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CO2-induced destabilization of pyrite-structured FeO2Hx in the lower mantle

CO2-induced destabilization of pyrite-structured FeO2Hx in the lower mantle Downloaded from https://academic.oup.com/nsr/article/5/6/870/4937983 by DeepDyve user on 20 July 2022 National Science Review 5: 870–877, 2018 RESEARCH ARTICLE doi: 10.1093/nsr/nwy032 Advance access publication 15 March 2018 GEOSCIENCES CO -induced destabilization of pyrite-structured FeO Hx 2 2 in the lower mantle 1,2,∗ 2 2 3 Eglantine Boulard , Franc¸ois Guyot , Nicolas Menguy , Alexandre Corgne , 4 5 2 Anne-Line Auzende , Jean-Philippe Perrillat and Guillaume Fiquet ABSTRACT Volatiles, such as carbon and water, modulate the Earth’s mantle rheology, partial melting and redox state, thereby playing a crucial role in the Earth’s internal dynamics. We experimentally show the transformation of goethite FeOOH in the presence of CO into a tetrahedral carbonate phase, Fe C O , at conditions 2 4 3 12 above 107 GPa—2300 K. At temperatures below 2300 K, no interactions are evidenced between goethite 1 and CO , and instead a pyrite-structured FeO H is formed as recently reported by Hu et al. (2016; 2017) 2 2 x Synchrotron SOLEIL, and Nishi et al. (2017). The interpretation is that, above a critical temperature, FeO H reacts with CO 91192 St Aubin, 2 x 2 France; Sorbonne and H , yielding Fe C O and H O. Our findings provide strong support for the stability of 2 4 3 12 2 Universite, ´ Museum ´ carbon-oxygen-bearing phases at lower-mantle conditions. In both subducting slabs and lower-mantle National d’Histoire lithologies, the tetrahedral carbonate Fe C O would replace the pyrite-structured FeO H through 4 3 12 2 x Naturelle, UMR CNRS carbonation of these phases. This reaction provides a new mechanism for hydrogen release as H O within 7590, IRD.—IMPMC, the deep lower mantle. Our study shows that the deep carbon and hydrogen cycles may be more complex 4 Place Jussieu, than previously thought, as they strongly depend on the control exerted by local mineralogical and chemical 75005 Paris, France; environments on the CO and H thermodynamic activities. 2 2 Instituto de Ciencias de la Tierra, Keywords: deep carbon cycle, FeOOH, high pressure Universidad Austral de Chile, 5090000 subduction are estimated to account for a flux of Valdivia, Chile; INTRODUCTION ISTerre, Universite´ 3.6 × 10 mol/year of carbon being returned into Water (H O) and carbon dioxide (CO ) both play 2 2 Grenoble Alpes, the deep mantle [3–5]. This quantity accounts for an important role in the history of the Earth, as they CNRS, F-38041 10–30 wt % of the carbon reservoir in the deep man- strongly influence the chemical and physical proper- Grenoble, France and tle [6]. Regarding the water cycle, Van Keken et al. 5 ties of minerals, melts and fluids. Distribution and Laboratoire de 13 [2] suggested that 4–6 × 10 mol/year of H O circulation of H O and CO between the Earth’s Geologie ´ de Lyon, 2 2 are recycled into the mantle through slab subduc- surface and the mantle have dominated the evolu- UMR CNRS 5276, tion. Dehydration of the slab accounts for the loss Universite´ Claude tion of the crust, the oceans and the atmosphere, of two-thirds of this amount of H O, while one- Bernard Lyon 1—ENS controlling several aspects of the Earth’s habitabil- third of the H O remains bounded to the slab (i.e. de Lyon, 69622 ity. It is therefore crucial to determine the stability ≈1.5 × 10 mol/year) reaching depths exceeding Villeurbanne, France and circulation of hydrous and CO -bearing miner- 240 km. Although this amount of H O entering the als in the Earth’s interior. Sedimentary material to- deep mantle may not appear very large, it provides a Corresponding gether with altered mafic and ultramafic rocks that mechanism for having significant amounts of water author: E-mail: eglan- constitute the subducted slabs represents the main tine.boulard@upmc.fr in the deep mantle. In addition, part of the CO and source for recycling of H O and CO as well as 2 2 H O present in the deep mantle may also originate other volatiles at great depth, possibly down to the from primitive mantle reservoirs [7], leading poten- Received 17 core—mantle boundary. The transport of H O and tially to fairly large amounts of these volatiles in the September 2017; CO via subducting slabs down to the transition Revised 28 January deep mantle. zone and to the lower mantle has been the subject 2018; Accepted 7 Because of its very low solubility in deep Earth’s of many studies but is still under debate [1,2]. As March 2018 minerals [8,9], carbon is expected to be present for the carbon cycle, carbonates preserved during The Author(s) 2018. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. All rights reserved. For permissions, plea se e-mail: journals.permissions@oup.com Downloaded from https://academic.oup.com/nsr/article/5/6/870/4937983 by DeepDyve user on 20 July 2022 RESEARCH ARTICLE Boulard et al. 871 as accessory phases in the mantle, either as oxi- transfer of H to the deep Earth. In their recent work, dized phases such as carbonates or CO and car- Hu et al. [34] suggested that a phase of FeO H com- 2 2 x bonated fluids or melts, or as reduced phases such position might indeed deliver H instead of water as diamonds or Fe-C alloys [10]. It is commonly when heated above a threshold temperature—a par- considered that the lower mantle is too reduc- ticularity due to valence changes of oxygen in this 2– 2– ing to host carbonates [11,12]. However, the rel- compound (from 2O to O )[37]. atively high oxygen fugacities prevailing in sub- After H O, CO is the second most important 2 2 ducting slabs might contribute to preserve oxidized volatile compound in the deep Earth. To get a more carbon-bearing phases in the deep mantle [13,14]. complete understanding of the H and C cycles in Moreover, it has recently been demonstrated that the deep Earth, it is necessary to know how deep carbonates at lower-mantle conditions adopt oxi- subducted materials can transport both C and H dized iron-bearing structures based on CO tetrahe- and identify the distinct species involved. In the dra that are associated with reduced carbon phases present study, we shed light on this crucial issue by [13,15–17]. Little is known about the stability of constraining experimentally the interactions of CO these new tetrahedral carbon-bearing phases but with potential carriers of H OorH at great depths. 