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Deep, hot, ancient melting recorded by ultralow oxygen fugacity in peridotites

Nature volume 631, pages 801–807 (2024)Cite this article

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Abstract

The oxygen fugacity (fO2) of convecting upper mantle recorded by ridge peridotites varies by more than four orders of magnitude1,2,3. Although much attention has been given to mechanisms that drive variations in mantle fO2 between tectonic settings1,3,4 and to comparisons of fO2 between modern rocks and ancient-mantle-derived rocks5,6,7,8,9,10, comparatively little has been done to understand the origins of the high variability in fO2 recorded by peridotites from modern mid-ocean ridge settings. Here we report the petrography and geochemistry of peridotites from the Gakkel Ridge and East Pacific Rise (EPR), including 16 new high-precision determinations of fO2. Refractory peridotites from the Gakkel Ridge record fO2 more than four orders of magnitude below the mantle average. With thermodynamic and mineral partitioning modelling, we show that excursions to ultralow fO2 can be produced by large degrees of melting at high potential temperature (Tp), beginning in the garnet field and continuing into the spinel field—conditions met during the generation of ancient komatiites but not modern basalts. This does not mean that ambient convecting upper mantle had a lower ferric to ferrous ratio in Archaean times than today nor that modern melting in the garnet field at hotspots produce reduced magmas. Instead, it implies that rafts of ancient, refractory, ultrareduced mantle continue to circulate in the modern mantle while contributing little to modern ridge volcanism.

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Fig. 1: Natural peridotite data.
Fig. 2: Modelling results.
Fig. 3: Comparison of natural data and models.
Fig. 4: Evolution of residues and liquids.

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Data availability

All new data and metadata necessary to reproduce our results are available through EarthChem (https://www.earthchem.org/) at https://doi.org/10.60520/IEDA/113225, using microprobe methods for spinel (https://doi.org/10.60520/IEDA/113226), olivine (https://doi.org/10.60520/IEDA/113227) and orthopyroxene (https://doi.org/10.60520/IEDA/113228).

Code availability

pMELTS modelling in this manuscript can be reproduced using alphaMELTS, which is publicly available at https://magmasource.caltech.edu/alphamelts/. We include instructions for reproducing our pMELTS modelling results in Methods and the Supplementary Information, and all input files and output data are included in the Supplementary Information. Empirical modelling can be reproduced using the provided code (CodeOcean, https://doi.org/10.24433/CO.9619937.v1) and parameters, and our output tables are tabulated in the Supplementary Information.

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Acknowledgements

We thank B. Wood for donating the spinel calibration suite to the Smithsonian Institution (NMNH catalogue no. 118320), T. Rose, R. Wardell and T. Gooding for laboratory support at the Smithsonian Institution, and D. Canil for fruitful discussions and comments that strengthened the manuscript. We acknowledge NSF-OCE 1433212 to E.C., NSF-OCE 1434199 and NSF-OCE 1620276 to J.M.W., NSF-OCE 1433212 and an NMNH Core Grant to E.C., and NSF OCE-1560088 to E.C. that supported Research Experiences for Undergraduates (REU) student M. Said (whom we also thank) during the pilot phase of this project. This research used samples and/or data provided by the Ocean Drilling Program (ODP).

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Authors and Affiliations

  1. Division of Natural Sciences, Nursing, and Mathematics, Berea College, Berea, KY, USA

    Suzanne K. Birner

  2. National Museum of Natural History, Smithsonian Institution, Washington, DC, USA

    Elizabeth Cottrell

  3. Department of Earth and Environmental Sciences, University of Minnesota Duluth, Duluth, MN, USA

    Fred A. Davis

  4. Department of Earth Sciences, University of Delaware, Newark, DE, USA

    Jessica M. Warren

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  1. Suzanne K. Birner
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Contributions

S.K.B.: conceptualization, methodology, software, validation, formal analysis, investigation, resources, data curation, writing—original draft, writing—review and editing, visualization, project administration. E.C.: conceptualization, methodology, resources, writing—original draft, writing—review and editing, supervision, project administration, funding acquisition. F.A.D.: methodology, validation, writing—review and editing. J.M.W.: conceptualization, resources, writing—review and editing, supervision, funding acquisition.

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Correspondence to Suzanne K. Birner.

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Extended data figures and tables

Extended Data Fig. 1 Extended geochemistry.

