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Electrical conductivity during incipient melting in the oceanic low-velocity zone

Nature volume 509, pages 81–85 (2014)Cite this article

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Abstract

The low-viscosity layer in the upper mantle, the asthenosphere, is a requirement for plate tectonics1. The seismic low velocities and the high electrical conductivities of the asthenosphere are attributed either to subsolidus, water-related defects in olivine minerals2,3,4 or to a few volume per cent of partial melt5,6,7,8, but these two interpretations have two shortcomings. First, the amount of water stored in olivine is not expected to be higher than 50 parts per million owing to partitioning with other mantle phases9 (including pargasite amphibole at moderate temperatures10) and partial melting at high temperatures9. Second, elevated melt volume fractions are impeded by the temperatures prevailing in the asthenosphere, which are too low, and by the melt mobility, which is high and can lead to gravitational segregation11,12. Here we determine the electrical conductivity of carbon-dioxide-rich and water-rich melts, typically produced at the onset of mantle melting. Electrical conductivity increases modestly with moderate amounts of water and carbon dioxide, but it increases drastically once the carbon dioxide content exceeds six weight per cent in the melt. Incipient melts, long-expected to prevail in the asthenosphere10,13,14,15, can therefore produce high electrical conductivities there. Taking into account variable degrees of depletion of the mantle in water and carbon dioxide, and their effect on the petrology of incipient melting, we calculated conductivity profiles across the asthenosphere for various tectonic plate ages. Several electrical discontinuities are predicted and match geophysical observations in a consistent petrological and geochemical framework. In moderately aged plates (more than five million years old), incipient melts probably trigger both the seismic low velocities and the high electrical conductivities in the upper part of the asthenosphere, whereas in young plates4, where seamount volcanism occurs6, a higher degree of melting is expected.

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Figure 1: Electrical conductivity of hydrous carbonated basalts, hydrated basalts and hydrous olivine.
Figure 2: Effect of incipient melt on the electrical conductivity of depleted and enriched carbonated peridotites.
Figure 3: Petrologically based conductivity profiles across the incipient melting region under the LAB for various ages.
Figure 4: The oceanic seismic low-velocity zone bracketed by the upper and lower boundaries of incipient melting.

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Acknowledgements

This work, part of the ElectroLith project, benefited from funding by the European Research Council (ERC project #279790) and the French agency for research (ANR project #2010 BLAN62101). S.H.-M. acknowledges support from the US NSF grant EAR1215800 and a grant from the University of Orleans. We thank David H. Green for comments.

Author information

Authors and Affiliations

  1. Université d’Orléans, Institut des Sciences de la Terre d’Orléans (ISTO), Unité mixte de recherche (UMR) 7327, 45071 Orléans, France ,

    David Sifré, Emmanuel Gardés, Malcolm Massuyeau, Leila Hashim, Saswata Hier-Majumder & Fabrice Gaillard

  2. Centre National de la Recherche Scientifique (CNRS)/ISTO, UMR 7327, 45071 Orléans, France ,

    David Sifré, Emmanuel Gardés, Malcolm Massuyeau, Leila Hashim, Saswata Hier-Majumder & Fabrice Gaillard

  3. Bureau des Recherches Géologiques et Minières, ISTO, UMR 7327, BP 36009, 45060 Orléans, France ,

    David Sifré, Emmanuel Gardés, Malcolm Massuyeau, Leila Hashim, Saswata Hier-Majumder & Fabrice Gaillard

  4. Commissariat à l’Energie Atomique-CNRS-École nationale supérieure d'ingénieurs de Caen-Université de Caen Basse Normandie, Centre de Recherche sur les Ions, les Matériaux et la Photonique, UMR 6252, BP 5133, 14070 Caen, France ,

    Emmanuel Gardés

  5. Department of Geology and Center for Scientific Computation and Applied Mathematical Modeling, University of Maryland, 20742, Maryland, USA

    Saswata Hier-Majumder

  6. Department of Earth Sciences, Royal Holloway University of London, Egham TW20 0EX, UK,

    Saswata Hier-Majumder

Authors
  1. David Sifré
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Contributions

F.G. led the project and wrote the first draft. All authors contributed equally to the writing of subsequent drafts. D.S. and F.G. developed the experimental set-up, and D.S. performed the conductivity measurements. S.H.-M. contributed to the discussion and provided editorial assistance with the manuscript. D.S. and L.H. produced Fig. 1, E.G. and L.H. produced Fig. 2, D.S. produced Fig. 3, and L.H. and M.M. produced Fig. 4.

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Correspondence to Fabrice Gaillard.

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Extended data figures and tables

Extended Data Figure 1 Set-up of electrical conductivity measurement using four wires.

a, Modified piston–cylinder assembly for electrical conductivity measurements using a four-wire configuration. The cored sample (in green) contains in its centre an inner electrode in platinum (in blue). A platinum foil (in blue) surrounds the sample, which extends upwards and downwards from the sample and corresponds to the outer electrode. The sample is sandwiched by machined MgO ceramics (in white). The electrode-sample assemblage is isolated from the graphite furnace by an Al2O3 jacket (in yellow). The four-electrode wires are emplaced using a four-hole Al2O3 tube (in orange). Two of these wires, that is, the thermocouple, are in contact with the inner electrode, whereas the outer electrode is in contact with two other wires by means of a top Ni plug (in red). b, SEM image of the assemblage of sample C after experiments (up to 1,463 °C and 3 GPa). We observed an average decrease of 20% compared to the initial cell geometry (corresponding to the porosity loss during melting). Cell geometry parameters (h, rin and rout in equation (2)) are determined from SEM images for each sample.

