# Enigmatic carbon isotopic variability in the oceanic upper mantle

Yves Moussallam, Estelle Rose-Koga, Cyril Aubaud, Guillaume Georgeais, Pierre Cartigny, Kenneth Koga, Jean-Luc Devidal, Peter Michael, Kei Shimizu, Alberto Saal

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## To cite this version:

Yves Moussallam, Estelle Rose-Koga, Cyril Aubaud, Guillaume Georgeais, Pierre Cartigny, et al. Enigmatic carbon isotopic variability in the oceanic upper mantle. Proceedings of the National Academy of Sciences of the United States of America, 2025, 122 (25), 10.1073/pnas.2502886122. insu-05162065

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https://insu.hal.science/insu-05162065v1

Submitted on 20 Aug 2025

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Main Manuscript for

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Enigmatic Carbon Isotopic Variability in the Oceanic Upper

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Mantle

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Yves Moussallam¹, Estelle F. Rose-Koga², Cyril Aubaud³, Guillaume Georgeais¹, Pierre

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Cartigny³, Kenneth T. Koga², Jean-Luc Devidal⁴, Peter J. Michael⁵, Kei Shimizu⁶, Alberto

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E. Saal⁷.

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¹ Lamont-Doherty Earth Observatory, Columbia University, New York, USA.

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² ISTO, UMR 7327, Université Orléans-CNRS-OSUC, 1A rue de la Férollerie, 45071 Orléans
cedex 2, France.

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³ Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, Laboratoire de
Géochimie des Isotopes Stables, 1 rue Jussieu, F-75005 Paris, France.

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⁴ Laboratoire Magmas et Volcans, CNRS, IRD, OPGC, Université Clermont Auvergne, 63000,
Clermont-Ferrand, France.

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⁵ Department of Geosciences, The University of Tulsa, 800 South Tucker Drive, Tulsa, OK
74104, USA.

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⁶ WiscSIMS, Department of Geosciences, University of Wisconsin-Madison, USA.

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⁷ Dept. of Earth, Environmental and Planetary Sciences, Brown University, USA.
20 **Corresponding author**: Yves Moussallam; yves.moussallam@ldeo.columbia.edu

21 **Author Contributions**: SIMS analyses (Y.M., E.R.K.); LA-ICPMS analyses (E.R.K., K.K., J-

22 L.D.); Sample collection and preparation (Y.M., C.A., G.G., P.M., D.G., A.S., K.S); Manuscript

23 writing (all authors); Visualization and data compilation (Y.M.); First draft (Y.M.)

24 **Competing Interest Statement**: None

25 **Classification**: Physical Science; Earth, Atmospheric, and Planetary Sciences.

26 **Keywords**: carbon isotopes; undersaturated melts; mid-ocean ridge basalts; upper mantle

27 **This PDF file includes**: Main Text; Figures 1 to 2

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29 **Abstract**

30 Unraveling the origin(s) of carbon on Earth has remained challenging, not only because of the

31 multiple isotopic fractionation episodes that may have occurred during planet formation

32 processes, but also because the end point of these processes, the current isotopic value of Earth's

33 deep carbon reservoirs, remains poorly constrained. Here we present Carbon isotopic

34 measurements on rare undegassed mid-ocean ridge basalts from the Pacific, Atlantic and Arctic

35 Oceans that have preserved the isotopic signature of their mantle source. We find that Earth's

36 present-day convecting upper mantle has variable δ¹³C value from ~-10 to -4‰, significantly

37 different from the δ¹³C value of peridotitic diamonds and with the highest values being restricted

38 to the Atlantic. Evidence for significant mantle heterogeneity contrasts with previous

39 assumptions and its origin remains puzzling being uncorrelated with geochemical markers

40 associated with neither subduction and surficial recycling processes nor lower mantle

41 contributions. The data do not preclude other causes such as primordial mantle heterogeneity.

42 We suggest that the δ¹³C value of the bulk silicate Earth may need to be revised.
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44 **Significance Statement**

45 Understanding the origin and distribution of carbon in Earth's mantle is crucial for deciphering
46 our planet's formation and evolution. We present a comprehensive dataset of δ¹³C measurements
47 from rare, undegassed mid-ocean ridge melts across the Pacific, Atlantic, and Arctic Oceans,
48 revealing significant heterogeneity in the carbon isotopic signature of Earth's upper mantle. Our
49 findings challenge the prevailing view that the bulk silicate Earth has a δ¹³C value of -5‰, as
50 previously constrained by diamonds. Instead, we propose that most of the upper mantle may be
51 around -8.5‰, with a distinct signature of -4‰ beneath the Atlantic Ocean. The observed
52 heterogeneity cannot be explained by conventional geochemical markers or known mantle
53 processes. Ancient, pre-plate tectonic heterogeneities may persist in the mantle.
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55 **Main Text**