2 2 their systematic association with reduced carbon We performed high-pressure and high-temperature suggests the idea that the mineralogies of the lower diamond-anvil cell (DAC) experiments to investi- mantle and D region may be more complex than gate the effects of a CO -rich medium on the trans- previously thought. Interestingly, carbonate-bearing formations of FeOOH at pressures and tempera- inclusions have also been reported in diamonds tures of the lower mantle. formed in the lower mantle. This suggests again the presence of carbonates in the deep Earth and a pos- sible coexistence of reduced and oxidized carbon RESULTS species [15,18,19]. Decarbonation reactions of car- bonates involving silicates (SiO and MgSiO )were A natural sample of crystalline α-FeOOH (Sup- 2 3 also reported to take place as shallow as ∼600 km in plementary Fig. 1) was loaded in CO and first depth (20 GPa) [20,21]. Such reactions could pro- pressurized up to 107 GPa in a DAC at ambient duce CO in the lower mantle. Given the current un- temperature. In situ X-Ray diffraction (XRD) pat- certainties on the phase diagram of CO at high pres- 2 terns showed a significant broadening of α-FeOOH sures, CO may be expressed as a solid CO -V phase 2 2 main diffraction reflections characteristic of incip- [21] or rather dissociate as C + O [22]. Carbon- ient amorphization. After laser heating at 2000 K ated fluids yet unknown at such conditions might for a few minutes, several changes in the diffrac- also contribute to CO transfer at large depth in the tion pattern were observed: the α-FeOOH phase mantle. In any case, large thermodynamic activities disappeared and two distinct phases were identified of CO are plausible in the lower mantle. (Fig. 1). The most intense diffraction peaks corre- A significant amount of water can be dissolved in spond to a cubic structure with extinctions of the nominally anhydrous minerals such as olivine, gar- two reflections 001 and 011 in agreement with a net and stishovite [23], as well as in high-pressure Pa-3 space group. Recently, Hu et al. [34] reported silicates such as wadsleyite and ringwoodite [24,25]. the transformation of FeOOH into a new Pa-3 cu- In addition, diverse dense hydrous silicates are sta- bic structure FeO H at similar pressure and tem- 2 x ble in mafic and ultramafic assemblages at upper- perature conditions. This phase is directly related to and lower-mantle conditions, such as phase A, phase the newly discovered pyrite-structured FeO perox- D, phase H and superhydrous phase B [26–30]. Fi- ide but is characterized by a larger unit cell volume nally, δ-AlOOH, a high-pressure form of diaspore [34,37]. FeO H can be interpreted as a solid solu- 2 x (α-AlOOH) with an orthorhombic symmetry very tion between pyrite-structure FeO and FeOOH. In close to that of the CaCl -type polymorph of SiO ,is addition, a pyrite-structured FeOOH oxyhydroxide 2 2 stable throughout the mantle and may be present in (i.e. FeO H with x = 1) was recently observed ex- 2 x suitably aluminous and hydrated lithologies [31,32]. perimentally by Nishi et al. [35]. It presents a struc- The high-pressure polymorph ε-FeOOH that shares ture close to the pyrite-type structure of AlOOH pre- the same structure with δ-AlOOH [33] might also dicted above 170 GPa by Tsuchiya and Tsuchiya store water in the mantle. Iron oxyhydroxides, in- [38]. Here, we measured a unit cell parameter of a cluding FeOOH and its polymorphs, are common at = 4.367 A at 107 GPa, which is significantly larger the surface of the Earth, where they are abundant in than that reported for FeO (a = 4.363 A at 76 GPa) soils and sediments. The incorporation of hydrogen by Hu et al. [37], but smaller than that reported for atoms in newly discovered iron oxyhydroxides with a FeOOH (a = 4.386 A at 109 GPa) in Nishi et al. pyrite structure [34–36] may thus contribute to the [35] (Fig. 2). It is thus probable that FeOOH in our Downloaded from https://academic.oup.com/nsr/article/5/6/870/4937983 by DeepDyve user on 20 July 2022 872 Natl Sci Rev, 2018, Vol. 5, No. 6 RESEARCH ARTICLE 2000 K 2300 K Calculated Observed Difference 4 5 6 7 8 9 10 11 12 13 4 5 6 7 8 9 10 11 12 13 14 2-theta 2-theta Figure 1. XRD patterns collected at 2000 K and at 2300 K and LeBail unit cell refinement of the two phases FeO H (red markers, space group Pa-3, a = 2 x 4.365(1)) and Fe C O (blue markers, space group P2, a = 9.697(2), b = 6.296(2), c = 5.726(1), beta = 92.94(2)). Red circles materialize the expected 4 3 12 peak position of the cubic phase according the XRD collected at lower temperature. (a) Calculated FeOOH Fe Observed a = 4.366828(56) H Difference X = 0.355356(257) Rp = 0.0049 wRp = 0.0030 x y 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 2-theta FeOOH this study (b) FeOOH (Nishi et al. 2017) FeOOH - FeOOH in Ar/Ne (Hu et al. 2017) FeOOH - Fe O + H O (Hu et al. 2017) 23.0 X 2 3 2 FeO - Fe O + O (Hu et al. 2016) 2 2 3 2 FeOOH (Liu et al. 2017) 22.5 Temperature (K) 22.0 21.5 21.0 20.5 60 80 100 120 Pressure (GPa) Figure 2. (a) Rietveld refinement of the XRD pattern collected at 107 GPa and 300 K after laser heating with a FeO Hpyrite- structured (right hand). (b) Unit cell volume of the pyrite-type structure measured experimentally as a function of pressure and temperature for FeO [37], FeO Hx ([31], from both Fe O +H O and FeOOH in Ar experiments and this study) and FeOOH 2 2 2 3 2 [35]. All the data reported here were collected after quenching the temperature. Counts Unit cell volume (Å /f.u.) Counts Counts Downloaded from https://academic.oup.com/nsr/article/5/6/870/4937983 by DeepDyve user on 20 July 2022 RESEARCH ARTICLE Boulard et al. 873 note that Fe in Fe C O is ferric iron Fe(III) as 4 3 12 Diamond anvil in FeOOH. After laser heating, we collected a pro- Sample file of diffraction patterns across the heated spot CO (Supplementary Fig. 2). The FeO H cubic phase 2 x was observed at the edge of the 2300-K heated spot only. Although it is theoretically possible that the reaction is kinetically restricted at lower tempera- CO - VI tures, the fact that a change