Article Decarbonation Reactions Involving Ankerite and under upper Mantle P,T-Parameters: Experimental Modeling

Yuliya V. Bataleva 1,* , Aleksei N. Kruk 1, Ivan D. Novoselov 1,2, Olga V. Furman 1,2 and Yuri N. Palyanov 1,*

1 Sobolev Institute of Geology and , Siberian Branch of Russian Academy of Sciences, Koptyug ave 3, 630090 Novosibirsk, Russia; [email protected] (A.N.K.); [email protected] (I.D.N.); [email protected] (O.V.F.) 2 Department of Geology and Geophysics, Novosibirsk State University, Pirogova str 2, 630090 Novosibirsk, Russia * Correspondence: [email protected] (Y.V.B.); [email protected] (Y.N.P.); Tel.: +7-383-330-75-01 (Y.N.P.)

 Received: 13 July 2020; Accepted: 11 August 2020; Published: 13 August 2020 

Abstract: An experimental study aimed at the modeling of dolomite- and ankerite-involving decarbonation reactions, resulting in the CO2 fluid release and crystallization of Ca, Mg, Fe garnets, was carried out at a wide range of pressures and temperatures of the upper mantle. Experiments were performed using a multi-anvil high-pressure apparatus of a “split-sphere” type, in CaMg(CO3)2-Al2O3-SiO2 and Ca(Mg,Fe)(CO3)2-Al2O3-SiO2 systems (pressures of 3.0, 6.3 and 7.5 GPa, temperature range of 950–1550 ◦C, hematite buffered high-pressure cell). It was experimentally shown that decarbonation in the dolomite-bearing system occurred at 1100 20 C (3.0 GPa), ± ◦ 1320 20 C (6.3 GPa), and 1450 20 C (7.5 GPa). As demonstrated by mass spectrometry, ± ◦ ± ◦ the fluid composition was pure CO2. Composition of synthesized garnet was Prp83Grs17, with main 1 Raman spectroscopic modes at 368–369, 559–562, and 912–920 cm− . Decarbonation reactions in the ankerite-bearing system were realized at 1000 20 C (3.0 GPa), 1250 20 C (6.3 GPa), ± ◦ ± ◦ and 1400 20 ◦C (7.5 GPa). As a result, the garnet of Grs25Alm40Prp35 composition with main Raman ± 1 peaks at 349–350, 552, and 906–907 cm− was crystallized. It has been experimentally shown that, in the Earth’s mantle, dolomite and ankerite enter decarbonation reactions to form Ca, Mg, Fe garnet + CO2 assemblage at temperatures ~175–500 ◦C lower than CaCO3 does at constant pressures.

Keywords: decarbonation reaction; dolomite; ankerite; garnet; CO2 fluid; experimental modeling; high-pressure experiment

1. Introduction Reconstruction of the global carbon cycle involves studies on natural carbonates and carbides stability, C-O-H fluid generation conditions, modeling of mantle metasomatic processes, and natural diamond formation, as well as the formation and evolution of carbonated rocks. One of the most important planetary-scale settings for studying and understanding the global carbon cycle is subduction. Under subduction conditions, carbon is transported to the Earth’s mantle as carbonates and organic material in marine sediments, altered oceanic crust, and basalts [1]. In the course of subduction, carbonates can undergo phase transitions and structure changes (Figure1a–c) [ 2,3], partial melting processes (Figure2)[ 4–6], decomposition (breakdown) [5–7], dissolution [8,9], or participate various carbonate-consuming reactions. These reactions include diamond-forming redox interactions between carbonates and highly reduced phases [10–12] as well as decarbonation reactions, which lead to the

