Letter | Published:

Sampling the volatile-rich transition zone beneath Bermuda

Abstract

Intraplate magmatic provinces found away from plate boundaries provide direct sampling of the composition and heterogeneity of the Earth’s mantle. The chemical heterogeneities that have been observed in the mantle are usually attributed to recycling during subduction1,2,3, which allows for the addition of volatiles and incompatible elements into the mantle. Although many intraplate volcanoes sample deep-mantle reservoirs—possibly at the core–mantle boundary4—not all intraplate volcanoes are deep-rooted5, and reservoirs in other, shallower boundary layers are likely to participate in magma generation. Here we present evidence that suggests Bermuda sampled a previously unknown mantle domain, characterized by silica-undersaturated melts that are substantially enriched in incompatible elements and volatiles, and a unique, extreme isotopic signature. To our knowledge, Bermuda records the most radiogenic 206Pb/204Pb isotopes that have been documented in an ocean basin (with 206Pb/204Pb ratios of 19.9–21.7) using high-precision methods. Together with low 207Pb/204Pb ratios (15.5–15.6) and relatively invariant Sr, Nd, and Hf isotopes, the data suggest that this source must be less than 650 million years old. We therefore interpret the Bermuda source as a previously unknown, transient mantle reservoir that resulted from the recycling and storage of incompatible elements and volatiles6,7,8 in the transition zone (between the upper and lower mantle), aided by the fractionation of lead in a mineral that is stable only in this boundary layer, such as K-hollandite9,10. We suggest that recent recycling into the transition zone, related to subduction events during the formation of Pangea, is the reason why this reservoir has only been found in the Atlantic Ocean. Our geodynamic models suggest that this boundary layer was sampled by disturbances related to mantle flow. Seismic studies and diamond inclusions6,7 have shown that recycled materials can be stored in the transition zone11. For the first time, to our knowledge, we show geochemical evidence that this storage is key to the generation of extreme isotopic domains that were previously thought to be related only to deep recycling.

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Data availability

Geochemical data for Bermuda lavas can be found online at the EarthChem database at https://doi.org/10.1594/IEDA/111282. All geochemical data—including olivine-spinel thermometry, magmatic water calculations and Pb-isotope modelling—can be found in the Supplementary Information.

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Acknowledgements

This study was supported by NSF OCE 1756349 and NSF EAR 1802012 to E.G. and NSF EAR-1249438 to E.A.J. The analytical work on EPMA facility in ISTerre, University Grenoble Alpes was supervised by V. Batanova and supported by grants of Institut Universitaire de France to A.V.S. and by the Richard Lounsbery Foundation to A.V.S. and E.G. S.E.M. thanks J. Dale, J. Trela, J. Berndt and L. Costello for help with sample collection, preparation and analyses. We are grateful to A. Hofmann for discussions. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Reviewer information

Nature thanks Gerya Taras and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information

S.E.M. prepared the samples, collected the bulk rock geochemistry data via X-ray fluorescence and laser ablation inductively coupled plasma–mass spectrometry, analysed data, wrote the paper and prepared figures and tables. E.G. conceived the project, analysed data and prepared figures. S.E.M., M.B. and P.B. performed the Pb, Sr, Nd and Hf isotopic analyses and P.B. conducted the Pb models. R.M. conducted the geodynamic models. E.A.J. performed the FTIR analyses of clinopyroxene. R.J.M. collected the 40Ar/39Ar age dates. A.V.S. performed the electron microprobe analyses of olivine. All authors participated in the discussion and interpretations of results and in the preparation of the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to Sarah E. Mazza or Esteban Gazel.

Extended data figures and tables

  1. Extended Data Fig. 1 Geologic map and core log for Bermuda.

    a, Bathymetric map of the northwestern Atlantic Ocean based on that from ref. 12, highlighting Bermuda and the Bermuda Rise. The predicted west–east hotspot track78 is shown as a red line. b, Sketch of the Bermuda core stratigraphy, based on the core log from Dalhousie University. The original core log corresponds to physical markings on the core, all given in inches and feet. Locations at which olivine (O) and phlogopite (Phl) phenocrysts have been observed are marked.

