The global mid-ocean ridge system is the most extensive magmatic system on our planet and is the site of 75 per cent of Earth’s volcanism1. The vertical extent of mid-ocean-ridge magmatic systems has been considered to be restricted: even at the ultraslow-spreading Gakkel mid-ocean ridge under the Arctic Ocean, where the lithosphere is thickest, crystallization depths of magmas that feed eruptions are thought to be less than nine kilometres2. These depths were determined using the volatile-element contents of melt inclusions, which are small volumes of magma that become trapped within crystallizing minerals. In studies of basaltic magmatic systems, olivine is the mineral of choice for this approach2,3,4,5,6. However, pressures derived from olivine-hosted melt inclusions are at odds with pressures derived from basalt major-element barometers7 and geophysical measurements of lithospheric thickness8. Here we present a comparative study of olivine- and plagioclase-hosted melt inclusions from the Gakkel mid-ocean ridge. We show that the volatile contents of plagioclase-hosted melt inclusions correspond to much higher crystallization pressures (with a mean value of 270 megapascals) than olivine-hosted melt inclusions (with a mean value of 145 megapascals). The highest recorded pressure that we find equates to a depth 16.4 kilometres below the seafloor. Such higher depths are consistent with both the thickness of the Gakkel mid-ocean ridge lithosphere and with pressures reconstructed from glass compositions. In contrast to previous studies using olivine-hosted melt inclusions, our results demonstrate that mid-ocean-ridge volcanoes may have magmatic roots deep in the lithospheric mantle, at least at ultraslow-spreading ridges.
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Source Data for all figures are provided with the paper and also in the Supplementary Information tables. The data are also available from EarthChem at https://doi.org/10.1594/IEDA/111315.
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We thank H. J. B. Dick for providing access to Gakkel Ridge samples, C-J. de Hoog for expert advice on SIMS analysis, D. Muir for assistance with the EDS analysis, and A. Oldroyd for help with sample preparation. We also thank Mark Behn for comments on the original manuscript. This research was supported by NERC grants NE/L002434/1 (to E.N.B.), NE/R001332/1 (to M.-A.M.) and NE/M000427/1 (to F.E.J) and IMF641/1017 (to C.J.L.) and by an AXA Professorship and Wolfson Merit Award (to K.V.C.).
The authors declare no competing interests.
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Peer review information Nature thanks Mark D. Behn and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
Green symbols show new plagioclase- and olivine-hosted melt-inclusion analysis; orange symbols show new plagioclase-hosted melt-inclusion data supplemented by olivine-hosted melt-inclusion data from ref. 3; purple and yellow symbols indicate data from ref. 2 and from ref. 3, respectively. AVR, axial volcanic ridge; SM, seamount; BR, basement ridge; DSF, deep seafloor. IBCAO bathymetric data are from ref. 45. The Global Multi-Resolution Topography (GMRT) synthesis46 base map underlies the IBCAO bathymetry. The map was made using GeoMapApp (http://www.geomapapp.org).
Extended Data Fig. 2 Textural complexity of olivine and plagioclase and melt-inclusion distribution.
Backscattered-electron images of plagioclase (a–c) and olivine (d–f). Plagioclase can be unzoned (c) or show patchy zoning (a) or reverse and normal zoning (b); plagioclase shows both internal (a, b) and external (c) resorption. Olivine is present in poly- and mono-mineral glomerocrysts (d, e) and as individual skeletal crystals (f); reverse (d) and normal (e) zoning is present. Scale bars are all 100 μm. Dashed red and yellow lines in a, b and d show the locations of resorption. Numbers correspond to the analysed melt inclusions highlighted in Supplementary Table 2. O and P refer to host olivine and plagioclase crystals, respectively; in d, a melt inclusion has been analysed from olivine within a polymineralic glomerocryst or clot.
Both plagioclase- and olivine-hosted melt inclusions show no relationship between melt inclusion size and CO2 content.
Within each sample, plagioclase records greater crystallization depths than olivine. The three individual samples are from the 31° E basement ridge (HLY0102-D95-11 and HLY0102-D48-SGB) and 3° E seamount (HLY0102-D27-8).
a, Phase map showing the association of the high-pressure olivine-hosted melt inclusion (MI) with plagioclase. White lines delineate grain boundaries. Plagioclase exhibits complex zoning (such as oscillatory (OZ) and patchy zoning (PZ) in b and c). Plagioclase exhibits both internal and external resorption (IR and ER, respectively) in b and c. Large amoeboid melt inclusions (c) also suggest the occurrence of resorption. Plagioclase melt inclusions were microcrystalline and hence were not analysed for their volatile contents. Scale bars are 500 μm. For information relating to phase map acquisition, see ref. 37.
Calibration curves are shown for H2O (a) and CO2 (b). The different coloured lines in each panel indicate different analytical sessions.
Plagioclase-hosted melt-inclusion compositions were empirically corrected for PEC (b). Host plagioclase compositions were added to the melt inclusions iteratively until the melt inclusions met the Al2O3 content (at a given Mg#) of the pseudo-liquid line of descent. The pseudo-liquid line of descent comprises two parts. First, a regression through Gakkel glass data36,37 (black line), and second, a line hand-picked to run along the top of the olivine-hosted melt inclusions2,3 and Gakkel glass data (red line) (a). A second PEC correction was undertaken using a different pseudo-liquid line of descent (green line) (c) that resulted in lower corrections. A comparison of pressures calculated following each of these PEC corrections shows that there is negligible difference between pressures calculated from the resulting melt compositions (d). Gakkel glass data were downloaded from the PetDB36 database (http://www.earthchem.org/petdb) on 15 July 2016.
There is no relationship between the magnitude of PEC correction and crystallization depth.
The majority of melt inclusions have SiO2 > 49 wt%, hence VolatileCalc pressures were calculated using a default SiO2 value (49 wt%; orange points). Where SiO2 < 49 wt%, specific VolatileCalc pressures were calculated (blue points); blue points correspond to orange points with no outline calculated with the default SiO2 content. VolatileCalc pressures are lower when the specific SiO2 (not the default 49 wt% SiO2) content of the melt inclusion is used.
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Bennett, E.N., Jenner, F.E., Millet, MA. et al. Deep roots for mid-ocean-ridge volcanoes revealed by plagioclase-hosted melt inclusions. Nature 572, 235–239 (2019). https://doi.org/10.1038/s41586-019-1448-0
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