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formation of a CO2 fluid and newly formed silicates/oxides [13–21]. These decarbonation reactions, Minerals 2020, 10, x FOR PEER REVIEW 2 of 17 occurring under mantle P,T-parameters, were experimentally studied in a number of works (Figure3):

reactions, which lead to the formation of a CO2 fluid and newly formed silicates/oxides. These MgCO + MgSiO Mg SiO + CO (1) decarbonation reactions, occurring under3 mantle3 P,T-parameters,↔ 2 4 were2 experimentally studied in a number of works (Figure 3): MgCO + SiO MgSiO + CO (2) 3 2 ↔ 3 2 MgCO3 + MgSiO3 ↔ Mg2SiO4 + CO2 [13,14] (1) CaMg(CO ) + 2SiO CaMgSi O + 2CO (3) 3 2 2 ↔ 2 6 2 MgСO3 + SiO2 ↔ MgSiO3 + СO2 [15,16] (2) (CaMg(CO3)2 + 4MgSiO3 2Mg2SiO4 + CaMgSi2O6 + 2CO2 (4) CaMg(CO3)2 + 2SiO↔2 ↔ CaМgSi2O6 + 2СO2 [17] (3) 3MgCO3 + Al2SiO5 + 2SiO2 Mg3Al2Si3O12 + 3CO2 (5) (CaMg(CO3)2 + 4MgSiO3 ↔ 2Mg2SiO↔4 + CaMgSi2O6 + 2CO2 [18,19] (4) 3MgCO + Al O + 3SiO Mg Al Si O + 3CO (6) 3MgCO33 + Al22SiO35 + 2SiO22 ↔↔ Mg3Al3 2Si23O123 + 123CO2 [20] 2 (5)

However, most existing3MgCO models3 + Al2O [322 + 3SiO–25]2 predict↔ Mg3Al that2Si3O12 decarbonation, + 3CO2 [16,21] involving Mg (6) and Ca carbonates, occurs at P,T-parameters higher that those supposed for most subduction settings (Figure3). However, most existing models [22–25] predict that decarbonation, involving Mg and Ca Accordingcarbonates, to these occurs models, at P,T-parameters most subducted higher carbon that in those the form supposed of Mg,Ca for most carbonates subduction is transported settings in the downward(Figure 3). According moving slab to these to great models, depths. most The subducted presence carbon of carbonates in the form in theof Mg,Ca mantle carbonates is confirmed is by bothtransported theoretical andin the experimental downward moving works slab [6,13 to– great15,17 depths.,19,20], The as well presence as numerous of carbonates findings in the of mantle carbonate inclusionsis confirmed in diamonds by both [26 theoretical–34]. Currently, and experimental in most works works on [6,13–15,17,19,20], experimental modeling as well as of numerous decarbonation reactionsfindings involving of carbonate natural inclusions carbonates in diamonds (1)–(4), [26–34]. the parameters Currently, ofin reactionmost works with on theexperimental formation of olivine,modeling ortho- of and decarbonation clinopyroxene reactions have involving been established. natural carbonates Moreover, (1)–(4), the positionthe parameters of the of decarbonation reaction with the formation of olivine, ortho- and clinopyroxene have been established. Moreover, the curves with the formation of garnet was experimentally determined only in the MgCO3-Al2O3-SiO2 position of the decarbonation curves with the formation of garnet was experimentally determined system [20,35] (Figure4). only in the MgCO3-Al2O3-SiO2 system [20,35] (Figure 4).

Figure 1. Experimentally determined P,T-diagrams of FeCO3 (a) (modified from [36–40]), CaCO3 (b) Figure 1. Experimentally determined P,T-diagrams of FeCO3 (a) (modified from [36–40]), CaCO3 (modified from [41,42]) and MgCO3 (c) (1—melting curve of MgCO3 [43]; 2—melting curve of (b) (modified from [41,42]) and MgCO3 (c) (1—melting curve of MgCO3 [43]; 2—melting curve of MgCO3 [44]; 3—decomposition of liquid MgCO3 into MgO and CO2 [43]; 4—calculated MgCO3 [44]; 3—decomposition of liquid MgCO3 into MgO and CO2 [43]; 4—calculated decomposition decomposition of MgCO3 [45]. HS—high-spin, LS—low-spin, HP—high-pressure, of MgCO [45]. HS—high-spin, LS—low-spin, HP—high-pressure, I,II,III,IV,V—CaCO phases. I,II,III,IV,V—CaCO3 3 phases. 3