  2. Extended Data Fig. 2 Examples of samples and mineralogy.

    a, Photograph of silica-saturated sample B1081, showing aphanitic texture with haematite and calcite veins. b, Photograph of silica-undersaturated sample B1644 showing porphyritic texture with clinopyroxene (Cpx). c, Photograph of silica-undersaturated sample B815 showing porphyritic texture and large phenocrysts of clinopyroxene (Cpx; up to 1 cm in length). d, Photograph of silica-undersaturated sample B193 showing aphanitic texture. e, Micrograph of sample B1908 in cross-polarized light, showing Ti-augite, apatite and phlogopite phenocrysts. f, Micrograph of sample B1908 in cross-polarized light, showing perovskite and nepheline phenocrysts. g, Olivine separates from B2299.

  3. Extended Data Fig. 3 40Ar/39Ar age spectra for samples B1908, B703, B1641 and B1036.

    40Ar/39Ar step-heating data for silica-undersaturated lavas from Bermuda. The phlogopite phenocrysts age spectrum records a magmatic age of approximately 30.9 Myr.

  4. Extended Data Fig. 4 Examples of trace-element ratios comparing Bermuda to HIMU, EM1, and EM2 mantle domains from ref. 79.

    a, Rb/Sr plotted against 1/Sr, showing Bermuda lavas spanning the range of HIMU, EM1, and EM2 domains. b, Ce/Pb plotted against U/Pb, showing Bermuda lavas as being more enriched than HIMU-derived lavas. c, K/U plotted against Pb/U, showing that Bermuda lavas are K-depleted. d, Ba/Th plotted against Rb/Th, showing that Bermuda lavas have fluid mobile element ratios similar to those of the HIMU domain. e, K/Nb plotted against K/U, showing that Bermuda lavas are K-depleted, with K depletions lower than previously reported for HIMU. f, Th/Pb plotted against U/Pb, showing that Bermuda is not characterized by sulfide fractionation, because Th and U are equally enriched and are more enriched than HIMU. EM1, enriched mantle I; EM2, enriched mantle II. 

  5. Extended Data Fig. 5 Experimental melts compared with Bermuda lavas.

    a–f, Proportions of TiO2 (wt%) (a), Al2O3 (wt%) (b), CaO (wt%) (c), K2O (wt%) (d), the K2O/Na2O ratio (e) and MgO (wt%) (f) plotted against SiO2 (wt%). Experimental data are from refs 22,49,50,80.

  6. Extended Data Fig. 6 Pb-isotope models for the Bermuda mantle source.

    a, b, 206Pb/204Pb–207Pb/204Pb and 206Pb/204Pb–208Pb/204Pb plots of Bermuda samples with measured values and age-corrected values (see legend). HIMU65 and Eastern Azores lavas81 are plotted for illustration. Azores MAR, Average NMAR and Iceland MAR end members (GEOROC database) are presented along with their linear regression, the Atlantic array. R3 corresponds to the measured value of sample B703, at t3 = 0 (today). ss, silica-saturated; su, silica-undersaturated. cf, Monte Carlo results derived from equation (1) for R1 values falling on the Atlantic array. Azores MAR, Average NMAR and Iceland MAR end members are presented at their respective model values. μ = 238U/204Pb, Ω = 232Th/204Pb, κ = 232Th/238U. μ and Ω values of Bermuda silica-undersaturated lavas are presented: dashed blue line, average; solid blue lines, ± 1 s.d.

  7. Extended Data Fig. 7 Modelling the μ of the Bermuda mantle source.

    Modelled μ compositions that are necessary to produce Bermuda’s most radiogenic 206Pb/204Pb sample (B703) starting from the average composition of the silica-undersaturated, least radiogenic samples. B703 (target) and the average silica-undersaturated samples have been age-corrected to time of eruption (30 Myr ago). Each line corresponds to possible source ages and the necessary μ for that source to produce an erupted lava that corresponds to B703. t = 40 Myr (70 Myr ago) corresponds to the calculated source age for Bermuda assuming that Bermuda represents an isochron, with a minimum μ < 420 necessary. t = 80 Myr (110 Myr ago) corresponds both to the error of the Bermuda isochron and the age of the oceanic lithosphere around Bermuda. T = 170 Myr (200 Myr ago) corresponds to the rifting of Pangea and the opening of the Atlantic in the region of Bermuda. t = 550 Myr (580 Myr ago) corresponds to the ideal source age that can evolve the least radiogenic sample to the most radiogenic sample with a μ of 50 (the actual μ for B703). Ma, million years ago.

