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Slab melting as a barrier to deep carbon subduction

Abstract

Interactions between crustal and mantle reservoirs dominate the surface inventory of volatile elements over geological time, moderating atmospheric composition and maintaining a life-supporting planet1. While volcanoes expel volatile components into surface reservoirs, subduction of oceanic crust is responsible for replenishment of mantle reservoirs2,3. Many natural, ‘superdeep’ diamonds originating in the deep upper mantle and transition zone host mineral inclusions, indicating an affinity to subducted oceanic crust4,5,6,7. Here we show that the majority of slab geotherms will intersect a deep depression along the melting curve of carbonated oceanic crust at depths of approximately 300 to 700 kilometres, creating a barrier to direct carbonate recycling into the deep mantle. Low-degree partial melts are alkaline carbonatites that are highly reactive with reduced ambient mantle, producing diamond. Many inclusions in superdeep diamonds are best explained by carbonate melt–peridotite reaction. A deep carbon barrier may dominate the recycling of carbon in the mantle and contribute to chemical and isotopic heterogeneity of the mantle reservoir.

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Figure 1: The melting curve of carbonated MORB compared to hot and cold subduction geotherms26.
Figure 2: Composition of majoritic garnet minerals from previous experimental studies, inclusions in diamonds and reaction experiments.
Figure 3: Composition of ferropericlase minerals from previous experimental studies, inclusions in diamonds and reaction experiments.
Figure 4: Schematic of the deep mantle carbon cycle.

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Acknowledgements

A.R.T. acknowledges the support of NERC grant NE/J500033/1. M.J.W. and S.C.K. acknowledge the support of NERC grant NE/J008583/1. We thank S. Kearns and B. Buse for their assistance performing electron probe microanalyses and J. Blundy for contributing ideas and expertise during discussions with the authors.

Author information

Authors and Affiliations

Authors

Contributions

A.R.T. designed, performed and analysed the experiments, gathered data from the literature and wrote the manuscript as part of his PhD studies. M.J.W. and S.C.K. provided training in experimental techniques, assisted during interpretation of results, provided advice and assisted with manuscript preparation in their roles as A.R.T.’s PhD supervisors. R.A.B. provided training and assistance with experimental techniques and sample preparation alongside contributing to the scientific content and preparation of the manuscript.

Corresponding author

Correspondence to Andrew R. Thomson.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Comparison of experimental compositions with natural rocks.

af, ‘Fresh’ MORB rocks (red field), ALL-MORB21 (red circle), altered MORB rocks41 (pale blue circles), exhumed blueschist, greenschist and/or eclogitic rocks (yellow circles) and starting material from this (dark blue circle) and previous studies (green circles) of carbonated MORB compositions. In a, rocks altered MORB and exhumed rock compositions that fall on the Mg-Fe side of the maj–cpx join from Extended Data Fig. 5 plot below the dashed line, compositions that lie on the Ca side of this join are plotted as orange circles with yellow outlines or purple circles with blue outlines and sit above the dashed curve. This confirms that magnesite will be the stable carbonate phase at high pressure in the vast majority of natural crustal rocks, as is the case for ATCM1. Data and corresponding references for this figure are provided in the online source data file.

Source data

Extended Data Figure 2 Experimental results/phase diagram and interpreted solidus position.

The reactions clinopyroxene + CO2 = dolomite + 2coesite and dolomite = magnesite + aragonite are from refs 22 and 23 respectively. The upper left curve is the anhydrous MORB solidus. Note that due to temperature gradients in experiments at 8 GPa, a small quantity of dolomite is observed coexisting with melt in one experiment above the solidus, present at the cold end of the capsule. arag, aragonite; CM, carbonatite melt; cpx, clinopyroxene; cs, coesite; dol, dolomite; gt, garnet; mag, magnesite; maj, majoritic garnet; Na carb, Na carbonate; ox, FeTi oxide; SM, silicate melt; st, stishovite.

Extended Data Figure 3 BSE images of experimental products.

a, 7.9 GPa, 1,250 °C; b, 7.9 GPa, 1,350 °C; c, 13.1 GPa, 1,350 °C; d, 13.1 GPa; 1,450 °C; e, 20.7 GPa, 1,100 °C; f, 20.7 GPa, 1,480 °C; g, 20.7 GPa, 1,600 °C; h, sandwich experiment, 20.7 GPa, 1,400 °C. Scale bars, 10 μm. CM, carbonatite melt; cpx, clinopyroxene; dol, dolomite; FeTi, FeTi oxide; gt, garnet; mag, magnesite.

Extended Data Figure 4 Composition of experimental melts from this study.

a, b, Experimental melts from selected previous studies marked with semi-transparent greyscale symbols. b, The effects of increasing pressure, temperature and the effect of contamination due to partial analysis of silicate minerals surrounding small melt pools are shown.

Extended Data Figure 5 The composition of experimental phases from this study projected into two quaternary plots.

a, b, [Ca]-[Mg+Fe2+]-[Si+Ti]-[Na+K] (a) and [Mg+Fe2+]-[Ca]-[Al+Fe3+]-[Na+K] (b). In both diagrams the grey fields are the compositional data projected onto the basal ternary. The red field is the range of natural MORB compositions projected onto the basal ternary. The yellow star plotted in the four-component system and projected onto the basal ternary is ATCM1 (our bulk composition) while the black stars are bulk compositions from previous studies25,26,27.

Extended Data Figure 6 BSE images of reaction experiments.

ad, G169 (a, b) and G177 (c, d). In both experiments a reaction zone and remaining carbonatite melt surrounds the unreacted peridotite region. a, An overview of G169. b, A close up of the reaction in G169 containing newly crystallized calcium perovskite, majorite, ferropericlase and ringwoodite minerals. c, A close up of the reaction products in G177, which consist of small bright calcium perovskites, new majorite that is often observed as a rim on relic peridotitic garnet and ringwoodite. d, An overview of G177. CaPv, calcium perovskite; fper, ferropericlase; maj, majorite; rw, ringwoodite; wad, wadsleyite.

Source data

Extended Data Figure 7 Raman spectra of minerals from reaction experiment G177 measured using a blue 455 cm−1 excitation laser.

The position of the main peaks in each collected spectrum have been labelled with their shift from the excitation laser in cm−1.

Extended Data Figure 8 Comparison of diamond-hosted calcium perovskite inclusions with experimental mineral compositions in MgO versus Ti number space.

Ti number = Ti/[Ca+Ti]. Data and corresponding references for this figure are provided in the online source data file.

Extended Data Table 1 Starting materials used in this and previous studies
Extended Data Table 2 Summary of run conditions and products for carbonated MORB melting experiments
Extended Data Table 3 Summary of reaction experiments run conditions and experimental products

Supplementary information

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This file contains Supplementary Tables 1-5. (XLSX 146 kb)

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Thomson, A., Walter, M., Kohn, S. et al. Slab melting as a barrier to deep carbon subduction. Nature 529, 76–79 (2016). https://doi.org/10.1038/nature16174

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