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OPEN

received: 30 August 2016
accepted: 07 December 2016
Published: 13 January 2017

Diamond formation in the deep
lower mantle: a high-pressure
reaction of MgCO3 and SiO2
Fumiya Maeda1, Eiji Ohtani1,2, Seiji Kamada1,3, Tatsuya Sakamaki1, Naohisa Hirao4 &Yasuo Ohishi4
Diamond is an evidence for carbon existing in the deep Earth. Some diamonds are considered tohave originated at various depth ranges from the mantle transition zone to the lower mantle. These
diamonds are expected to carry significant information about the deep Earth. Here, we determined
the phase relations in the MgCO3-SiO2 system up to 152 GPa and 3,100 K using a double sided laserheated diamond anvil cell combined with in situ synchrotron X-ray diffraction. MgCO3 transforms frommagnesite to the high-pressure polymorph of MgCO3, phase II, above 80 GPa. A reaction between
MgCO3 phase II and SiO2 (CaCl2-type SiO2 or seifertite) to form diamond and MgSiO3 (bridgmanite or
post-perovsktite) was identified in the deep lower mantle conditions. These observations suggested
that the reaction of the MgCO3 phase II with SiO2 causes formation of super-deep diamond in cold slabs
descending into the deep lower mantle.
Carbon is circulated around the surface and interior of the Earth with subducting slabs and volcanic eruptions;subduction carries carbon-bearing rocks to the Earth’s interior and volcanic eruption expels carbon-bearing gas,lavas and rocks from the interior of the Earth1. The flux of subducted carbon within oceanic plates is estimated tobe more than 5 Tmol/yr, almost twice as large as the expelled-carbon flux, 2–3 Tmol/yr, through arc magmatism2.
This difference suggests the existence of carbon reservoirs in the deep Earth1,2.
One source of direct evidence for deep carbon is carbon-bearing samples originating from the Earth’s interior.
Diamond is evidence of quite a deeper-origin carbon. In particular, some diamonds, called ‘super-deep diamond’,
are thought to arise from the mantle transition zone or the lower mantle3–8. The inclusions in super-deep diamond may supply information on the lithology, water content, and/or elemental distribution in deep parts of the
Earth3–8.
Subducting slabs play a key role for carrying carbon-bearing phases into the deep Earth and forming
super-deep diamond3–5. Altered rocks in the oceanic crust contains the large amount of carbon such as organiccarbon or carbonate minerals, which can be the deep reservoir of subducted carbon9,10. These carbonate minerals or melts may change to diamond if they become unstable in the Earth during the subduction process3,5,11–13.
The stability of carbonates has been investigated using high-pressure experiments and ab initio calculations, andMgCO3 magnesite is determined to be stable under the high-pressure and high-temperature conditions expectedduring subduction11–20. The existence of carbonate (or carbonatite melt) in the deep mantle is supported by thediscovery of carbonate as inclusions in diamonds originating from the mantle transition zone and/or the lower
mantle3,5.
Since SiO2 is one of the abundant components and also is an important phase in deeply subducted slabs21–23,
the MgCO3-SiO2 system may be applied to the slabs descending into the lower mantle. SiO2 phases may change toits high-pressure polymorph, such as coesite, stishovite, CaCl2-type phase and seifertite24,25. Magnesite is expected
to break down to CO2 or diamond by reacting with silica minerals in the MgCO3-SiO2 system in subducting
process12,13,26,27 although the detail of its phase relation has not yet been clarified. Knowledge of the reactionsin this system at high pressure and high temperature may provide important insights into the carbon-related
processes in the deep mantle, such as the origin of super-deep diamond and melting or oxidation by release of1
Department of Earth Science, Graduate School of Science, Tohoku University, Sendai, 980-8578, Japan. 2V.S.
Sobolev Institute of Geology and Mineralogy, SB RAS, Novosibirsk, 630090, Russia. 3Frontier Research Institutefor Interdisciplinary Sciences, Tohoku University, Sendai, 980-8578, Japan. 4Japan Synchrotron Radiation ResearchInstitute, Sayo, Hyogo 679-5198, Japan. Correspondence and requests for materials should be addressed to F.M.
(email: f.maeda@dc.tohoku.ac.jp)

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volatile components13,28. We used a laser-heated diamond anvil cell (LHDAC) combined with a high-pressure andhigh-temperature in situ synchrotron X-ray diffraction (XRD) technique to quantify the phase relations of the
MgCO3-SiO2 system down to the lowermost-mantle conditions. Our objective is to clarify the behavior of carbon
in the lower mantle and to model the origin of super-deep diamond.