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FigureFigure 2. 2.Experimentally Experimentally determined determined parameters parameters of melting of melting and decomposition and decomposition of Mg, Ca, of Fe Mg, carbonates: Ca, Fe 1—calcite–aragonitecarbonates: 1—calcite–aragonite transition [41 transition]; 2—magnesite [41]; 2—magnesite+ aragonite = +dolomite aragonite [7 ];= 3—sideritedolomite [7];+ aragonite 3—=

ankerite+ aragonite [7]; 4—aragonite–CaCO = ankerite [7]; 4—aragonite–CaCO3 (R3_m) transition3 [ (R3_m)46]; 5—siderite transition melting [46]; and 5—siderite decomposition melting [38 , and39]; 6—CaCOdecomposition3 melting [38,39]; [47]; 6—CaCO 7, 8—magnesite3 melting melting [47]; 7, and 8—magnesite decomposition melting [6,43]. and Sd—siderite, decomposition LSd—liquid [6,43]. FeCOSd—siderite,3, Mt—magnetite, LSd—liquid Gr—graphite, FeCO3, Mt—magnetite, Ms—magnesite, Gr—graphite, Pc—periclase, L Ms—magnesite,Ms—liquid MgCO Pс—periclase,3, Cc—calcite, Arg—aragonite,LMs—liquid MgCO Ank—ankerite,3, Cc—calcite, LCc—liquid Arg—aragonite, CaCO Ank—ankerite,3. LCc—liquid CaCO3.

FigureFigure 3.3. P,TP,T diagramdiagram withwith previouslypreviously constrainedconstrained decarbonationdecarbonation curves,curves, resultingresulting inin thethe formationformation

ofof CO2 fluidfluid and and (1) (1) diopside diopside [17], [17], (2) (2) orthopyroxene orthopyroxene [15,19], [15,19 ],(3) (3) pyrope pyrope [20], [20 (4)], (4)olivine olivine [13,14], [13,14 (5)], (5)periclase periclase [6]. [6 The]. The graphite-diamond graphite-diamond direct direct transition transition line line (6) (6) is is given given according according to to [48]. [48]. Ms—magnesite,Ms—magnesite, Dol—dolomite, Di—diopside, Pc—periclase, Opx—orthopyroxene (enstatite), Ol—olivineOl—olivine (forsterite),(forsterite), Coe—coesite,Coe—coesite, Ky—kyanite,Ky—kyanite, Prp—pyrope.Prp—pyrope.

Thus, it seems relevant to perform experimental modeling of decarbonation reactions involving natural Mg,Ca carbonates (dolomite, ankerite), and associated with the formation of garnets and CO2 fluid, and to determine the position of the corresponding decarbonation curves at a wide range of pressures and temperatures of the upper mantle.