56 **INTRODUCTION**

57 To elucidate the provenance of elements on Earth, cosmochemists compare the stable isotopic
58 compositions of these elements in various solar system bodies (meteorites, comets, planets, and
59 the Sun) to those of Earth (e.g.,(1–3). However, for carbon, this straightforward comparison is
60 complicated by the likely isotopic fractionation during Earth's formation and the poorly
61 constrained present bulk carbon isotopic composition of our planet (e.g., (4) in addition to the
62 large variability recorded in meteorites (e.g., (5). Most of Earth's carbon resides in its deep
63 interior (e.g., (6), and while direct measurement of the core's carbon isotopic signature may
64 remain infeasible, such measurements for the mantle, and thus the bulk silicate Earth (BSE), are
65 attainable. To date, our estimates of the BSE's δ¹³C have relied on two primary data sources. The
first is derived from diamond studies, which exhibit a wide range of carbon isotope signatures (from -41‰ to +5‰ in δ¹³C, all values given relative to Pee Dee Belemnite), yet display a distinct mode at -5‰ ± 1‰ (e.g., (7, 8), widely interpreted as the initial mantle signature (e.g., (9). The second source is carbon isotope measurements of CO₂ gas trapped in vesicles from two rare 'popping rock' samples (2πD43 and OT03-09) collected at 14°N and 34°N along the Mid-Atlantic Ridge (10–12). These samples are thought to retain all CO₂ degassed from the melt during magma ascent and indicate an initial mantle δ¹³C of -4‰. The 'popping rock' samples are exceptional because most mid-ocean ridge basalts (MORB) normally lose most of their initial CO₂ during ascent and degassing, and the significant gas-melt isotopic fractionation factor (13–15) results in δ¹³C values in the melt that deviate substantially from those of their sources (e.g., (16). In addition to these few samples, rare instances of ultradepleted MORBs, undersaturated and undegassed with respect to CO₂ have been documented across all ocean basins (17–20). The δ¹³C signatures of these melts remain unaltered during ascent, and thus, should directly characterize their source. An initial set of δ¹³C measurements was conducted on a suite of CO₂-undersaturated olivine-hosted melt inclusions from the Indian Ocean (21) that yielded a δ¹³C value of -7.5‰ (±0.7‰; 1σ), strikingly different from that of the Atlantic popping rocks. Here, we present an extended dataset of δ¹³C measurements on CO₂-undersaturated basalt glasses and melt inclusions from the Pacific, Atlantic, and Arctic oceans.

## RESULTS

δ¹³C measurements were performed by secondary ion mass spectrometry using a Cameca IMS 1280 ion microprobe at CRPG-CNRS-Nancy, France. The results of our analyses are presented in Table S1 to S3, and alongside data from the 'popping rock' and Indian Ocean samples in
Figures 1 and 2. While our analytical approach is still novel, the results align remarkably well with established methodologies. For instance, the δ¹³C value of sample GN09-02 (Garrett Transform Fault) was previously measured using step-heating C-extraction in a vacuum line (a bulk analysis technique) by ref. (22), who reported a δ¹³C value of -10.8‰ (±2.1‰; 1σ). This value is within error margins of our reported value of -9.9‰ (±0.8‰; 1σ). It is noteworthy also that the δ¹³C value of our Mid-Atlantic Ridge sample (RC2806 1D-1g), measured at -3.6‰ (±1.1‰; 1σ), aligns remarkably well with the previously reported δ¹³C values of the Mid-Atlantic 'popping rock' vesicles (-4‰) (23). The consistency between these two independent techniques reinforces confidence in our results.

Figure 1 illustrates a striking heterogeneity in the δ¹³C values of the upper mantle underlying oceanic ridges. Data from the Mid-Atlantic Ridge, including literature values for 'popping rock' and our undegassed samples, cluster around an upper mantle δ¹³C signature of -3.9‰ (±0.4‰; 1σ). In contrast, Mid-Ocean Ridge data for the Pacific indicate a distinct upper mantle δ¹³C signature of -9.1‰ (±1.3‰; 1σ), or -8.5‰ (±1.3‰; 1σ) when considering data from the Rano-Rahi seamounts. Samples from the Gakkel Ridge (Arctic Mid-Ocean Ridge) point to δ¹³C values of -8.5‰ (±0.2‰; 1σ), while data from the Indian Ocean Mid-Ocean Ridge suggest a signature of -7.5‰ (±0.7‰; 1σ).