Results

We observed the phase relation of MgCO3 and SiO2 system up to 152 GPa and 3,100 K (see Supplementary Tables S1 and S3). Diamond and bridgmanite may be formed through the following reactions12,13,28:
SiO2 + MgCO3 → MgSiO3 + CO2 .

(1)

CO2 → C + O2 .

(2)

SiO2 + MgCO3 → MgSiO3 + C + O2 .

(3)

Or, the following reaction is also possible:

Bridgmanite and diamond were observed using XRD, in a run product recovered from 83 GPa and 1,780 K(Fig. 1a). The X-ray pattern was taken at an ambient condition without DAC, of which surface showed no damage
after the experiments. The high-pressure phase of CO2 was not detected in most runs, matching the previous
studies on the MgCO3-SiO2 system12,13. We successfully detected the high-pressure phase of CO2, CO2-VI29 in
one run made by in situ X-ray diffraction at 83 GPa and 1,780 K (see Supplementary Table S1). Thus, occurrenceof reactions (1) and (2) or (3) were thus confirmed.
The structures of the MgCO3 high-pressure phases and their phase transition boundary are controversial
and the data on their compression behaviors are very limited. A recent ab initio study reported a monoclinic
post-magnesite phase, MgCO3 phase II (C2/m) at 300 K and pressures from 82 to 138 GPa16. On the other hand,
the latest study reported the stabilization of another post-magnesite phase, having space group P-1, at 300 K
and 85–101 GPa, which transforms to MgCO3 phase II (C2/m) at 101 GPa and 300 K19. We conducted an in situ
XRD study of MgCO3 using a double sided laser-heated diamond anvil cell in the pressure range from 85 GPa
to 132 GPa at about 2,500–3,000 K. Diffraction peaks after heating corresponded to the MgCO3 phase II16 in thesame pressure range (see Supplementary Fig. S1 and Supplementary Table S2). The volumes of MgCO3 phase II
in each run were estimated by fitting XRD patterns (see Supplementary Table S2). We then fitted the estimatedvolumes using the second order Birch-Murnaghan equations of state (BMEOS). As a result, the unit-cell volume
at ambient condition (V0) and isothermal bulk modulus (K300K, 0) of magnesite phase II were estimated to be
498.9(5) Å3 and 154.9(7) GPa, respectively (K’ =​ 4; fixed). These values are consistent with the V0 and KT0 calculated by ref. 16, V0 =​ 503.36 Å3 (Z =​ 12) and K0 =​ 156.76 GPa using the third order BMEOS (K’ =​  4.12).
MgCO3 phase II was identified in MgCO3-SiO2 system at pressures above 100 GPa by comparing the XRD
patterns of the runs in the MgCO3-SiO2 system with the results of MgCO3 compression (see Supplementary Inf
ormation Table S2). This enabled us to distinguish the MgSiO3 bridgmanite/post-perovskite phase from MgCO3
phase II in the XRD patterns (Fig. 1b). The MgSiO3 post-perovskite phase is thought to be formed by either
reaction (1) or (3), where MgCO3 and SiO2 are considered to be MgCO3 phase II and seifertite, respectively.
XRD spots (111) indicating diamond were also observed in some runs conducted at pressures greater than
100 GPa (see Supplementary Fig. S2). The number of diamond spots was limited and their intensities were weak.
They did not appear in all 1D XRD patterns at high pressure. However, diamond created in high-pressure andhigh-temperature conditions was confirmed in the recovered run products (Fig. 1a and c).
Figure 2 shows the phase diagram of the MgCO3-SiO2 system based on the present and previous studies. The
temperature of magnesite decarbonation is consistent with ref. 13 but higher than ref. 12 up to 70 GPa. We discovered the phase transformations from CaCl2-type SiO2 +​  MgCO3 phase II to bridgmanite +​  diamond +​  O2 andfrom seifertite +​  MgCO3 phase II to bridgmanite/post-perovskite +​  diamond +​  O2 for the first time in this experiment. The decomposition boundary of CO2 has a steep gradient in the present phase diagram of MgCO3-SiO2
system which is not consistent with the results of any previous studies on CO2, such as decomposition28 or phasetransition of CO230.

