suite of the mCm to be derived for the 5 distinct model scenarios.

Extended Data Table 1 Time-averaged (mean ± 1 standard deviation, SD) CO2/ST ratios in volcanic gases
Extended Data Table 2 Major and trace element composition for volcanic rocks of the 12 volcano sites
Extended Data Table 3 Derived parental melt CO2 and source mantle carbon contents
Extended Data Table 4 Trace element composition of the metasomatic C-rich melts

Supplementary information

Supplementary information

Supplementary discussion and Fig. 1.

Source data

Source Data Fig. 1

Parental melt CO2, Ba and Nb for 14 volcanoes. Melt inclusions CO2, Ba and Nb data from literature. Calculated end-members for metasomatic C-rich melts.

Source Data Fig. 2

CO2/ST, Sr/Nd and Sr/Sm mass ratios of 12 volcanoes and DMM.

Source Data Fig. 3

Source mantle C concentration, melting pressure and melting depth of 12 volcanoes, DMM and from literature.

Source Data Fig. 4

Parental melt CO2 and Sr concentrations, Sr/Nd and Sr/Sm mass ratios of 12 volcanoes, calculated end-members for metasomatic C-rich melts and from literature.

Source Data Fig. 5

CO2/ST, Sr/Nd and Sr/Sm mass ratios of 12 volcanoes, DMM, MORBs and calculated end-members for metasomatic C-rich melts. Calculate PM-normalized trace elements of the end-members for metasomatic C-rich melts.

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Aiuppa, A., Casetta, F., Coltorti, M. et al. Carbon concentration increases with depth of melting in Earth’s upper mantle. Nat. Geosci. 14, 697–703 (2021). https://doi.org/10.1038/s41561-021-00797-y

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