# DISCUSSION

To investigate the cause of the observed mantle carbon isotope heterogeneity we turn to trace element systematics. CO₂ is highly incompatible during partial melting and the ratio of CO₂ to refractory trace elements with similar incompatibility (e.g., Ba or Nb) in CO₂-undersaturated

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melts is commonly used as an indication of the CO₂ content of the source region (e.g., 17, 19, 24). We observe no correlation between δ¹³C and CO₂/Ba (Fig. 2A) or any CO₂ to other incompatible refractory trace elements ratios, nor do we observe any significant correlation between δ¹³C and CO₂ content of the melt or calculated CO₂ content of the source (see supplementary information Fig. S5). This indicates that the upper mantle carbon isotopic signature is uncorrelated with its CO₂ content. Consequently, processes governing mantle CO₂ content may differ from those driving the mantle carbon isotopic signature. The lack of correlation between δ¹³C and radiogenic isotope ratios such as ²⁰⁶Pb/²⁰⁴Pb, (Fig. 2B), ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, or ¹⁷⁶Hf/¹⁷⁷Hf (Table S3), commonly used to categorize MORB lavas into variably enriched or depleted components (e.g., (25), suggests that the mantle δ¹³C values are not directly linked to processes leading to these types of chemical enrichment or depletion of the mantle source (commonly attributed to recycling of crustal components). We also found no correlation with incompatible trace element enrichment (see supplementary information Fig. S4 and S5). Finally, the absence of a significant correlation between δ¹³C and ³He/⁴He ratios (Fig. 2C) indicates that the contribution of primordial mantle material (e.g., (26) is not a significant factor in determining the observed carbon isotopic compositions.

Our previous study (21) reported unexpectedly negative δ¹³C values of CO₂-undersaturated melts from the Indian Ocean compared to the Atlantic popping rocks. In it, we proposed that this signature might reflect upper mantle contamination by subducted fluids (following the model of ref. 27). The study predicted that future measurements should fall under an umbrella defined by mixing lines between a “normal” mantle component and variably metasomatized mantle components, with carbon isotopic signatures ranging from pure organic carbon (≈25‰) to pure
carbonate (≈0‰) contamination. Note that the subduction metasomatized mantle endmembers considered in this scenario are mantle components fluxed only with subduction fluids, but they are neither incorporating the subducted sediments nor the subducted oceanic crust. Our new measurements fit within this conceptual framework if we consider the Nb/U versus δ¹³C distribution (Fig. 2D). However, other trace element ratios such as Th/Nb and H₂O/Ce, which have also been proposed as tracers of subduction fluids (27), show no correlation with δ¹³C (Fig. 2E). Therefore, available data do not support ‘subducted fluids’ as the mechanism behind the observed variability in mantle δ¹³C signatures.

Excluding the Atlantic Ocean, all other δ¹³C values from undegassed mid-ocean ridge samples from the Arctic, Pacific and Indian oceans, are almost all within error, with a mean of -8.8‰ (±1.2‰; 1σ). This suggests that most of the upper mantle is likely homogeneous around this value, with the exception of the region beneath the Atlantic Ocean, which exhibits a distinct signature at -3.9‰ (±0.4‰; 1σ) (Fig. 1). The presence of spatially constrained mantle domains in carbon isotope is intriguing and will need to be confirmed as more samples become available.
We note that a mantle δ¹³C value around -8.5‰ is consistent with data from degassed MORB samples (see supplementary information Fig. S3) although these offer little constraints. The current dataset, although still limited, indicates that significant portions of the upper mantle exhibit a carbon isotopic signature that is notably more negative than the canonical mean value of -5‰ observed in peridotitic diamonds and kimberlites (Fig. 3). While the carbon isotopic signature of peridotitic diamonds is often considered representative of the entire Earth's upper mantle, these diamonds are found exclusively in continental, cratonic settings. As a result, they offer insights into the isotopic signature of the upper mantle beneath these specific regions,
which constitute less than 10% of the Earth's surface. In contrast, approximately 70% of the Earth's surface is covered by oceanic crust, and the carbon isotopic signature of the upper mantle beneath mid-ocean ridges may offer a more representative view of the Earth's mantle as a whole.

Peridotitic diamonds may also be biased toward sampling mostly older and/or metasomatized mantle domains.

