Diamonds sampled by plumes from the core–mantle boundary

Journal name:
Nature
Volume:
466,
Pages:
352–355
Date published:
DOI:
doi:10.1038/nature09216
Received
Accepted

Diamonds are formed under high pressure more than 150kilometres deep in the Earth’s mantle and are brought to the surface mainly by volcanic rocks called kimberlites. Several thousand kimberlites have been mapped on various scales1, 2, 3, 4, but it is the distribution of kimberlites in the very old cratons (stable areas of the continental lithosphere that are more than 2.5 billion years old and 300 kilometres thick or more5) that have generated the most interest, because kimberlites from those areas are the major carriers of economically viable diamond resources. Kimberlites, which are themselves derived from depths of more than 150 kilometres, provide invaluable information on the composition of the deep subcontinental mantle lithosphere, and on melting and metasomatic processes at or near the interface with the underlying flowing mantle. Here we use plate reconstructions and tomographic images to show that the edges of the largest heterogeneities in the deepest mantle, stable for at least 200 million years and possibly for 540 million years, seem to have controlled the eruption of most Phanerozoic kimberlites. We infer that future exploration for kimberlites and their included diamonds should therefore be concentrated in continents with old cratons that once overlay these plume-generation zones at the core–mantle boundary.

At a glance

Figures

  1. Reconstructed large igneous provinces and kimberlites for the past 320[thinsp]Myr with respect to shear-wave anomalies at the base of the mantle.
    Figure 1: Reconstructed large igneous provinces and kimberlites for the past 320Myr with respect to shear-wave anomalies at the base of the mantle.

    The deep mantle (2,800km on the SMEAN tomography model20) is dominated by two LLSVPs beneath Africa and the Pacific. The 1% slow contour (approximating to the PGZs) is shown as a thick red line. 80% of all reconstructed kimberlite locations (black dots) of the past 320Myr erupted near or over the sub-African PGZ. The most ‘anomalous’ kimberlites (17%) are from Canada (white dots). Present-day continents are shown as a background, to illustrate the distribution of hotspots classified as being of deep-plume origin18 and present-day shear-wave velocity anomalies (percentage δvS), and bear no geographical relationship to reconstructed kimberlites or large igneous provinces.

  2. Late-Jurassic plate reconstruction of continents and kimberlite locations draped on the SMEAN model.
    Figure 2: Late-Jurassic plate reconstruction of continents and kimberlite locations draped on the SMEAN model.

    We reconstructed kimberlite locations with eruption ages between 155 and 165Myr ago to the average of 160Myr ago. Reconstructed kimberlite locations are found near the edges of the African LLSVP (near the 1% slow contour, which is shown as a thick red line) and at the old cratons in North America4, northwestern Africa, South Africa (the Kalahari craton1) and Australia30. The most important cratons for kimberlite eruption since the Carboniferous period are shaded in grey.

  3. Devonian and Cambrian period plate reconstructions draped on the SMEAN model.
    Figure 3: Devonian and Cambrian period plate reconstructions draped on the SMEAN model.

    We reconstructed kimberlite locations with eruption ages between 350 and 360Myr ago to the average of 355Myr ago, and ages between 500 and 510Myr ago to the average of 505Myr ago; they all fall close to vertically above the SMEAN model −1% contours (PGZs), shown as thick red lines.

  4. Reconstructed Palaeozoic kimberlites from Laurentia (North America, Canada), Siberia and core Gondwana draped on the SMEAN model.
    Figure 4: Reconstructed Palaeozoic kimberlites from Laurentia (North America, Canada), Siberia and core Gondwana draped on the SMEAN model.

    The SMEAN model -1% contour is shown as a thick red line.