occurred abruptly at 3600 H O - ice X 1g T around 2300 K (FeO H disappears within a few 2g 2 x seconds) more probably pinpoints to a thermo- dynamic boundary. The pyrite-structured FeO H 3400 2 x would be stable only at relatively low temperatures in the Fe-O-C-H system provided CO thermody- namic activities are high enough. Further studies should verify this point. Raman spectra were also collected at ambi- ent temperature and high pressure in the 300– −1 1300 cm range. As presented in Fig. 3, when col- lected in areas where the CO loading gas was pure, the spectra reveal only one low intensity mode at 700 800 900 1000 1100 1200 1300 −1 ∼1123 cm , which corresponds to the most in- -1 Frequency (cm ) tense mode A of CO -VI [41]. In the solid sam- 1g 2 ple area, an additional mode was detected at ∼930 Figure 3. Raman spectra collected after laser heating in the CO area (in black) and −1 cm assigned to the T mode from the high- 2g sample area (in red). pressure phase of H Oice-X[42,43]. No Raman ac- tive modes associated with Fe C O could be ob- 4 3 12 served, which may be due to high fluorescence back- ground of the diamond from the DAC. study underwent partial dehydrogenation. A similar Transmission electron microscopy (TEM) anal- unit cell volume based on a FeO H formula was re- 2 x yses of a thin section extracted from the recov- ported at the same P-T conditions by Hu et al. [34], for which, using their calibration, they deduced x = ered sample are reported in Fig. 4. Semi-quantitative 0.66. Because this calibration is not only built using chemical analyses (XEDS) showed a homogenous experimental data, but also incorporates theoretical composition with carbon, iron and oxygen with Fe- results, which are known to either overestimate or O atomic proportions consistently with Fe C O 4 3 12 underestimate unit cell volume, uncertainty on the (Fig. 4A). The sample was unstable under elec- exact amount of hydrogen ‘x’ present in FeO H may tron beam, and selective area electron diffraction 2 x be high. To account for this, we simply refer to this (SAED) revealed the presence of two patterns phase as FeO H . (Fig. 4B): γ -Fe O maghemite coexisting with a 2 3 2 x In the XRD pattern, the less intense diffrac- phase characterized by a 6-fold symmetry diffraction tion peaks can be assigned to an already discovered pattern that could not be indexed. It is probable that, carbon-rich phase stable at these P-T conditions: under the electron beam, Fe C O underwent car- 4 3 12 Fe C O [15] (Fig. 1). Among the five structures bon loss to form γ -Fe O together with a second 4 3 12 2 3 proposed in literature [15,17,39,40], we found that phase still containing carbon. Note that the observed only the monoclinic structure reported in [15]al- texture, often observed in cases of irradiation dam- lowed us to assign all of the observed diffraction ages, is in agreement with amorphization and de- peaks. Although ex situ analyses of the hydrogen con- volatilization of the sample under the electron beam (Fig. 4C). tent of this phase would be necessary, the fact that we measured unit cell parameters in very good agree- ment with that reported in [15] for a hydrogen- free composition leads us to propose an Fe C O 4 3 12 DISCUSSION stoichiometry. Upon heating at higher temperature, diffraction peaks of Fe C O increase in intensity This study demonstrates that, at pressures of about 4 3 12 at the expense of the FeO H cubic phase, which 110 GPa and upon laser heating, a chemical reaction 2 x fully disappears above 2300 K (Fig. 1). Neither iron occurs between FeO H and CO yielding a tetra- 2 x 2 oxides (e.g. Fe O ,Fe O ,Fe O ,Fe O )nor hedral carbonate Fe C O . The transformation 3 4 2 3 4 5 13 19 4 3 12 diamond were observed in these experiments. We from the initial goethite FeOOH with increasing Intensity a.u. Downloaded from https://academic.oup.com/nsr/article/5/6/870/4937983 by DeepDyve user on 20 July 2022 874 Natl Sci Rev, 2018, Vol. 5, No. 6 RESEARCH ARTICLE which might have interesting consequences for oxy- (a) gen fugacity at large depth and by consequence at the Fe Earth’s surface. The P-T conditions at which FeO H and 2 x Fe C O have been observed are presented in 4 3 12 Element Atomic (%) Fig. 5 along with mantle geotherms and hypothet- C (K) 12 ical slab geotherms [44,45]. The exact chemistry O (K) 59 Fe (K) 29 and stability of the high-pressure pyrite-structured FeO H are still controversial: Nishi et al. [35] 2 x propose a pyrite-structured oxyhydroxide FeOOH that is stable down to the core–mantle boundary and might undergo dehydration in the D layer, Fe whereas Hu et al. [34] and Liu et al. [46] suggest a pyrite-structured peroxide/hydride FeO H that 2 x Fe would undergo progressive dehydrogenation from about 1800-km depth down to the core–mantle boundary. However, our present study demon- 0 5 10 15 20 strates that the presence of CO , produced for Energy (KeV) example by decarbonation reactions involving (b) (C) (c) silicate phases, could completely alter these inter- pretations. Indeed, the pyrite-structured FeO H 2 x would react with CO to form a high-pressure 2.55 Å 2-42 carbon-bearing phase Fe C O at P-T conditions 4 3 12 0-22 of the lower-mantle geotherm, as well as of those 0-44 of a ‘hot’ slab path (such as Central America slabs 2-20 [47]), and even on geotherms of cold slabs close 4-40 2.96 Å to the core–mantle boundary. Unfortunately, 1.70 Å we currently lack thermodynamic constraints to 1.48 Å evaluate the activity of CO in the mantle and its stability relative to carbonates or C-reduced species. This should be addressed in the future to confirm Figure 4. (a) Semi-quantitative chemical analyses (EDX); (b) electron diffraction of that Fe C O -forming reaction actually takes 4 3 12 Fe O maghemite (white markers) together with an unknown phase (yellow markers); 2 3 place in the mantle. Although the thermodynamic and (c) TEM picture of the sample after analyses. stability of tetrahedral carbonates with respect to reduced carbon phases is still unknown, it appears temperature can be schematized as: that Fe C O tetrahedral carbonate is an excellent 4 3 12 candidate for a stable carbon host in the lower FeOOH => FeO H + 1/2 (1 − x) H (1) 2 x 2 mantle [15,40]. The carbonation reaction (R3) is associated with