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Figure 4.FigureP,T diagram 4. P,T diagram with previously with previously experimentally experimentally constrained constrained and and calculated calculated decarbonation decarbonation curves, curves, resulting in the formation of CO2 fluid and garnet solid solutions [20]; a—activity of resulting in the formation of CO fluid and garnet solid solutions [20]; a—activity of component, component, Ms—magnesite, Ky—kyanite,2 Co—coesite, Prp—pyrope, Grs—grossular. Ms—magnesite, Ky—kyanite, Co—coesite, Prp—pyrope, Grs—grossular. 2. Materials and Methods Thus, it seems relevant to perform experimental modeling of decarbonation reactions involving Experimental modeling of decarbonation reactions involving ankerite and dolomite was natural Mg,Ca carbonates (dolomite, ankerite), and associated with the formation of garnets and CO2 performed in the Ca(Mg,Fe)(СО3)2-SiO2-Al2O3 and CaMg(СО3)2-SiO2-Al2O3 systems using a fluid, andmulti-anvil to determine high-pressure the position split-sphere of theapparatus corresponding (BARS) [49]. decarbonation The experiments curves were carried at a wide out at range of pressurespressures and temperatures of 3.0, 6.3, and of 7.5 the GPa, upper in the mantle. temperature range of 950–1550 °C and for durations from 15 min to 60 h. Methodological features of the assembly, the design of the high-pressure cell, as well 2. Materials and Methods as data onFigure the pressure 4. P,T diagram and withtemperature previously calibration experimentally have constrained been published and calculated previously decarbonation [21,50–52]. As initial reagents, we used natural ankerite (Ca(Mg,Fe)(СО3)2 (Mésage Mine, Saint-Pierre-de-Mésage, Experimentalcurves, modeling resulting in of the decarbonation formation of CO reactions2 fluid and involving garnet solid ankerite solutions [20]; and a—activity dolomite of was performed France) andcomponent, dolomite Ms—magnesite, (CaMg(СО Ky—kyanite,3)2 (Satka, Urals, Co—coesite, Russian Prp—pyrope, Federation), Grs—grossular. as well as synthetic SiO2 and in theAl Ca(Mg,Fe)(CO2O3 with a purity3) 2of-SiO 99.99%.2-Al Raman2O3 and spectra CaMg(CO of these natural3)2-SiO carbonates2-Al2O3 aresystems shown in using Figure a 5. multi-anvilThe high-pressuremolar2. Materials proportions split-sphere and Methods of theapparatus starting (BARS)materials [49 ensure]. The garnet experiments and CO2 fluid were formation carried out when at the pressures of 3.0, 6.3,interaction andExperimental 7.5 is GPa,complete in modeling the(Table temperature 1). of The decarbonation starting range reagents reactions of 950–1550 were involving ground◦C andankerite thoroughly for and durations dolomite homogenized. from was 15 min to 60 h. MethodologicalTakingperformed into account in features the previous Ca(Mg,Fe)(СО of theexperience assembly,3)2-SiO of2-Al the2O the3 studies and design CaMg(СО in carbonate-oxide of the3)2-SiO high-pressure2-Al2O 3systems systems cell,under using as high well a P as data on the pressureand multi-anvilT [53–55], and Pthigh-pressure temperature was chosen split-sphere as calibrationthe capsule apparatus material. have (BARS) been [49]. published The experiments previously were carried [21,50 out–52 at]. As initial pressures of 3.0, 6.3, and 7.5 GPa, in the temperature range of 950–1550 °C and for durations from reagents, we15 usedmin to natural60 h. Methodological ankerite (Ca(Mg,Fe)(CO features of the assembly,3)2 (M theésage design Mine, of the Saint-Pierre-de-Mhigh-pressure cell, as wellésage, France) and dolomiteas data (CaMg(CO on the pressure3)2 (Satka, and temperature Urals, Russian calibration Federation), have been published as well previously as synthetic [21,50–52]. SiO As2 and Al2O3 with a purityinitial of reagents, 99.99%. we Raman used natural spectra ankerite of these (Ca(Mg,Fe)(СО natural carbonates3)2 (Mésage Mine, are Saint-Pierre-de-Mésage, shown in Figure5. The molar France) and dolomite (CaMg(СО3)2 (Satka, Urals, Russian Federation), as well as synthetic SiO2 and proportions of the starting materials ensure garnet and CO fluid formation when the interaction is Al2O3 with a purity of 99.99%. Raman spectra of these natural 2carbonates are shown in Figure 5. The complete (Tablemolar 1 proportions). The starting of the startingreagents materials were groundensure garnet and and thoroughly CO2 fluid homogenized. formation when the Taking into account previousinteraction experience is complete (Table of the 1). studies The starting incarbonate-oxide reagents were ground systems and thoroughly under homogenized. high P and T [53–55], Pt was chosenTaking as into the account capsule previous material. experience of the studies in carbonate-oxide systems under high P and T [53–55], Pt was chosen as the capsule material.

Figure 5. Raman spectra of starting materials—natural dolomite (a) and ankerite (b).

Figure 5. FigureRaman 5. Raman spectra spectra of starting of starting materials—natural materials—natural dolomite dolomite (a) and ( aankerite) and ankerite(b). (b).