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Author information

Affiliations

  1. Physics of Geological Processes and Geosciences, University of Oslo, Blindern, 0316 Oslo, Norway

    • Trond H. Torsvik &
    • Bernhard Steinberger
  2. Centre for Geodynamics, Geological Survey of Norway, Leiv Eirikssons vei 39, 7491 Trondheim, Norway

    • Trond H. Torsvik &
    • Bernhard Steinberger
  3. School of Geosciences, University of the Witwatersrand, Wits 2050, South Africa

    • Trond H. Torsvik,
    • Kevin Burke,
    • Susan J. Webb &
    • Lewis D. Ashwal
  4. Department of Geosciences, University of Houston, 312 Science and Research 1, Houston, Texas 77204-5007, USA

    • Kevin Burke
  5. Helmholtz Centre Potsdam, German Research Centre for Geosciences, 14473 Potsdam, Germany

    • Bernhard Steinberger

Contributions

T.H.T. and K.B. developed the conceptual idea for the study, B.S. developed statistical methods and tests and S.J.W. and L.D.A. assembled input data. All authors contributed to discussions and writing of the manuscript.

Competing financial interests

The authors declare no competing financial interests.

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  1. Supplementary Information (10.5M)

    This file contains Supplementary Information, References, Supplementary Table S1 and Supplementary Figures S1-S8 with legends.

Comments

  1. Report this comment #15566

    Steven Shirey said:

    Diamonds Are Accidental Passengers in Kimberlite Magma

    Steven B. Shirey^1^ and D. Graham Pearson^2^

    ^1^Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC USA, shirey@dtm.ciw.edu

    ^2^Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, T6G 2E3 Canada, gdpearson@ualberta.ca

    Arising from: Torsvik, T.H., K. Burke, B. Steinberger, S.J. Webb, and L.D. Ashwal, Diamonds sampled by plumes from the core-mantle boundary. Nature, 2010. 466: p. 352-355.

    The recent article by Torsvik et al.[1] has received intense coverage by implying that diamonds are related to kimberlites from lower mantle plumes and that this relationship might have usefulness in exploration. The high interest been created chiefly by the misleading use of the word ‘diamond’ in the title and first sentences of their paper. Here we show that diamonds are rarely related to their kimberlite host so that the best exploration scheme for diamonds is to focus on the deep geological evolution and structure of the ancient continental interior regions, known as cratons.

    Diamonds in kimberlite come from three sources: the continental lithosphere (~90%); the deep convecting mantle (<10%); and as fibrous overgrowths on the first variety (Table 1). The ages of diamonds are determined by radioactive decay schemes such as Sm-Nd, Rb-Sr, and Re-Os applied to mineral inclusions[2], or via the aggregation of lattice-hosted nitrogen[3]. Kimberlite ages can be determined by radioactive decay schemes applied to phenocrysts such as Rb-Sr and Ar-Ar on phlogopite[4], U-Pb on perovskite[4] and U-Pb and (U-Th)/He on zircon)[4,5]. Lithospheric diamonds, by far the dominant type, show a large age contrast with their kimberlitic host magma (800 myr up to 3500 myr; Table 1). This age difference clearly negates a genetic link between kimberlite and diamond, making the timing and origin of the host irrelevant to the timing and origin of its diamond cargo. The diamond cargo is just an accidental sampling of the ancient mantle lithospheric keel in which the diamonds have been stored for long geological time periods. Diamond distribution within cratonic keels is a complicated result of often overlapping processes that introduced diamond-forming fluids to the lithosphere. These processes include subduction, continental collision, and sublithospheric magmatism resulting from continental break-up and reassembly[6,7,8].

    The spatial mismatch between large low-velocity shear wave province (LLVSP) positions identified by Torsvik et al.[1] and known diamond occurrences, especially those that contain superdeep diamonds, also underscores the accidental nature of sampling by kimberlite. If the link between diamonds and plumes exists, as alluded to in the Torsvik et al.[1] paper, we should expect the deepest-derived, youngest, non-lithospheric diamonds (superdeep diamonds; Table 1) to bear the most direct spatial relationship to the LLVSP. These diamonds originate at depths ranging from the shallow mantle transition zone to the top of the lower mantle [9] (Table 1) and are the most likely diamonds to be involved with any deeply derived mantle upwellings related to LLVSP. No such relationship exists. Furthermore, we note that any spatial associations between surface kimberlite eruptions and the very deep-seated phenomena of the sort highlighted by Torsvik et al.[1], cannot be directly viewed as cause-and-effect relationships.