Discussion

Since the reactions between MgCO3 and SiO2 may be expected in deeply subducted slabs13,26,27, we should
consider the phase relations in the MgCO3-SiO2 system on subducting depth-temperature paths. The phaseboundaries of the MgCO3-SiO2 system are shown with several slab geotherms31 in Fig. 3. The slab geotherms in
Fig. 3a are models based on the geological observations32 and mineralogical hypothesis31 in the present Earth.
The super-deep diamonds form Juina kimberlite were considered to be related to Gondwana subduction andformed at 150 Ma5. Therefore, the colder geotherms like the modern Earth may be useful to be compared with
the MgCO3-SiO2 reactions. The reactions in this system provide important information for understanding thestability limits of MgCO3 in deep subduction.
The reaction between MgCO3 and SiO2 may not occur down to the top of the lower mantle but can occur at
depths from 1,000 to 2,000 km in various subducting slabs. In “hot” and “cold” slabs31, which are comparable
to Mexico and the various Pacific subduction paths respectively32, magnesite can react with stishovite to form
bridgmanite and CO2 at depths of around 1,700 km, and subsequently CO2 can decompose to diamond and O2 at
the greater depths. Therefore, super-deep diamond could be formed at depths greater than 1,700 km in cold slabs.
The path of “very cold” slabs31 such as Tonga subduction32 may pass through the ‘bottleneck’ region in the phase
diagram at ~80 GPa corresponding to the depth near the MgCO3 magnesite-phase II boundary (~1,900 km). MgCO3
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Figure 1. XRD patterns of the reaction products in the MgCO3-SiO2 system. (a) XRD patterns obtained
under the ambient condition for the samples recovered from 83.1(5) GPa and 1,780(210) K. The X-ray pattern
was taken at ambient condition without DAC, of which surface showed no damage after the experiment. The
abbreviations represent as follow: St: stishovite, Brd: bridgmanite, Dia: diamond, Pt: platinum, W: tungsten(gasket), NaCl: sodium chloride (pressure medium). (b) In situ XRD patterns obtained at 145.5 (31) GPa and2,700 (360) K. (c) 2D XRD images of a sample recovered from 145.5 (31) GPa and 2,700 (360) K obtained atambient condition without DAC. The abbreviations represent as follow: Se: seifertite, Mgs: magnesite, MgsII:
MgCO3 phase II, PPv: MgSiO3 post-perovskite phase, Dia: diamond, Pt: platinum, W: tungsten (gasket).

in such slabs may descend to the base of the lower mantle because the stability field of MgSiO3 +​  diamond +​  O2
is relatively narrow or absent in the pressure-temperature path of very cold slabs (Fig. 3a). If MgCO3 is able
to survive beyond 1,900-km depth, MgCO3 phase II will be formed and subsequently decompose to MgSiO3
post-perovskite phase +​  diamond +​  O2 by a reaction with seifertite at the base of the lower mantle due to heatingfrom the outer core.
Slabs in the early Earth, Archean/Proterozoic age, might descend into the hotter mantle than the modern adiabat33.
The hotter subduction could restrict the MgCO3 subduction into the deep lower mantle because of the reaction(1) in the shallower mantle (Fig. 3). In this case, CO2 might be a significant carbon carrier in the subducted slabs
although its stability has been still controversial under high pressure and high temperature conditions28,30. If earlyslab temperatures had been higher than the modern mantle adiabat33, the formation of CO2 fluid28 or melting
in the MgCO3-SiO2 system26 would be expected down to the mantle transition zone. These phenomena might
prevent the subduction of carbon-bearing phases into the lower mantle. This might be the reason why the reportsof the old super-deep diamonds are absent.

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Figure 2. Phase diagram of the MgCO3-SiO2 system. The closed and open symbols represent the results ofthe present study and ref. 13, respectively. The colors of the symbols show the observed phases as follow: blue:
MgCO3 +​  SiO2, green: MgSiO3 ±​  MgCO3 ±​  SiO2 (+​CO2), red: MgSiO3 +​  C ±​  MgCO3 ±​  SiO2. The present phaseboundaries in the MgCO3-SiO2 system are shown by black solid and dotted lines. The non-equilibrium phases
are shown as a small font in the index column above the phase diagram. The referred phase boundaries between
CO2-V and ionic CO230 and between CO2-V and C +​  O228 are gray dotted lines, shown by ‘Y11’ and ‘L11’,
respectively. The phase boundaries of SiO2 stishovite (St)-to-CaCl2-type SiO2 (CS)24 and CaCl2-type SiO2 (CS)to-seifertite (Se)25, the phase boundary of MgSiO3 bridgmanite (Br)-to-post-perovskite phase (PPv)37, and thoseof MgCO3 magnesite-to-MgCO3 liquid and MgCO3 liquid-to-MgO +​  CO220 are shown by gray solid lines.