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Table 1. Weights of initial reagents.

Weight, mg System P, GPa Carbonate SiO2 Al2O3 3.0 5.0 3.2 1.8 CaMg(CO3)2-SiO2-Al2O3 6.3 5.0 3.2 1.8 Minerals 2020, 10, 715 7.5 4.0 2.6 1.4 5 of 17 3.0 5.2 3.1 1.8 Ca(Mg,Fe)(CO3)2-SiO2-Al2O3 6.3 5.2 3.1 1.8 Table 1. Weights7.5 of initial reagents. 4.1 2.5 1.4

Weight, mg The volume of reactionSystem capsules was selectedP, GPa to ensure that all necessary analytical methods Carbonate SiO Al O could be employed, taking into account the size of the high-pressure cell.2 The internal2 3 diameter of the Pt capsules for experiments at 3.0 and 6.33.0 GPa was 1.5 5.0mm at a length 3.2 of 6 mm, 1.8 and at 7.5 GPa, CaMg(CO ) -SiO -Al O 6.3 5.0 3.2 1.8 1.5 mm at a length of 4 mm.3 2 2 2 3 7.5 4.0 2.6 1.4 There is a problem of hydrogen diffusion into the reaction volume through the walls of capsules in the high-temperature high-pressure3.0 experimental 5.2 studies 3.1[56,57]. Hydrogen 1.8 diffusion Ca(Mg,Fe)(CO3)2-SiO2-Al2O3 6.3 5.2 3.1 1.8 can significantly decrease the oxygen fugacity7.5 in capsules, 4.1 change the 2.5 composition 1.4 of the fluid in the reaction volume, and lead to a shift in the decarbonation curves in the P,T-field (Figure 6а). To prevent the effect of hydrogen diffusion on the course of the experiment, in this study, we used a The volume of reaction capsules was selected to ensure that all necessary analytical methods specially designed high-pressure cell with a hematite buffer container [51]. The effective time of this could be employed, taking into account the size of the high-pressure cell. The internal diameter of buffer at temperatures below 1200 °C is more than 150 h, and at 1500 °C, it is around 5 h. After the the Pt capsules for experiments at 3.0 and 6.3 GPa was 1.5 mm at a length of 6 mm, and at 7.5 GPa, experiments, control studies of the chemical composition of the hematite buffer container were carried1.5 mm out. at a lengthIn all cases, of 4 mm. the material of the buffer container was analyzed; it was represented by There is a problem of hydrogen diffusion into the reaction volume through the walls of capsules in hematite and magnetite, which indicates the effectiveness of the hematite container throughout the entirethe high-temperature experiment. The high-pressure duration of the experimental experiments studies for each [56, 57temperature]. Hydrogen was diff selectedusion can based significantly on the decrease the oxygen fugacity in capsules, change the composition of the fluid in the reaction volume, time of effective action of the hematite container. The experimental temperatures were selected accordingand lead to the a shift calculations in the decarbonation (Figure 6b) [58,59]. curves in the P,T-field (Figure6a). To prevent the e ffect of hydrogenThe phase diff andusion chemical on the course compositions of the experiment, of the final in this samples study, were we used determined a specially by designed energy dispersivehigh-pressure spectroscopy cell with a (Tescan hematite MIRA3 buffer containerLMU scanning [51]. The electron effective microscope, time of this TESCAN, buffer at temperaturesBrno, Czech Republic)below 1200 and◦C is microprobe more than 150 analysis h, and (Camebax-micro at 1500 ◦C, it is around analyzer, 5 h. After CAMECA, the experiments, Gennevilliers, control France). studies of the chemical composition of the hematite buffer container were carried out. In all cases, the material Standards used for the analyses of garnet were pyrope (for SiO2 and Al2O3 contents), ferrous spessartineof the buffer (for container FeO and was MnO), analyzed; and itdiopside was represented (for MgO by and hematite CaO). andSilicate, magnetite, carbonate, which and indicates oxide mineralthe effectiveness phases were of the analyzed hematite at container an accelerating throughout voltage the of entire 20 kV, experiment. a probe current The duration of 20 nA, of the a countingexperiments time for of each 10 s temperature on each analytical was selected line, andbased an on electron the time beam of eff diameterective action of 2–4 of the μm. hematite Phase relationshipscontainer. The in experimental the samples temperatureswere studied wereby means selected of scanning according electron to the calculations microscopy (Figure (SEM).6b) [ 58,59].