    Another significant problem for the Torsvik et al.[1] plume-kimberlite-diamond link is that no LLVSP can be matched to the prolific diamond-bearing kimberlite fields of the Canadian Shield, the richest of which is centered on the Slave craton. Over 350 Slave craton kimberlites have been found[10]. They erupted in two age groupings at 45-74 and 435-542 Ma[4] and both groups contain superdeep diamonds[11,12,13]. The young age of most of these kimberlites (45-74 Ma) implies that errors in the longitude correction should be small relative to other sample groups. Given the large number of Slave craton kimberlites, their high grade, and the presence of superdeep diamonds within them, the non-match to the LLVSP (actually mentioned by Torsvik et al.[1]) seems a fatal exception to their model.

    Diamonds are almost exclusively found in kimberlite because it is the magma type that ascends rapidly enough under conditions favorable for preventing the magmatic digestion of diamonds during transport. Single-crystal diamonds generally bear no genetic relationship to the kimberlitic host magma in which they are found nor to the mantle plume that created the kimberlites. We therefore argue that continental age, structure, and tectonic history remain as the more relevant exploration tools for diamonds rather than position above a low velocity zone on the core mantle boundary.

    Table 1: Summary of characteristics of diamond in kimberlite

    Diamond type common . Occurrence of . . . . Depth of diamond . Host material for original . Age difference with
    in kimberlite^a^ . .. diamond type (%)^b^ . growth (km) . .. . growth of diamond . . .. . . kimberlitic magma (myr)

    peridotitic . . . . . . . 60% . . . . . . . . 120 to 160 . . . . lithospheric mantle . . . . 800 to 3400^c^

    eclogitic . . . . . . . . 30% . . . . . . . . 120 to 160 . . . . lithospheric mantle . . . . 800 to 2900^d^

    superdeep . . . . . . . . 10% . . . . . . . . 500 to 1000 . .. . convecting mantle . . . . . 100^e^

    fibrous . . . . . . common locally .. . . . . 120 to 160 . . . . kimberlite magma . . .. . . equal to kimberlite^f^

    polycrystalline . . usually rare .. . . . . . 120 to 160 . . . . lithospheric mantle . . . . young (?)^g^


    Entries in table constructed from the summarized data in published review papers[2,9,16,21,22].
    ^a^ Peridotitic diamonds are those that contain mineral inclusions similar in composition to the main minerals in the mantle (olivine, orthopyroxene, clinopyroxene, garnet). Eclogitic diamonds are those that contain mineral inclusions (garnet, clinopyroxene) similar in composition to the high-pressure form of basalt known as eclogite. Superdeep diamonds are those that contain inclusions of high-pressure phases[9]. Fibrous diamonds are cloudy, non-gem diamonds that grow alone or as overgrowths on gem-quality, older diamonds. Polycrystalline diamonds are aggregates of mm-sized diamond crystals.
    ^b^ Estimates made from silicate inclusions only[21]. Neither fibrous diamonds nor polycrystalline diamonds (framesite) are gem quality. Of these types, only fibrous diamonds are thought to grow from kimberlite.
    ^c^ Excludes peridotitic sulfide pair from Koffiefontein[14];
    ^d^ gt+cpx eclogitic isochron from Premier also omitted as only example that does not fit age range[20];
    ^e^ Superdeep diamonds are poorly studied for age; only one determination has been made at present[15].
    ^f^ Incorporation of kimberlitic components, isotopic signatures similar to the present convecting mantle, and low N aggregation all support an age equal to the kimberlite[16,17].
    ^g^ Petrologic studies support aggregation of polycrystalline diamonds close to time of kimberlite eruption[18,19]


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