release of H O. Therefore, the carbonation reaction provides a new mechanism for releasing hydrogen into the deep mantle as H O. It adds up to dehydration reactions that take place 4FeO H + 3CO + 2 1 − x H => Fe C O ( ) 2 x 2 2 4 3 12 at shallower depths in subduction settings and to the progressive dehydrogenation of FeOOH at + 2HO(2) about 1800-km depth [34,46]. Similarly to carbon [12,48], H would be oxidized to produce OH which can be summed up as: 3+ or H O through the reduction of Fe in silicate minerals during mantle upwelling. Such release 4FeOOH + 3CO => Fe C O + 2H O. 2 4 3 12 2 of OH or H O could trigger partial melting, since (3) H O is much more soluble in silicate melts than H 2 2 If the local thermodynamic activity of H is too [49,50]. In hot subducting slabs, the carbonation low for reaction (2) to proceed, other reactions are reaction from oxyhydroxide may take place as possible, such as: shallow as 1200 km [15], before any transformation of α -FeOOH into FeO H . In environments rich in 2 x 4FeO H + 3CO => Fe C O + 2xH O 2 x 2 4 3 12 2 iron oxides such as hematite Fe O (e.g. in banded 2 3 + 1 − x O , (4) iron formation lithology), Fe O may directly react ( ) 2 3 Counts Downloaded from https://academic.oup.com/nsr/article/5/6/870/4937983 by DeepDyve user on 20 July 2022 RESEARCH ARTICLE Boulard et al. 875 MATERIALS AND METHODS Experiments were conducted using symmetric Mao- Bell-type DAC equipped with 300/100-μm beveled culet diamonds and rhenium gasket with 30-μm starting diameter hole and 25-μm starting thickness. Natural sample of crystalline goethite (α-FeOOH) from lateritic soil in Central African Republic was provided by the collection of University Pierre et Marie Curie. FeOOH was loaded in CO together with a ruby ball (see Supplementary Fig. 1 for an XRD characterization before the experiment). The CO gas was loaded using high-pressure gas- loading apparatus at room temperature and 600 bars. FeOOH was isolated from diamonds by CO , preventing reactions with diamonds. In situ angle-dispersive XRD measurements were performed on the high-pressure beamline ID-27 at the European Synchrotron Radiation Facility (ESRF) using a monochromatic incident X-ray beam of 0.3738-A wavelength. Before the experi- ments, the X-ray spot, spectrometer entrance and the heating laser spot were carefully aligned. The sample was first pressurized to its target pressure (107 GPa) before laser heating. Pressure was mea- Figure 5. P-T conditions at which the different phases have been observed experimen- sured using ruby fluorescence before and after laser tally: pyrite-structured FeO from Hu et al. [37], pyrite-structured FeO H from this study 2 2 x heating at room temperature by the use of a blue as well as from Hu et al. [34] from both Fe O +H O and FeOOH in Ar/Ne experiments, 2 3 2 laser [53]. XRD peaks from the Re-gasket collected pyrite-structured FeOOH from Nishi et al. [35] and Fe C O from the present study as 4 3 12 across the diamond culet gave pressures between well as from FeO + CO experiments in Boulard et al. [15]. The mantle geotherms from 101 and 107 GPa. Two YAG lasers with excellent [59] (O(08)) and [60] (A(82)), as well as hypothetical P-T paths for a ‘very cold slab’, ‘cold power stability were aligned on both sides of the slab’ and a ‘hot slab’ [45,61] are also represented for comparison. sample, which produce hot spots larger than 20-μm (FWHM) diameter. Temperatures were obtained with CO [15] without implication of FeOOH 2 by fitting the sample thermal emission spectrum in the chemical reaction. In this latter scenario, from the central 2 × 2 μm of the hotspot to the carbon and hydrogen would be both transported Planck’s function using the wavelength range 600– in the deep mantle without dehydrogenation due 900 nm. Reflective lenses were used for measure- to carbonation (although it is possible that slow ment in order to prevent any chromatic aberration dehydrogenation of FeO H takes place [34]). The 2 x [54]. The monochromatic X-ray beam was focused degree of coupling between the deep carbon and to 3 × 3 micron. This is smaller than the laser heating hydrogen cycles is therefore strongly dependent on spot in order to reduce both the radial and axial tem- the local mineralogical and chemical environment. perature gradients. Typical exposure time was 30 s Because carbonates are also potential oxidized at high pressures and high temperatures. The diffrac- carbon carriers, additional studies on the interac- tion images were integrated with the Fit2d soft- tion between carbonates and FeOOH should be ware [57]. The 1D diffraction patterns were treated carried out in order to provide a comprehensive with the General Structure Analysis System (GSAS) model for the deep-mantle carbon and water cycles. software package [ 58] using the Rietveld or Lebail Nevertheless, the transformation reported here methods to identify the different phases and refine would prevent the production of FeH ,which is x lattice parameters. During heating and XRD acquisi- expected by the reaction of iron alloy from the core tion, temperature was measured continuously. Tem- and hydrous phase at the core–mantle boundary perature uncertainties are estimated to be of about [28,35,51]. This might have favored transfer of 150 K [55]. At high temperatures, thermal pressure carbon to the core rather than of hydrogen during corrections are of the order of +10–15% of the ini- early Earth differentiation and therefore provide a tial pressure [56]. mechanism for high amounts of C in an O-rich core Raman spectra were collected at high pressure [52]. and ambient temperature after transformation of the Downloaded from https://academic.oup.com/nsr/article/5/6/870/4937983 by DeepDyve user on 20 July 2022 876 Natl Sci Rev, 2018, Vol. 5, No. 6 RESEARCH ARTICLE sample. We used a Jobin–Yvon HR-460 spectrom- 6. Javoy M. The major volatile elements of the Earth: their origin, eter with monochromator with 1500 gratings/mm, behavior, and fate. Geophys Res Lett 1997; 24: 177–80. equipped with an Andor CCD camera. Raman 7. Ohtani E. Hydrous minerals and the storage of water in the deep signal was excited using the 514.5-nm wavelength mantle. Chem Geol 2015; 418: 6–15. of an Ar laser, delivering 300 mW focused into a 8. 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CO2-induced destabilization of pyrite-structured FeO2Hx in the lower mantle