Geochemistry results for ridge peridotites in this study. Data for residual (non-melt-influenced) samples from the SWIR are from ref. 2, Gakkel Ridge olivine and orthopyroxene data are from ref. 32, all other data are from this study. a, Olivine Fo# versus spinel Cr#. Refractory harzburgites record Mg-rich compositions, consistent with their refractory nature. b, \({X}_{{\rm{Fe}}}^{M1}{X}_{{\rm{Fe}}}^{M2}\) in orthopyroxene versus spinel Cr#. Refractory harzburgites record Fe-poor compositions, consistent with their refractory nature. c, Fe3+/ΣFe ratios in spinel versus spinel Cr#. The Fe3+/ΣFe ratios in spinel are the primary driver of fO2 variations. Several Gakkel Ridge refractory harzburgites record values within an error of zero. d, TiO2 in spinel versus spinel Cr#. The low TiO2 content of these peridotites is one indication that they have not interacted with melts. One sample from the SWIR (KN162-9 D56-33) records slightly elevated TiO2 but does not show the other signs of melt infiltration identified in ref. 25. e, Temperature versus spinel Cr#. We determined temperature using the spinel–olivine Fe–Mg exchange thermometer in ref. 68. No trends are seen between temperature and sample type. f, Yb in orthopyroxene versus spinel Cr#. Refractory samples record both low Yb and high Cr#, both consistent with large degrees of melt extraction. Gakkel Ridge rare earth element data are from ref. 32.

Extended Data Fig. 2 Importance of the normalizing buffer (QFM versus FFM).

The importance of the normalizing buffer when interpreting fO2. a,b, Absolute log(fO2) values for the QFM buffer, the FFM buffer and a peridotite residue undergoing near-fractional adiabatic decompression melting at Tp = 1,350 °C. Absolute fO2 was calculated for pMELTS and empirical model outputs using the empirical spinel oxybarometer of refs. 36,37. QFM was calculated at each pressure and temperature using the formulation in ref. 70; FFM was calculated using the pure-phase Gibbs free energy component of the spl–olv–opx oxybarometer as formulated in refs. 36,71 and described in ref. 37. c,d, log(fO2) for the same peridotite residue, calculated relative to the QFM and FFM buffers. In the subsolidus portion of the spinel stability field, relative fO2 is approximately constant relative to FFM, while apparently increasing relative to QFM. In this case, ΔFFM is a more useful formulation, demonstrating that subsolidus exchange between minerals does not lead to shifts in fO2 in this region51. By contrast, the apparent increase in fO2 as seen when normalizing to QFM is misleading, as this change simply reflects the divergence of the QFM and FFM buffers in P–T space, as illustrated in panels a and b. Relative to QFM, fO2 begins to turn over around 3–4 GPa in the garnet stability field. At depths shallower than this maximum, fO2 is controlled primarily by the passive dilution of ferric iron in pyroxene and spinel during the garnet-to-spinel transition51. At depths deeper than this maximum, fO2 is instead primarily controlled by the stabilization of the ferric iron-bearing garnet endmember skiagite with increasing pressure4,55.

Extended Data Fig. 3 Main-text figures plotted relative to the QFM buffer.

These figures are equivalent to Figs. 2a,b and 3 but illustrate fO2 relative to the more commonly used QFM buffer, rather than the FFM buffer. Relative to QFM, fO2 seems to increase during melting in the spinel stability field. However, this increase is an artefact of the divergence in P–T space between the FFM and QFM buffers (see Extended Data Fig. 2) and does not represent a true oxidative process.

Extended Data Fig. 4 Graphical output of pMELTS model runs.

Melt-allowed runs: Row 1: mass fraction of each phase. At higher Tp, melting initiates deeper. Both garnet-out and cpx-out shift to higher pressures at higher Tp, although garnet-out stays approximately constant once melting begins in the garnet field. Clinopyroxene mode decreases substantially at higher temperatures, in contrast to melt-suppressed runs, in which clinopyroxene mode increases. Row 2: distribution of Fe2O3 between phases. Most of the Fe3+ lost to the melt comes from the clinopyroxene phase. Row 3: ratio of ferric to total iron in bulk rock and solid phases. Whereas the bulk solid Fe3+/ΣFe ratio decreases only slightly at higher Tp, Fe3+/ΣFe ratios decrease substantially in spinel and pyroxene as the potential temperature increases, both in the garnet field and the spinel field. Melt-suppressed runs: Row 1: mass fraction of each phase. At higher Tp, orthopyroxene dissolves into clinopyroxene, garnet mode decreases and both garnet-out and orthopyroxene-out move to higher pressures. Plagioclase, which does not appear in melting-allowed runs, appears in melting-suppressed runs at low pressures owing to retained Al3+ that would otherwise have been lost to the melt phase. Row 2: distribution of Fe2O3 between phases. At higher Tp, more Fe2O3 is hosted by clinopyroxene and less is hosted by orthopyroxene and spinel. Spinel exists at high pressures in pMELTS models as it is the only solid phase in the model that can incorporate Cr. The pMELTS model does not allow incorporation of Fe3+ into garnet, although garnet is probably a notable host of Fe3+ in the mantle. Row 3: ratio of ferric to total iron in each phase. Although the bulk rock Fe3+/ΣFe ratio is constant across runs, at higher Tp, Fe3+/ΣFe ratios decrease in spinel and clinopyroxene while staying approximately constant in orthopyroxene, reflecting transfer of Fe2+ from other phases, such as olivine and garnet, to spinel and clinopyroxene.