In conclusion, we found that carbon isotope composition in worldwide CO₂-undersaturated melts from mid-ocean ridges, show striking heterogeneities (from -3.6 to -10.5‰) that cannot be ascribed to degassing but instead reveal mantle domains with distinct carbon isotopic compositions. We found no significant correlation between the mantle δ¹³C signatures and indicators of mantle (i) CO₂ content, (ii) degree of radiogenic isotopes enrichment, (iii) extent of partial melting, (iv) scale of mixing with primordial material, nor (v) degree of influence by subduction fluids. Our data indicate that a δ¹³C value of -4‰ can no longer be considered representative of the 'normal' present-day MORB mantle. This value is still representative of the mantle underneath the Atlantic Ocean, but samples from all other ocean basins display values between -7.5‰ and -10‰, with a mean of -8.4‰ (±1.1‰; 1σ). We suggest that this latter value may be more representative of the global upper mantle δ¹³C signature, outside of the Atlantic while the δ¹³C value of -5‰ preserved in peridotitic diamonds may only be representative of sub-cratonic mantle domains. The mechanism behind the observed variability in oceanic mantle δ¹³C signatures and its possible grouping in distinct mantle domains remains undetermined. The hypothesis that they might reflect old, pre-plate tectonic heterogeneities cannot at this point be discarded.
181 MATERIALS AND METHODS

182 Our sample suite consists of (i) basaltic glasses from mid-oceans ridges, including the Equatorial
183 Mid-Atlantic ridge (RC2806 1D-1g), East Pacific Rise (EN112 7D-2; EN113 13D-1; EN113
184 26D-2), Gakkel Ridge (PS59-199-003; HLY0102-055-019) and Garrett transform fault (GN09-
185 02) (18), (ii) melt inclusions from a mid-ocean ridge basalt from the Siqueiros transform fault
186 (A25-D20-1) (17), and (iii) basaltic glasses from the Rano Rahi seamounts (GL08-68-4, GL08-
187 72-1 and GL08-73-4) (28). The δ¹³C of all samples was measured by secondary ion mass
188 spectrometry, building on the recent development of this analytical technique for silicate glasses
189 (29). Trace element concentrations were analyzed by laser ablation-inductively coupled plasma-
190 mass spectrometry. All analytical details are provided in the supplementary materials.

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192 ACKNOWLEDGMENTS

193 We thank Nordine Bouden and Johan Villeneuve for their invaluable help during the ion probe
194 analyses at CRPG Nancy. All the staff of the CRPG-CNRS French National SIMS facility is
195 thank for their never-failing support. Y.M acknowledges support from the US National Science
196 Foundation under Award No. EAR-2407264. E.R-K thank Saskia Erdmann (ISTO-CNRS) for
197 her support during the LA-ICPMS measurements. We are very grateful to Katherine Kelley and
198 Bill White for their review, comments and suggestions on a previous version of this manuscript.

199

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<figure><img src="image_3.png" /><figcaption>**Figure 1:** Mercator projection world map and upper right inset N-polar projection map, showing the location and δ¹³C (‰) value of CO₂-undersaturated MORB melts from this study and ref. 21 together with δ¹³C (‰) value of vesicles from the “popping rock” samples reported in ref. 23. The δ¹³C values reported are in each case directly representative of that of the mantle source at each location, indicating a strong mantle heterogeneity in δ¹³C between ocean basins. Base maps layers are from the Global Multi-Resolution Topography (GMRT) synthesis, Lamont-Doherty Earth Observatory, Columbia University.</figcaption></figure>

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<figure><img src="image_4.png" /><figcaption>**Figure 2:** δ<sup>13</sup>C (‰) value of CO<sub>2</sub>-undersaturated MORB and of vesicles from the "popping rock" samples compared to **A.** The glass CO<sub>2</sub>/Ba ratio, a proxy for CO<sub>2</sub> abundance in the mantle. **B.** The glass <sup>206</sup>Pb/<sup>204</sup>Pb ratio, a proxy of chemical enrichment or depletion of the</figcaption></figure>

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298 mantle. The $^{206}$Pb/$^{204}$Pb values of the bulk silicate Earth (BSE), depleted MORB mantle (DMM) and prevalent mantle" (PREMA) are shown along the y-axis. C. The glass $^{3}$He/$^{4}$He ratio, a proxy for primordial mantle contribution. D. The glass Nb/U ratio, a proxy of mantle metasomatism by subduction fluids. Mixing lines from ref.(21) between a "normal" MORB mantle and two possible subduction-fluid metasomatized mantles endmembers (from left to right; pure organic, and pure marine carbonate) are shown. E. The glass H$_{2}$O/Ce ratio, another proxy of mantle metasomatism by subduction fluids. Error bars show 95% confidence intervals.

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**Figure 3:** Carbon isotope composition (δ¹³C) of CO₂-undersaturated MORBs (n= 9; this study and ref. (21)) compared to popping rocks (n=2; ref. (23)), all diamonds (n= 4440; database from ref. (30)), Peridotitic diamonds only (n=708; literature database from ref. (31)) and kimberlites (n=65; database from ref. (32)). **A.** Shows 1‰ binning interval histograms between 0 and -12‰. **B.** Shows box and whisker diagram with horizontal lines representing the median of each population, boxes show interquartile range and whiskers show full distribution excluding “outliers” assuming a normal distribution.

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