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Downloaded from https://academic.oup.com/nsr/article/5/6/870/4937983 by DeepDyve user on 20 July 2022 National Science Review 5: 870–877, 2018 RESEARCH ARTICLE doi: 10.1093/nsr/nwy032 Advance access publication 15 March 2018 GEOSCIENCES CO -induced destabilization of pyrite-structured FeO Hx 2 2 in the lower mantle 1,2,∗ 2 2 3 Eglantine Boulard , Franc¸ois Guyot , Nicolas Menguy , Alexandre Corgne , 4 5 2 Anne-Line Auzende , Jean-Philippe Perrillat and Guillaume Fiquet ABSTRACT Volatiles, such as carbon and water, modulate the Earth’s mantle rheology, partial melting and redox state, thereby playing a crucial role in the Earth’s internal dynamics. We experimentally show the transformation of goethite FeOOH in the presence of CO into a tetrahedral carbonate phase, Fe C O , at conditions 2 4 3 12 above 107 GPa—2300 K. At temperatures below 2300 K, no interactions are evidenced between goethite 1 and CO , and instead a pyrite-structured FeO H is formed as recently reported by Hu et al. (2016; 2017) 2 2 x Synchrotron SOLEIL, and Nishi et al. (2017). The interpretation is that, above a critical temperature, FeO H reacts with CO 91192 St Aubin, 2 x 2 France; Sorbonne and H , yielding Fe C O and H O. Our findings provide strong support for the stability of 2 4 3 12 2 Universite, ´ Museum ´ carbon-oxygen-bearing phases at lower-mantle conditions. In both subducting slabs and lower-mantle National d’Histoire lithologies, the tetrahedral carbonate Fe C O would replace the pyrite-structured FeO H through 4 3 12 2 x Naturelle, UMR CNRS carbonation of these phases. This reaction provides a new mechanism for hydrogen release as H O within 7590, IRD.—IMPMC, the deep lower mantle. Our study shows that the deep carbon and hydrogen cycles may be more complex 4 Place Jussieu, than previously thought, as they strongly depend on the control exerted by local mineralogical and chemical 75005 Paris, France; environments on the CO and H thermodynamic activities. 2 2 Instituto de Ciencias de la Tierra, Keywords: deep carbon cycle, FeOOH, high pressure Universidad Austral de Chile, 5090000 subduction are estimated to account for a flux of Valdivia, Chile; INTRODUCTION ISTerre, Universite´ 3.6 × 10 mol/year of carbon being returned into Water (H O) and carbon dioxide (CO ) both play 2 2 Grenoble Alpes, the deep mantle [3–5]. This quantity accounts for an important role in the history of the Earth, as they CNRS, F-38041 10–30 wt % of the carbon reservoir in the deep man- strongly influence the chemical and physical proper- Grenoble, France and tle [6]. Regarding the water cycle, Van Keken et al. 5 ties of minerals, melts and fluids. Distribution and Laboratoire de 13 [2] suggested that 4–6 × 10 mol/year of H O circulation of H O and CO between the Earth’s Geologie ´ de Lyon, 2 2 are recycled into the mantle through slab subduc- surface and the mantle have dominated the evolu- UMR CNRS 5276, tion. Dehydration of the slab accounts for the loss Universite´ Claude tion of the crust, the oceans and the atmosphere, of two-thirds of this amount of H O, while one- Bernard Lyon 1—ENS controlling several aspects of the Earth’s habitabil- third of the H O remains bounded to the slab (i.e. de Lyon, 69622 ity. It is therefore crucial to determine the stability ≈1.5 × 10 mol/year) reaching depths exceeding Villeurbanne, France and circulation of hydrous and CO -bearing miner- 240 km. Although this amount of H O entering the als in the Earth’s interior. Sedimentary material to- deep mantle may not appear very large, it provides a Corresponding gether with altered mafic and ultramafic rocks that mechanism for having significant amounts of water author: E-mail: eglan- constitute the subducted slabs represents the main tine.boulard@upmc.fr in the deep mantle. In addition, part of the CO and source for recycling of H O and CO as well as 2 2 H O present in the deep mantle may also originate other volatiles at great depth, possibly down to the from primitive mantle reservoirs [7], leading poten- Received 17 core—mantle boundary. The transport of H O and tially to fairly large amounts of these volatiles in the September 2017; CO via subducting slabs down to the transition Revised 28 January deep mantle. zone and to the lower mantle has been the subject 2018; Accepted 7 Because of its very low solubility in deep Earth’s of many studies but is still under debate [1,2]. As March 2018 minerals [8,9], carbon is expected to be present for the carbon cycle, carbonates preserved during The Author(s) 2018. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. All rights reserved. For permissions, plea se e-mail: journals.permissions@oup.com Downloaded from https://academic.oup.com/nsr/article/5/6/870/4937983 by DeepDyve user on 20 July 2022 RESEARCH ARTICLE Boulard et al. 871 as accessory phases in the mantle, either as oxi- transfer of H to the deep Earth. In their recent work, dized phases such as carbonates or CO and car- Hu et al. [34] suggested that a phase of FeO H com- 2 2 x bonated fluids or melts, or as reduced phases such position might indeed deliver H instead of water as diamonds or Fe-C alloys [10]. It is commonly when heated above a threshold temperature—a par- considered that the lower mantle is too reduc- ticularity due to valence changes of oxygen in this 2– 2– ing to host carbonates [11,12]. However, the rel- compound (from 2O to O )[37]. atively high oxygen fugacities prevailing in sub- After H O, CO is the second most important 2 2 ducting slabs might contribute to preserve oxidized volatile compound in the deep Earth. To get a more carbon-bearing phases in the deep mantle [13,14]. complete understanding of the H and C cycles in Moreover, it has recently been demonstrated that the deep Earth, it is necessary to know how deep carbonates at lower-mantle conditions adopt oxi- subducted materials can transport both C and H dized iron-bearing structures based on CO tetrahe- and identify the distinct species involved. In the dra that are associated with reduced carbon phases present study, we shed light on this crucial issue by [13,15–17]. Little is known about the stability of constraining experimentally the interactions of CO these new tetrahedral carbon-bearing phases but with potential carriers of H OorH at great depths. 