Extended Data Fig. 5 Graphical output of empirical model runs.

Melt-allowed runs: Row 1: mass fraction of each phase. At higher Tp, melting initiates deeper and cpx-out shifts to higher pressures. Orthopyroxene mode increases slightly, whereas garnet mode decreases slightly at higher Tp, to accommodate a greater proportion of Al-bearing (Tschermak’s) pyroxene components. Row 2: distribution of Fe2O3 between phases. Most of the Fe3+ lost to the melt comes from the clinopyroxene phase. Row 3: ratio of ferric to total iron between phases. Although the bulk solid Fe3+/ΣFe ratio decreases only slightly at higher Tp, Fe3+/ΣFe ratios decrease substantially in spinel as potential temperature increases. Melt-suppressed runs: Row 1: mass fraction of each phase. At higher Tp, garnet mode decreases. Row 2: distribution of Fe2O3 between phases. At higher Tp, slightly more Fe2O3 is hosted by orthopyroxene and slightly less is hosted by garnet. Spinel exists in small quantities (0.05 wt%) at high pressures in the empirical model to facilitate Fe3+ partitioning calculations (see Methods section ‘Phases and components’). However, much less spinel exists in the garnet field in the empirical model than in the pMELTS model, and because the empirical model also allows ferric iron to incorporate into garnet, the amount of Fe2O3 hosted by spinel in the garnet field is negligible (in contrast to pMELTS, in which more than a third of the rock’s Fe2O3 is hosted by spinel in the garnet field). Row 3: ratio of ferric to total iron in each phase. Although bulk rock Fe3+/ΣFe is constant across all runs, at higher Tp, Fe3+/ΣFe ratios increase slightly in spinel, while staying approximately constant in orthopyroxene and clinopyroxene, probably reflecting transfer of Fe2+ from other phases, such as olivine to spinel. When calculating fO2, this slight increase in Fe3+/ΣFe ratio in spinel is outweighed by the effect of temperature on magnetite activity, leading to lower fO2 at higher potential temperature (as shown in Fig. 2).

Extended Data Fig. 6 Effects of temperature, melting and garnet.

Effects of temperature, melting and garnet presence on fO2 during isentropic decompression of a peridotite residue in the pMELTS model (row 1) and our empirical model (row 2). Column a, garnet-suppressed, melt-suppressed runs. In both pMELTS and our empirical model, we observe little effect on fO2 as a function of either pressure or potential temperature. Subsolidus pMELTS runs truncate at opx-out, as orthopyroxene is exhausted during subsolidus reactions that dissolve orthopyroxene into clinopyroxene. Subsolidus empirical model runs end at an arbitrary pressure of 0.75 GPa. Column b, garnet-allowed, melt-suppressed runs. When garnet is allowed to form at high pressures, fO2 is higher relative to the garnet-suppressed case. In both pMELTS, which puts no ferric iron in garnet, and in our empirical model, which does put ferric iron in garnet, garnet takes in less ferric iron than the equilibrium pyroxenes and spinel. Consequently, high modal garnet concentrates ferric iron in the pyroxenes and spinel, and fO2 increases. During decompression from the garnet field to the spinel field, fO2 decreases because the concentration of ferric iron in spinel and pyroxene decreases (ferric iron is ‘diluted’ in these phases) as their modal proportions grow. Because bulk composition remains constant, runs end at the same fO2 in the low-pressure spinel field region as they do in the garnet-suppressed, melt-suppressed case. In melt-suppressed runs, potential temperature plays a small role in varying fO2, with the effect of temperature being more pronounced in pMELTS than in the empirical model. Column c, garnet-suppressed, melt-allowed runs. Both pMELTS and the empirical model generate more reduced residues at similar degrees of melt extraction (for example, clinopyroxene-out) when melting occurs at higher Tp compared with melting at lower Tp. Column d, garnet-allowed, melt-allowed runs. These runs are equivalent to those shown in Fig. 2. In both pMELTS and the empirical model, hot melting that begins in the garnet field results in further fO2 reduction relative to hot melting when garnet is suppressed. This demonstrates that high-temperature melting and garnet-field melting each play a role in developing the extremely reduced residues we observe in this study.