FigureFigure 6. 6. T-ƒOT-ƒ2 O diagram2 diagram (a) with (a) with lines lines of buffer of bu equilibriaffer equilibria (according (according to [60–62]), to [60 as–62 well]), as as well the decarbonationas the decarbonation reaction [53] reaction and P,T [53 ] diagram and P,T (b) diagram with the (b theoretical) with the positions theoretical of decarbonation positions of reactionsdecarbonation involving reactions ankerite involving and ankerite dolomite, and as dolomite, calculated as calculated in this inpaper this paper[58,59]. [58 MH,59]. (magnetite-hematite),MH (magnetite-hematite), FMQ (fayalite-magnetite-quartz), FMQ (fayalite-magnetite-quartz), IW (-wüstite), IW (iron-wüstite), CCO—buffer CCO—bu equilibria;ffer Ms—magnesite,equilibria; Ms—magnesite, Coe—coesite, Coe—coesite, Crn—corundum, Crn—corundum, Prp—pyrope, Prp—pyrope, Mgt—magnetite, Mgt—magnetite, Ru—rutile, Ru—rutile, Ilm—ilmenite,Ilm—ilmenite, Dm—diamond. Dm—diamond.

The phase and chemical compositions of the final samples were determined by energy dispersive spectroscopy (Tescan MIRA3 LMU scanning electron microscope, TESCAN, Brno, Czech Republic) and microprobe analysis (Camebax-micro analyzer, CAMECA, Gennevilliers, France). Standards used for the analyses of garnet were pyrope (for SiO2 and Al2O3 contents), ferrous (for FeO and MnO), and diopside (for MgO and CaO). Silicate, carbonate, and oxide phases were analyzed Minerals 2020, 10, 715 6 of 17 at an accelerating voltage of 20 kV, a probe current of 20 nA, a counting time of 10 s on each analytical line, and an electron beam diameter of 2–4 µm. Phase relationships in the samples were studied by means of scanning electron microscopy (SEM). At the preliminary stage of the experiments, we calculated the theoretical positions of the decarbonation curves (Figure6b) according to data given in [ 20] on carbonation curves for grossular, pyrope, and “diluted” grossular. This technique implied ideal mixing for the garnet solid solution. The lines obtained as a result would be true for the complete decarbonation (all ankerite or dolomite is decomposed). The structural features of the obtained garnet and carbonate were studied by Raman spectroscopy (Jobin Yvon LabRAM HR800 spectrometer equipped with an Olympus BX41 stereo microscope, Horiba Jobin Yvon S.A.S., Lonjumeau, France). An He-Cd laser with a wavelength of 325 nm (Laser Quantum, Stockport, UK) was used as an excitation source. To monitor the effectiveness of the hematite buffer, the composition of the fluid phase was identified by mass spectrometry. For this, the platinum capsule after the experiment was placed in a vacuum device connected to a sample injection system in a Delta V Advantage mass spectrometer (Thermo Fisher Scientific, Bremen, Germany) and equipped with a special mechanism for puncturing samples. After preliminary pumping out of the device with the sample to a pressure of 2.7 10 2 mbar, guaranteeing the absence of atmospheric gases × − in the device, the capsule was punctured and the gas released at room temperature was let into the mass spectrometer analyzer. Analytical studies were performed at the Sobolev Institute of Geology and Mineralogy, SB RAS, and at the Analytical Center for multi-elemental and isotope research, SB RAS.