2 2 their systematic association with reduced carbon We performed high-pressure and high-temperature suggests the idea that the mineralogies of the lower diamond-anvil cell (DAC) experiments to investi- mantle and D region may be more complex than gate the effects of a CO -rich medium on the trans- previously thought. Interestingly, carbonate-bearing formations of FeOOH at pressures and tempera- inclusions have also been reported in diamonds tures of the lower mantle. formed in the lower mantle. This suggests again the presence of carbonates in the deep Earth and a pos- sible coexistence of reduced and oxidized carbon RESULTS species [15,18,19]. Decarbonation reactions of car- bonates involving silicates (SiO and MgSiO )were A natural sample of crystalline α-FeOOH (Sup- 2 3 also reported to take place as shallow as ∼600 km in plementary Fig. 1) was loaded in CO and first depth (20 GPa) [20,21]. Such reactions could pro- pressurized up to 107 GPa in a DAC at ambient duce CO in the lower mantle. Given the current un- temperature. In situ X-Ray diffraction (XRD) pat- certainties on the phase diagram of CO at high pres- 2 terns showed a significant broadening of α-FeOOH sures, CO may be expressed as a solid CO -V phase 2 2 main diffraction reflections characteristic of incip- [21] or rather dissociate as C + O [22]. Carbon- ient amorphization. After laser heating at 2000 K ated fluids yet unknown at such conditions might for a few minutes, several changes in the diffrac- also contribute to CO transfer at large depth in the tion pattern were observed: the α-FeOOH phase mantle. In any case, large thermodynamic activities disappeared and two distinct phases were identified of CO are plausible in the lower mantle. (Fig. 1). The most intense diffraction peaks corre- A significant amount of water can be dissolved in spond to a cubic structure with extinctions of the nominally anhydrous minerals such as olivine, gar- two reflections 001 and 011 in agreement with a net and stishovite [23], as well as in high-pressure Pa-3 space group. Recently, Hu et al. [34] reported silicates such as wadsleyite and ringwoodite [24,25]. the transformation of FeOOH into a new Pa-3 cu- In addition, diverse dense hydrous silicates are sta- bic structure FeO H at similar pressure and tem- 2 x ble in mafic and ultramafic assemblages at upper- perature conditions. This phase is directly related to and lower-mantle conditions, such as phase A, phase the newly discovered pyrite-structured FeO perox- D, phase H and superhydrous phase B [26–30]. Fi- ide but is characterized by a larger unit cell volume nally, δ-AlOOH, a high-pressure form of diaspore [34,37]. FeO H can be interpreted as a solid solu- 2 x (α-AlOOH) with an orthorhombic symmetry very tion between pyrite-structure FeO and FeOOH. In close to that of the CaCl -type polymorph of SiO ,is addition, a pyrite-structured FeOOH oxyhydroxide 2 2 stable throughout the mantle and may be present in (i.e. FeO H with x = 1) was recently observed ex- 2 x suitably aluminous and hydrated lithologies [31,32]. perimentally by Nishi et al. [35]. It presents a struc- The high-pressure polymorph ε-FeOOH that shares ture close to the pyrite-type structure of AlOOH pre- the same structure with δ-AlOOH [33] might also dicted above 170 GPa by Tsuchiya and Tsuchiya store water in the mantle. Iron oxyhydroxides, in- [38]. Here, we measured a unit cell parameter of a cluding FeOOH and its polymorphs, are common at = 4.367 A at 107 GPa, which is significantly larger the surface of the Earth, where they are abundant in than that reported for FeO (a = 4.363 A at 76 GPa) soils and sediments. The incorporation of hydrogen by Hu et al. [37], but smaller than that reported for atoms in newly discovered iron oxyhydroxides with a FeOOH (a = 4.386 A at 109 GPa) in Nishi et al. pyrite structure [34–36] may thus contribute to the [35] (Fig. 2). It is thus probable that FeOOH in our Downloaded from https://academic.oup.com/nsr/article/5/6/870/4937983 by DeepDyve user on 20 July 2022 872 Natl Sci Rev, 2018, Vol. 5, No. 6 RESEARCH ARTICLE 2000 K 2300 K Calculated Observed Difference 4 5 6 7 8 9 10 11 12 13 4 5 6 7 8 9 10 11 12 13 14 2-theta 2-theta Figure 1. XRD patterns collected at 2000 K and at 2300 K and LeBail unit cell refinement of the two phases FeO H (red markers, space group Pa-3, a = 2 x 4.365(1)) and Fe C O (blue markers, space group P2, a = 9.697(2), b = 6.296(2), c = 5.726(1), beta = 92.94(2)). Red circles materialize the expected 4 3 12 peak position of the cubic phase according the XRD collected at lower temperature. (a) Calculated FeOOH Fe Observed a = 4.366828(56) H Difference X = 0.355356(257) Rp = 0.0049 wRp = 0.0030 x y 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 2-theta FeOOH this study (b) FeOOH (Nishi et al. 2017) FeOOH - FeOOH in Ar/Ne (Hu et al. 2017) FeOOH - Fe O + H O (Hu et al. 2017) 23.0 X 2 3 2 FeO - Fe O + O (Hu et al. 2016) 2 2 3 2 FeOOH (Liu et al. 2017) 22.5 Temperature (K) 22.0 21.5 21.0 20.5 60 80 100 120 Pressure (GPa) Figure 2. (a) Rietveld refinement of the XRD pattern collected at 107 GPa and 300 K after laser heating with a FeO Hpyrite- structured (right hand). (b) Unit cell volume of the pyrite-type structure measured experimentally as a function of pressure and temperature for FeO [37], FeO Hx ([31], from both Fe O +H O and FeOOH in Ar experiments and this study) and FeOOH 2 2 2 3 2 [35]. All the data reported here were collected after quenching the temperature. Counts Unit cell volume (Å /f.u.) Counts Counts Downloaded from https://academic.oup.com/nsr/article/5/6/870/4937983 by DeepDyve user on 20 July 2022 RESEARCH ARTICLE Boulard et al. 873 note that Fe in Fe C O is ferric iron Fe(III) as 4 3 12 Diamond anvil in FeOOH. After laser heating, we collected a pro- Sample file of diffraction patterns across the heated spot CO (Supplementary Fig. 2). The FeO H cubic phase 2 x was observed at the edge of the 2300-K heated spot only. Although it is theoretically possible that the reaction is kinetically restricted at lower tempera- CO - VI tures, the fact that a change occurred abruptly at 3600 H O - ice X 