Extended Data Fig. 7 Mechanism for reduction in residue fO2.

Bulk Fe2O3 partition coefficients (a,b), bulk Fe2O3 in wt% (c,d) and mineral modes (e–h) of peridotite residues output from pMELTS and our empirical model plotted against percent total melt extracted (F). Model output illustrates the mechanisms for fO2 reduction during melt extraction from garnet peridotite at high potential temperature. Dotted purple lines show model output for a modern ridge potential temperature (Tp = 1,350 °C) at which melting begins in the spinel field, after garnet-out. Solid orange lines show model output for a hotter potential temperature (Tp = 1,550 °C) at which melting begins in the garnet stability field. Dotted orange lines show model output for this same hot potential temperature (Tp = 1,550 °C) but with garnet stability suppressed, so the residue remains spinel peridotite at all pressures. The hot melting curves for pMELTS do not extend to F = 0 because pMELTS predicts that the peridotite is above its solidus at 4 GPa for Tp = 1,550 °C. All model output ends at clinopyroxene-out. Bulk Fe3+ extraction (a–d) occurs when Fe3+ is removed from the solid during melting owing to the incompatibility of Fe3+ in the bulk solid. Concentrations of Fe3+ in residual minerals also decrease when the relative modal abundances of pyroxenes and spinel (e–h) increase (Fe3+ ‘dilution’). The two models are largely in agreement that both high temperatures and residual garnet are important factors in generating reduced spinel peridotite residues, although the models differ in the weights of the temperature and garnet effects.

Extended Data Fig. 8 Refractory rafts along a modern adiabat.

Modelling results from pMELTS demonstrating the effect of bringing refractory residues back up along a modern geotherm. Dashed lines represent subsolidus conditions and solid lines represent conditions in which melt is present. a, P–T paths for pMELTS models at constant composition (DMM) and varying Tp. Subsolidus field is shown in grey and the location of garnet-out is shown as a black line. Shaded orange stars along the 1,550 °C Tp path represent locations of 10–35% melting. These bulk solid compositions are used in panels c and d to investigate the effect of previous melt depletion on fO2. b, Oxygen fugacity of the residual solid versus spinel Cr# for the portions of the paths in panel a that exist in the spinel stability field. Model output at higher Tp records much lower fO2 than the model output at lower Tp. Spinel Cr#s consistent with low-fO2 peridotites at the SWIR and Gakkel Ridge (Cr# ≈ 40–60) require roughly 30–35% melting along the 1,550 °C Tp path. c, Bulk compositions corresponding to the shaded orange stars in panels a and b, representing varying degrees of previous melt depletion, brought up along a modern 1,350 °C adiabat. Evolution of DMM is shown for comparison (equivalent to the 1,350 °C adiabat in panel a). d, Oxygen fugacity versus spinel Cr# for varying degrees of previous depletion, brought up along a modern 1,350 °C adiabat. Evolution of DMM is shown for comparison (equivalent to the 1,350 °C adiabat in panel b). Peridotite residues that melted at high temperature during previous melting events retain their reduced signature when brought up beneath the modern ridge axis. Circles represent conditions at garnet-out (2–3 GPa; see panel c); however, the residues do not evolve substantially in Cr# and fO2 between garnet out and the solidus at pressures shallower than about 1 GPa. After the onset of melting, Cr# and fO2 begin to increase. Because these refractory residues do not begin to re-melt until such shallow conditions, they are unlikely to contribute substantially to ridge melts.

Extended Data Fig. 9 Schematic.

Schematic representation of the development of fO2 heterogeneities owing to ancient melting events. a,b, Ancient hot mantle, in which melting initiates in the garnet field. a, Ancient fertile mantle ascends along a hot adiabat, producing large degrees of melting in the garnet stability field and a refractory peridotite residue. b, During garnet-field melting, fO2 is reduced relative to an equivalent, melt-suppressed assemblage. c–f, Modern cool mantle, consisting primarily of fertile material with rafts of previously melted mantle. c, Rafts of refractory material produced by ancient melting (as shown in panel a) may re-ascend along a cool adiabat but will not experience notable further melting owing to their refractory nature. d, Along a cool adiabat, fertile ambient mantle material melts in the spinel stability field. The extent of melting is primarily a function of spreading rate, which determines the pressure at which melting stops. e,f, Modern, fertile mantle will not substantially change in fO2 during melting, although spinel Cr# will increase as melting continues25. Rafts of ancient, refractory material will retain their high spinel Cr# and low fO2 signature produced during ancient melting events.