The reaction in the “very cold” slab may play an important role in formation of super-deep diamond in the
deep lower mantle. Reference 5 summarized the mineral inclusions in diamond, and estimated the formation
depths of diamonds in the mantle. Some diamonds from São Luiz, Juina province, Brazil have inclusions oforthopyroxene which are a psuedomorph of bridgmanite and TAPP (tetragonal almandine-pyrope phase) suggesting the top of the lower mantle, 660–750 km depths. They also suggested that some diamonds with inclusions of aluminous bridgmanite pseudomorph from the same locality, Juina, Brazil, might be originated at thedepths greater than 750 km, although the exact depth limit was not estimated. On the other hand, Diamonds
containing the iron-rich (Mg,Fe)O inclusions6,8 were considered to be originated from the deep lower mantle(1,700–2,900 km) because of iron enrichment by the spin transition at depths greater than about 1,700 km34–36,
or by the phase transition from bridgmanite to post perovskite at around 2,700-km depth37–41, although it is a
debated matter whether iron-rich (Mg,Fe)O inclusions are the signature of the bottom of the lower mantle or not.
The mechanism of diamond formation discovered here may explain the origin of super-deep diamond fromthe deep lower mantle such as those reported from the limited locality in Brazil5,6,8. Although we did not consider
the effects of fO242,43 in the reaction (3) because of the technical difficulty for controlling fO2 in the DAC experiments, we can expect the above reaction in the presence of iron in the average lower mantle with the fO2 condition
below the iron-wüstite buffer44. When metallic iron exists in the system, iron oxide can be formed by the reaction
with O2 in the run products. The role of iron is also important in formation of high-pressure polymorphs ofcarbonate18,45,46. The latest studies reported that the iron-magnesium carbonate transforms into the several newphases: Fe4(CO4)3 phase having unknown structure18, orthorhombic (Mg,Fe)CO3 phase II45, or the unquenchable phases with unexpected stoichiometry (Mg2Fe2(C4O13) +​  Fe13O19) that coexist under the lower mantle46.
Therefore, we may need to modify the present phase relation further in the iron bearing system. However, we canconsider that the reaction of diamond formation could also occur in the iron-bearing system since iron oxides
and/or FeO-bearing bridgmanite and ferropericlase can be formed in deep subducted slabs.
Released oxygen will oxidize the ferrous iron in mantle minerals or the metallic iron penetrated from the outer
core at the core-mantle boundary28,47. The oxidation of metallic iron can generate FeO. As a result, the iron-richmagnesiowüstite of (Mg,Fe)O6,8 would be formed as inclusions in the deepest diamonds, although it is not a single process for generation of FeO enrichment in magnesiowustite as inclusions in diamond and other processesof the redox change in the shallower depths could also have generated similar iron enrichment. Future studies
are needed to elucidate the ultra-deep iron-carbon redox coupling processes and their influence on formation ofsuper-deep diamond.

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Figure 3. Change of carbon-bearing phases in the slabs descending into the lower mantle. (a) The phaseboundaries in the MgCO3-SiO2 system are denoted with the Earth’s geotherms. The gray solid zone shows
the mantle adiabat33. The blue and red solid arrows are the depth-temperature paths of Tonga and Mexico
subduction zones, where the coldest and hottest slabs were modeled in ref. 32, respectively. The hypotheticalmodels of the “hot”, “cold” and “very cold” slabs31 are shown by the orange, light blue and blue arrows,
respectively, and the zone of each color is extrapolation of each path. The series of paths of cold-slab geotherm
below the mantle transition zone represent the case of the fast subduction, stagnant and the slow subduction31.
(b) The schematic model of the super-deep diamond’s formation. The orange, light blue and blue lines represent
the “hot”, “cold” and “very cold” subducting slabs31, respectively. The green circles and diamond-shaped symbols
indicate the possible depth of CO2 and diamond formation, respectively.