  8. Extended Data Fig. 8 Standard reproducibility for Pb isotopes.

    Recommended USGS standard values (BHVO-2, AGV-2 and BCR-2) compared with standards run during this study for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb. Recommended standard values are from refs 63,82 and the most recent GEOREM preferred values. We did not leach any of our standards. It is important to note that ref. 63 reports that all the USGS standards are variably contaminated during processing, and therefore we plot the residues and leachate data for these USGS standards. a, b, Pb isotopes for BHVO-2 showing that our 206Pb/204Pb ratios are within error of the other values, but our 207Pb/204Pb ratios are lower than those in refs 63,82, yet within error of the GEOREM recommended values and the residues of leaching from ref. 63. In ref. 63 it was noted that the BHVO-2 and BHVO-1 are contaminated and that, upon leaching, their residues converge. c, d, Pb isotopes for AGV-2 showing that our data are within error of those in ref. 63 and the GEOREM recommended values. On the basis of AGV-2, our Pb isotope data are both accurate and highly precise (the duplicates overlap on the symbol size). e, f, Pb isotopes for BCR-2 showing that our 207Pb/204Pb values are identical to those of refs 63,82. As in the BHVO-2, our data are closer to the residues of leaching reported in ref. 63. Ours and literature data plot on a mixing line with the residues of leaching for BCR-2 being highly variable, pointing to heterogeneity in these standards.

Supplementary information

  1. Supplementary Table 1

    40Ar/39Ar age dating for Bermuda samples B1908, B703, B1641, and B1036. Table contains sample information, analytical conditions, constants used in geochronological calculations, and raw data from step-heating data collection.

  2. Supplementary Table 2

    Bulk rock geochemistry, olivine-spinel mineral geochemistry, and FTIR and mineral geochemistry for clinopyroxene. Bulk rock geochemistry includes sample description (depth in core in feet, phenocrysts present, age if applicable), major oxides (wt%), trace elements (ppm), 176Hf/177Hf, 143Nd/144Nd, 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb isotopic ratios. Olivine (Ol) – Spinel (Sp) thermometry includes major oxides (wt%) for olivine and spinel inclusions, as well as the calculations for olivine crystallization temperatures. Magmatic water from clinopyroxene (Cpx) includes FTIR results and calculations for OH in clinopyroxene grains, and the major oxide (wt%) composition. Final H2O in the magma calculations are also presented.

  3. Supplementary Table 3

    Monte Carlo modeling for Pb isotope source composition from Bermuda. Includes measured 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb isotopic compositions and age corrected values for time of eruption (30 Myr), Monte Carlo simulations, and the results from the modeling.

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Fig. 1: Map, geodynamic and schematic models of Bermuda.
Fig. 2: Bulk rock geochemistry of Bermuda.
Fig. 3: Trace-element data plotted against fosterite content in Bermuda olivine.
Fig. 4: Pb, Sr, Nd and Hf isotopic composition of Bermuda lavas.
Extended Data Fig. 1: Geologic map and core log for Bermuda.
Extended Data Fig. 2: Examples of samples and mineralogy.
Extended Data Fig. 3: 40Ar/39Ar age spectra for samples B1908, B703, B1641 and B1036.
Extended Data Fig. 4: Examples of trace-element ratios comparing Bermuda to HIMU, EM1, and EM2 mantle domains from ref. 79.
Extended Data Fig. 5: Experimental melts compared with Bermuda lavas.
Extended Data Fig. 6: Pb-isotope models for the Bermuda mantle source.
Extended Data Fig. 7: Modelling the μ of the Bermuda mantle source.
Extended Data Fig. 8: Standard reproducibility for Pb isotopes.

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