Extended Data Fig. 10 Liquid fO2.

Instantaneous/aggregated liquid fO2 calculated for model runs. Differences between dotted lines (instantaneous liquids) and solid lines (aggregated liquids) demonstrate the homogenizing effect of melt aggregation. Although the residue, and thus instantaneous liquids, may reach very low fO2 at high degrees of melting and high potential temperature, the aggregated liquids are less reduced owing to the influence of early, more oxidized liquids. Row 1 shows the raw pMELTS output, which calculates liquid fO2 from the algorithm in ref. 79. Depending on P–T conditions, instantaneous liquids (dotted lines) may have higher or lower fO2 than the fO2 returned by the solid-phase assemblage using the ‘alternative-fO2’ tag in pMELTS (residue**) (see Methods). We also calculate fO2 values from pMELTS solid-phase output using spinel oxybarometry (residue*), as applied in the main-text figures. Because ref. 79 is not well calibrated on our model compositions, we used two other methods for calculating liquid fO2: (1) the compositional framework of ref. 84 combined with the ref. 79 pressure term (rows 2 and 4) and (2) the model of ref. 87, which revises ref. 84 and applies the pressure term of ref. 88 (rows 3 and 5). These frameworks offer an improvement, although large uncertainties remain (see Methods). As well as uncertainty related to translation between melt composition and melt fO2, we emphasize that neither model forces instantaneous liquids to be in fO2 equilibrium with their residues (see Methods and ref. 24 for discussion), despite that requirement in nature. Furthermore, calculated liquid fO2 values from pMELTS and our empirical model are particularly uncertain at high Tp, at which Fe3+-partitioning and fO2-compositional relationships are less well constrained. Finally, liquids in our empirical model do not extend beyond cpx-out and so contributions from shallow pressures are absent. Despite these caveats and uncertainties, we expect that high-temperature, garnet-field melting could lead to aggregated liquids that are slightly more reduced than aggregated liquids from spinel-field melting, although not as reduced as the peridotite residues observed in this study owing to melt aggregation and homogenization.

Extended Data Table 1 Phases and components (empirical model)
Full size table
Extended Data Table 2 Constraints and degrees of freedom (empirical model)
Full size table
Extended Data Table 3 Garnet-out reactions (empirical model)
Full size table

Supplementary information

Supplementary Methods

Supplementary Methods Sections 1–4: Section 1 Statistical evaluation of SWIR/Gakkel Ridge samples; Section 2 pMELTS modelling – steps for reproducing calculations; Section 3 Empirical modelling – steps for reproducing calculations; and Section 4 Empirical modelling – further details.

Peer Review file

Supplementary Table 1

Oxygen fugacity of SWIR, Gakkel Ridge and Hess Deep peridotites. Mineral parameters and oxygen fugacity data for residual lherzolites and refractory harzburgites. SWIR fO2 data are from ref. 2; Gakkel Ridge and Hess Deep fO2 data are new to this study.

Supplementary Table 2

Electron microprobe analyses of spinel correction standards. Data for spinel correction standards, which were run at the beginning and end of each electron microprobe analysis session. Unknown spinel Fe3+/∑Fe ratios were corrected using the Cr#-based correction method in ref. 36, revised in ref. 37.

Supplementary Table 3

pMELTS run conditions. Summary of pMELTS model runs, including potential temperature, input composition and phases suppressed.

Supplementary Table 4

pMELTS output. Tabulated model output for all pMELTS runs. Includes modal data and oxide data for each phase, as well as system parameters, such as pressure, temperature, entropy and oxygen fugacity.

Supplementary Table 5

Empirical model output. Tabulated model output for all empirical model runs. Includes modal data and oxide data for each phase, as well as system parameters, such as pressure, temperature and oxygen fugacity.

Supplementary Data

This zipped file contains the pMELTS input files and their descriptions.

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Birner, S.K., Cottrell, E., Davis, F.A. et al. Deep, hot, ancient melting recorded by ultralow oxygen fugacity in peridotites. Nature 631, 801–807 (2024). https://doi.org/10.1038/s41586-024-07603-w

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