Methods

A natural single crystal of magnesite (Bahia, Brazil, Mg0.994Ca0.003Fe0.003CO3) and a reagent quartz (reagent grade,Woko) were ground to fine powders in an agate mortar for 1 hour. The powders were mixed 1:1 by mole fraction
and ground in an agate mortar for 30 min to homogenize the mixture. High-pressure and high-temperatureexperiments were conducted using a double sided LHDAC. The culet diameters of the diamond anvils were
between 100 and 350 μ​m. The sample chamber drilled in rhenium or tungsten gaskets ranges from 30 to 100 μ​m
in diameter and from 40 to 80 μ​m in the thickness depending on the culet sizes. NaCl or SiO2 glass was used as thepressure medium and thermal insulator.
Platinum was included in the sample chamber as a laser absorber. We used three sample configurations to
obtain stable temperatures at high pressures: Run Ch1-3-layer-chamber, Run Ch2-5-layer-chamber and Run
Ch3-Pt/W-doughnut-chamber (Supplementary Table 1). In Run Ch1, 5 wt. % platinum powder (platinum black,purity 99.9%, Mitsuwa Chemicals) was mixed with magnesite and quartz starting material (Ch1) and this was
sandwiched by a pressure medium. On the other hand, in Run Ch2, the magnesite and quartz mixture was sandwiched by platinum foils (purity 99.9%, Nilaco). Run Ch2 aimed to reduce the temperature gradient at the heating
spot during double-sided laser heating.
The samples were heated by double-sided fiber lasers (SPI LASER) of wavelength 1.090 μ​m at Tohoku
University in Sendai, Japan and 1.070 μ​m at BL10XU of SPring-8 in Hyogo, Japan. The heating duration was
10–120 minutes. Temperature was determined by fitting the emission spectra from the heated samples to thegray-body radiation formula.
In situ synchrotron X-ray diffraction (XRD) was conducted to identify the experimental products. We
acquired the XRD patterns of the samples at high pressure up to 152 GPa in the temperature range from 300 K
to 3,100 K at BL10XU of SPring-8. The collimator was 20 μ​m in diameter and the typical wavelength of X-ray
was 0.41414(9) Å. An imaging plate (Rigaku, R-AXIS IV++) and a charge coupled device (Bruker, AXS SMART
APEX) detectors were used for acquiring the XRD patterns. The pressures of in situ experiments were determined based on the equation of state of Pt48. Thermal pressure was estimated based on Mie–Grüneisen–Debye
model49,50. In Runs Ch3-1 and Ch2-5, the sample was heated at Tohoku University and observed after temperature quenching at high pressure using XRD at BL10XU of SPring-8. In Ch3-1, the sample was recovered to theambient condition at Tohoku University after the temperature quenching and pressure determination using rubyfluorescence method51. Since thermal pressure could not be estimated in Runs Ch3-1 and Ch2-5, we considered
the pressure errors up to ±​10 GPa for pressures at high temperature during laser heating. This is almost equivalent to the maximum pressure increase by laser heating in the in situ XRD experiments.

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The high-pressure phase transition of MgCO3 was confirmed in two series of runs, Ms-1 and Ms-2 in addition to the MgCO3-SiO2 system (Supplementary Table 2). These runs were conducted using a membrane-type
diamond anvil cell. The natural magnesite and Pt powder were sandwiched by the same magnesite in the sample chamber. Pt powder was used as a pressure maker and laser absorber. Magnesite was compressed to about
100 GPa, and then pre-heated to temperature of less than 1,500 K using CO2 laser at Tohoku University in orderto crystallize the compressed sample in Run Ms-1. We conducted laser heating at 2,500–3,000 K for 5–20 min inorder to obtain the obvious XRD patterns of the MgCO3 high-pressure phase using SPI fiber laser in two series ofruns, Ms-1 and Ms-2. XRD patterns were acquired at high pressure and 300 K after quenching.
Temperatures in some experiments increased suddenly to above 3,000 K in the Runs Ch1-1, Ch1-2, Ch1-3
and Ch 2-1 using platinum as a laser absorber (Supplementary Table 1). This phenomenon may correspond to the‘temperature jump’ reported by ref. 13. The sudden temperature increase was also observed at high pressures andabove 2,000 K when a Pt powder (Run Ch1) and Pt foil (Run Ch2) was used as the laser absorber. These results
indicate that large temperature fluctuation may be caused by the volume change following reactions, phase transitions and/or melting of the sample.

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Acknowledgements

This work was supported by Grant-in-Aid awards for Scientific Research to E.O. (numbers 22000002 and
15H05748) from the Ministry of Education, Culture, Sports, Science, and Technology of Japanese Government.
E.O. was also supported by Russian Science Foundation, Project 14B25.31.0032. The synchrotron radiation
experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation ResearchInstitute (Proposal No. 2013B0104, 2014A0104, 2014B0104, 2015A0104, 2015B0104). F.M. was supported by theInternational Joint Graduate Program in Earth and Environmental Science (GP-EES), Tohoku Univirstiy.

Author Contributions

F.M., S.K., T.S., E.O., N.H. and Y.O. performed XRD measurements. F.M., S.K., E.O. and T.S. made arguments
on geological applications of the experimental results and wrote the paper. All authors discussed the results andcommented on the manuscript.

Additional Information

Supplementary information accompanies this paper at http://www.nature.com/srep
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Maeda, F. et al. Diamond formation in the deep lower mantle: a high-pressure reactionof MgCO3 and SiO2. Sci. Rep. 7, 40602; doi: 10.1038/srep40602 (2017).
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Scientific Reports | 7:40602 | DOI: 10.1038/srep40602

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