1g T around 2300 K (FeO H disappears within a few 2g 2 x seconds) more probably pinpoints to a thermo- dynamic boundary. The pyrite-structured FeO H 3400 2 x would be stable only at relatively low temperatures in the Fe-O-C-H system provided CO thermody- namic activities are high enough. Further studies should verify this point. Raman spectra were also collected at ambi- ent temperature and high pressure in the 300– −1 1300 cm range. As presented in Fig. 3, when col- lected in areas where the CO loading gas was pure, the spectra reveal only one low intensity mode at 700 800 900 1000 1100 1200 1300 −1 ∼1123 cm , which corresponds to the most in- -1 Frequency (cm ) tense mode A of CO -VI [41]. In the solid sam- 1g 2 ple area, an additional mode was detected at ∼930 Figure 3. Raman spectra collected after laser heating in the CO area (in black) and −1 cm assigned to the T mode from the high- 2g sample area (in red). pressure phase of H Oice-X[42,43]. No Raman ac- tive modes associated with Fe C O could be ob- 4 3 12 served, which may be due to high fluorescence back- ground of the diamond from the DAC. study underwent partial dehydrogenation. A similar Transmission electron microscopy (TEM) anal- unit cell volume based on a FeO H formula was re- 2 x yses of a thin section extracted from the recov- ported at the same P-T conditions by Hu et al. [34], for which, using their calibration, they deduced x = ered sample are reported in Fig. 4. Semi-quantitative 0.66. Because this calibration is not only built using chemical analyses (XEDS) showed a homogenous experimental data, but also incorporates theoretical composition with carbon, iron and oxygen with Fe- results, which are known to either overestimate or O atomic proportions consistently with Fe C O 4 3 12 underestimate unit cell volume, uncertainty on the (Fig. 4A). The sample was unstable under elec- exact amount of hydrogen ‘x’ present in FeO H may tron beam, and selective area electron diffraction 2 x be high. To account for this, we simply refer to this (SAED) revealed the presence of two patterns phase as FeO H . (Fig. 4B): γ -Fe O maghemite coexisting with a 2 3 2 x In the XRD pattern, the less intense diffrac- phase characterized by a 6-fold symmetry diffraction tion peaks can be assigned to an already discovered pattern that could not be indexed. It is probable that, carbon-rich phase stable at these P-T conditions: under the electron beam, Fe C O underwent car- 4 3 12 Fe C O [15] (Fig. 1). Among the five structures bon loss to form γ -Fe O together with a second 4 3 12 2 3 proposed in literature [15,17,39,40], we found that phase still containing carbon. Note that the observed only the monoclinic structure reported in [15]al- texture, often observed in cases of irradiation dam- lowed us to assign all of the observed diffraction ages, is in agreement with amorphization and de- peaks. Although ex situ analyses of the hydrogen con- volatilization of the sample under the electron beam (Fig. 4C). tent of this phase would be necessary, the fact that we measured unit cell parameters in very good agree- ment with that reported in [15] for a hydrogen- free composition leads us to propose an Fe C O 4 3 12 DISCUSSION stoichiometry. Upon heating at higher temperature, diffraction peaks of Fe C O increase in intensity This study demonstrates that, at pressures of about 4 3 12 at the expense of the FeO H cubic phase, which 110 GPa and upon laser heating, a chemical reaction 2 x fully disappears above 2300 K (Fig. 1). Neither iron occurs between FeO H and CO yielding a tetra- 2 x 2 oxides (e.g. Fe O ,Fe O ,Fe O ,Fe O )nor hedral carbonate Fe C O . The transformation 3 4 2 3 4 5 13 19 4 3 12 diamond were observed in these experiments. We from the initial goethite FeOOH with increasing Intensity a.u. Downloaded from https://academic.oup.com/nsr/article/5/6/870/4937983 by DeepDyve user on 20 July 2022 874 Natl Sci Rev, 2018, Vol. 5, No. 6 RESEARCH ARTICLE which might have interesting consequences for oxy- (a) gen fugacity at large depth and by consequence at the Fe Earth’s surface. The P-T conditions at which FeO H and 2 x Fe C O have been observed are presented in 4 3 12 Element Atomic (%) Fig. 5 along with mantle geotherms and hypothet- C (K) 12 ical slab geotherms [44,45]. The exact chemistry O (K) 59 Fe (K) 29 and stability of the high-pressure pyrite-structured FeO H are still controversial: Nishi et al. [35] 2 x propose a pyrite-structured oxyhydroxide FeOOH that is stable down to the core–mantle boundary and might undergo dehydration in the D layer, Fe whereas Hu et al. [34] and Liu et al. [46] suggest a pyrite-structured peroxide/hydride FeO H that 2 x Fe would undergo progressive dehydrogenation from about 1800-km depth down to the core–mantle boundary. However, our present study demon- 0 5 10 15 20 strates that the presence of CO , produced for Energy (KeV) example by decarbonation reactions involving (b) (C) (c) silicate phases, could completely alter these inter- pretations. Indeed, the pyrite-structured FeO H 2 x would react with CO to form a high-pressure 2.55 Å 2-42 carbon-bearing phase Fe C O at P-T conditions 4 3 12 0-22 of the lower-mantle geotherm, as well as of those 0-44 of a ‘hot’ slab path (such as Central America slabs 2-20 [47]), and even on geotherms of cold slabs close 4-40 2.96 Å to the core–mantle boundary. Unfortunately, 1.70 Å we currently lack thermodynamic constraints to 1.48 Å evaluate the activity of CO in the mantle and its stability relative to carbonates or C-reduced species. This should be addressed in the future to confirm Figure 4. (a) Semi-quantitative chemical analyses (EDX); (b) electron diffraction of that Fe C O -forming reaction actually takes 4 3 12 Fe O maghemite (white markers) together with an unknown phase (yellow markers); 2 3 place in the mantle. Although the thermodynamic and (c) TEM picture of the sample after analyses. stability of tetrahedral carbonates with respect to reduced carbon phases is still unknown, it appears temperature can be schematized as: that Fe C O tetrahedral carbonate is an excellent 4 3 12 candidate for a stable carbon host in the lower FeOOH => FeO H + 1/2 (1 − x) H (1) 2 x 2 mantle [15,40]. The carbonation reaction (R3) is associated with release of H O. Therefore, the carbonation