Extended Data Figure 2 Measured resistance of molten carbonate versus nickel.

a, The electrical cell resistance versus temperature. We show the resistance of a sample made of nickel measured using either a two-wire set-up (empty diamond) or a four-wire set-up (red diamond). There are several orders of magnitude of difference between the two measurements, showing that the two-wire setup is not suitable at all for conductive materials. We also show the resistance of carbonate in a four-wire set-up (sample C, molten at T > 1,230 °C; green triangle). b, Impedance spectra obtained on molten carbonate (sample C) at 3 GPa as a function of temperature. Impedance spectra show vertical lines, indicating an inductance-dominated signal for all temperatures. The resistance is taken from the intercept with the horizontal axis. Data are obtained at frequencies ranging from 19,905 to 315,479 Hz. The black line represents an impedance spectrum of a nickel sample (blank) obtained with a four-wire configuration at 1,464 °C.

Extended Data Figure 3 Electrical conductivity measurements.

a, Electrical conductivity versus reciprocal temperature measured on carbonated melts and hydrous carbonated basalts. Samples: a carbonated melt (C), a hydrous carbonated melt (HC), and three hydrous carbonated basalts with H2O contents ranging from 4.43 to 9.22 wt% (HCB-9, HCB-7 and HCB-4) and CO2 contents ranging from 10.39 to 23.32 wt%. To complete Fig. 1, we distinguished heating–cooling temperature cycles and reported error bars. Large solid symbols, heating cycle (H1); open symbols, cooling cycle (C1); small solid symbols, second heating cycle (H2) (compare with Extended Data Table 2a). The error bars include uncertainties in the geometrical factors of the samples and in the measured resistance. b, Compensation plots showing the correlation between activation energy, Ea, and pre-exponential terms, ln(σ0). Hydrous basalts (HB) are from the experimental data set of ref. 7 between 1,200 and 1,500 °C, and the data point for the dry basalt (B) is from ref. 32. The dry carbonated melt (C), the hydrous carbonated melts (HC) and the hydrous carbonated basalts (HCB) are from this study (see Extended Data Table 2b for the Arrhenius parameters).

Extended Data Figure 4 The incipient melt effect on the electrical conductivity of an H2O-enriched, CO2-free peridotite.

This figure completes the scenarios illustrated in Fig. 2. The conductivity of partially molten peridotite, in which H2O partitions between minerals and melt (Methods), is reported as a function of melt content and temperature for CO2-free peridotite with 500 p.p.m. H2O (log values; conductivity increases from cold to warm colours). The discontinuity at T = 1,070 °C is due to pargasite amphibole breakdown (Extended Data Fig. 5) that redistribute H2O between NAMs and the melt as explained in the Methods. Melt H2O contents (blue if pargasite out, green if pargasite in) are tabulated above the panel.

Extended Data Figure 5 Melting curves for different bulk peridotitic systems as functions of temperature and depth.

The solidus of dry peridotite (black curve) is calculated from ref. 53. The dehydration solidus of nominally anhydrous peridotite at 200 p.p.m. H2O (blue curve) is modelled from ref. 9. The dehydration solidus of pargasite lherzolite is based on ref. 10. The nominally anhydrous carbonated, fertile peridotite solidus is based on ref. 54 and references therein (green curve). The H2O-undersaturated carbonated, fertile peridotite curve (purple curve) corresponds to the solidus of a pyrolite with 0.5–2.5 wt% CO2 and 0.3 wt% H2O (ref. 14). For pressures ≤1.7 GPa, carbonated melts are unstable and gaseous CO2 prevails. We connected the melting curve of CO2-bearing peridotite to that of the dry peridotite at low pressures, which slightly differs from previously published phase diagrams. We considered that, for P ≤ 1.7 GPa, gaseous CO2 must have a negligible influence on the peridotite solidus due to the small solubility of CO2 in basaltic melts55. Similarly, at low pressures, the H2O-undersaturated carbonated, fertile peridotite solidus was connected to the dehydration solidus of nominally anhydrous peridotite (considering peridotite with 200 p.p.m. H2O), neglecting the presence of pargasite owing to the NAM’s H2O capacity storage.

Extended Data Figure 6 Phase equilibria control on H2O–CO2 partitioning, ultimately resulting in a change in conductivity as shown in Fig. 3.

We show changes in H2O content in olivine (left), melt fraction (centre) and melt CO2/H2O (right) for the 70-Myr age used for calculation in Fig. 3. We use two illustrative compositions: bulk with 200 p.p.m. H2O and 500 p.p.m. CO2 and bulk with 500 p.p.m. H2O and 500 p.p.m. CO2.

Extended Data Table 1 Chemical composition of samples before and after electrical conductivity measurements
Full size table
Extended Data Table 2 Temperature range of electrical conductivity measurements and adjusted Arrhenius parameters
Full size table
Extended Data Table 3 Parameters used for equations (3) and (4)
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Sifré, D., Gardés, E., Massuyeau, M. et al. Electrical conductivity during incipient melting in the oceanic low-velocity zone. Nature 509, 81–85 (2014). https://doi.org/10.1038/nature13245

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