reaction provides a new mechanism for releasing hydrogen into the deep mantle as H O. It adds up to dehydration reactions that take place 4FeO H + 3CO + 2 1 − x H => Fe C O ( ) 2 x 2 2 4 3 12 at shallower depths in subduction settings and to the progressive dehydrogenation of FeOOH at + 2HO(2) about 1800-km depth [34,46]. Similarly to carbon [12,48], H would be oxidized to produce OH which can be summed up as: 3+ or H O through the reduction of Fe in silicate minerals during mantle upwelling. Such release 4FeOOH + 3CO => Fe C O + 2H O. 2 4 3 12 2 of OH or H O could trigger partial melting, since (3) H O is much more soluble in silicate melts than H 2 2 If the local thermodynamic activity of H is too [49,50]. In hot subducting slabs, the carbonation low for reaction (2) to proceed, other reactions are reaction from oxyhydroxide may take place as possible, such as: shallow as 1200 km [15], before any transformation of α -FeOOH into FeO H . In environments rich in 2 x 4FeO H + 3CO => Fe C O + 2xH O 2 x 2 4 3 12 2 iron oxides such as hematite Fe O (e.g. in banded 2 3 + 1 − x O , (4) iron formation lithology), Fe O may directly react ( ) 2 3 Counts Downloaded from https://academic.oup.com/nsr/article/5/6/870/4937983 by DeepDyve user on 20 July 2022 RESEARCH ARTICLE Boulard et al. 875 MATERIALS AND METHODS Experiments were conducted using symmetric Mao- Bell-type DAC equipped with 300/100-μm beveled culet diamonds and rhenium gasket with 30-μm starting diameter hole and 25-μm starting thickness. Natural sample of crystalline goethite (α-FeOOH) from lateritic soil in Central African Republic was provided by the collection of University Pierre et Marie Curie. FeOOH was loaded in CO together with a ruby ball (see Supplementary Fig. 1 for an XRD characterization before the experiment). The CO gas was loaded using high-pressure gas- loading apparatus at room temperature and 600 bars. FeOOH was isolated from diamonds by CO , preventing reactions with diamonds. In situ angle-dispersive XRD measurements were performed on the high-pressure beamline ID-27 at the European Synchrotron Radiation Facility (ESRF) using a monochromatic incident X-ray beam of 0.3738-A wavelength. Before the experi- ments, the X-ray spot, spectrometer entrance and the heating laser spot were carefully aligned. The sample was first pressurized to its target pressure (107 GPa) before laser heating. Pressure was mea- Figure 5. P-T conditions at which the different phases have been observed experimen- sured using ruby fluorescence before and after laser tally: pyrite-structured FeO from Hu et al. [37], pyrite-structured FeO H from this study 2 2 x heating at room temperature by the use of a blue as well as from Hu et al. [34] from both Fe O +H O and FeOOH in Ar/Ne experiments, 2 3 2 laser [53]. XRD peaks from the Re-gasket collected pyrite-structured FeOOH from Nishi et al. [35] and Fe C O from the present study as 4 3 12 across the diamond culet gave pressures between well as from FeO + CO experiments in Boulard et al. [15]. The mantle geotherms from 101 and 107 GPa. Two YAG lasers with excellent [59] (O(08)) and [60] (A(82)), as well as hypothetical P-T paths for a ‘very cold slab’, ‘cold power stability were aligned on both sides of the slab’ and a ‘hot slab’ [45,61] are also represented for comparison. sample, which produce hot spots larger than 20-μm (FWHM) diameter. Temperatures were obtained with CO [15] without implication of FeOOH 2 by fitting the sample thermal emission spectrum in the chemical reaction. In this latter scenario, from the central 2 × 2 μm of the hotspot to the carbon and hydrogen would be both transported Planck’s function using the wavelength range 600– in the deep mantle without dehydrogenation due 900 nm. Reflective lenses were used for measure- to carbonation (although it is possible that slow ment in order to prevent any chromatic aberration dehydrogenation of FeO H takes place [34]). The 2 x [54]. The monochromatic X-ray beam was focused degree of coupling between the deep carbon and to 3 × 3 micron. This is smaller than the laser heating hydrogen cycles is therefore strongly dependent on spot in order to reduce both the radial and axial tem- the local mineralogical and chemical environment. perature gradients. Typical exposure time was 30 s Because carbonates are also potential oxidized at high pressures and high temperatures. The diffrac- carbon carriers, additional studies on the interac- tion images were integrated with the Fit2d soft- tion between carbonates and FeOOH should be ware [57]. The 1D diffraction patterns were treated carried out in order to provide a comprehensive with the General Structure Analysis System (GSAS) model for the deep-mantle carbon and water cycles. software package [ 58] using the Rietveld or Lebail Nevertheless, the transformation reported here methods to identify the different phases and refine would prevent the production of FeH ,which is x lattice parameters. During heating and XRD acquisi- expected by the reaction of iron alloy from the core tion, temperature was measured continuously. Tem- and hydrous phase at the core–mantle boundary perature uncertainties are estimated to be of about [28,35,51]. This might have favored transfer of 150 K [55]. At high temperatures, thermal pressure carbon to the core rather than of hydrogen during corrections are of the order of +10–15% of the ini- early Earth differentiation and therefore provide a tial pressure [56]. mechanism for high amounts of C in an O-rich core Raman spectra were collected at high pressure [52]. and ambient temperature after transformation of the Downloaded from https://academic.oup.com/nsr/article/5/6/870/4937983 by DeepDyve user on 20 July 2022 876 Natl Sci Rev, 2018, Vol. 5, No. 6 RESEARCH ARTICLE sample. We used a Jobin–Yvon HR-460 spectrom- 6. Javoy M. The major volatile elements of the Earth: their origin, eter with monochromator with 1500 gratings/mm, behavior, and fate. Geophys Res Lett 1997; 24: 177–80. equipped with an Andor CCD camera. Raman 7. Ohtani E. Hydrous minerals and the storage of water in the deep signal was excited using the 514.5-nm wavelength mantle. Chem Geol 2015; 418: 6–15. of an Ar laser, delivering 300 mW focused into a 8. 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