3. Results The experimental parameters and the results obtained are presented in Table2. Taking into account the previously developed approach [20], the main criterion for the decarbonation reaction to occur is the appearance of garnet and CO2 fluid in the reaction volume. The formation of these phases was accompanied by a decrease in the amount of ankerite or dolomite and oxides in the reaction volume (Figure7a,b). The partial preservation of carbonate and oxides in the samples is a consequence of the incomplete passage of the decarbonation reaction during the effective time of the hematite buffer.

Table 2. Experimental parameters and results.

Run N System P, GPa T, ◦C t, h Final Mineral Phases 1744-A 3.0 950 60 Ky, Ank, Arg, Coe, Crn 1738-A 3.0 1050 60 Grt, Ky, Ank, Coe, Crn 2117-A 6.3 1100 40 Ky, Ank, Coe, Crn 2119-A 6.3 1200 40 Ky, Ank, Coe, Crn 2115-A 6.3 1300 20 Grt, Ky, Ank, Coe, Crn Ca(Mg,Fe)(CO3)2-SiO2-Al2O3 2113-A 6.3 1400 10 Grt, Ky, Ank, Coe, Crn 2137-A 7.5 1150 60 Ky, Ank, Coe, Crn 2135-A 7.5 1250 40 Ky, Ank, Coe, Crn 2141-A 7.5 1350 10 Ky, Ank, Coe, Crn 2145-A 7.5 1450 10 Grt, Ky, Ank, Coe, Crn 1743-D 3.0 1050 60 Ky, Dol, Coe, Crn 2122-D 3.0 1150 60 Grt, Ky, Dol, Coe, Crn 2160-D 6.3 1200 40 Ky, Dol, Coe, Crn 2155-D 6.3 1250 40 Ky, Dol, Coe, Crn CaMg(CO3)2-SiO2-Al2O3 2128-D 6.3 1350 20 Grt, Ky, Dol, Coe, Crn 2120-D 6.3 1450 10 Grt, Ky, Dol, Coe 2161-D 7.5 1450 10 Ky, Dol, Coe, Crn 2157-D 7.5 1550 15 min Grt, Coe, Crn Grt—garnet, Ank—ankerite, Arg—aragonite, Dol—dolomite, Coe—coesite, Crn—corundum, Ky—kyanite.

At temperatures above the onset of the decarbonation reaction (Figures7c–f and8a), grossular–pyrope–almandine and kyanite were formed in the samples. The compositions of the obtained mineral phases are shown in Table3. The resulting CO 2 fluid segregated and formed cavities Minerals 2020, 10, 715 7 of 17 Minerals 2020, 10, x FOR PEER REVIEW 7 of 17 in theobtained entire volume mineral of phases the samples; are shown the sizein Table and shape3. The of resulting the cavities CO2 depends fluid segregated on the temperature and formed and, accordingly,cavitiesMinerals 2020 thein the, depth10, x entire FOR of PEER thevolume REVIEW decarbonation of the samples; reactions. the size At and the initialshape ofstage the of cavities the system depends decarbonation, 7on of the 17 temperature and, accordingly, the depth of the decarbonation reactions. At the initial stage of the the fluid was in the interstitial space, and with increasing temperature, it formed rounded cavities with systemobtained decarbonation, mineral phases the are fluid shown was in in the Table interstitial 3. The resulting space, and CO with2 fluid increasing segregated temperature, and formed it sizes from 10 to 150 µm. formedcavities rounded in the entirecavities volume with sizes of the from samples; 10 to 150 the μm. size and shape of the cavities depends on the temperature and, accordingly, the depth of the decarbonation reactions. At the initial stage of the system decarbonation, the fluid was in the interstitial space, and with increasing temperature, it formed rounded cavities with sizes from 10 to 150 μm.

FigureFigure 7. SEM 7. SEM micrographs micrographs (BSE (BSE regime) regime) of polished polished sample sample fragments fragments after after experiments experiments in in Ca(Mg,Fe)(CO3)2-SiO2-Al2O3 system: (a) polycrystalline aggregate of ankerite, aragonite, coesite, Ca(Mg,Fe)(CO3)2-SiO2-Al2O3 system: (a) polycrystalline aggregate of ankerite, aragonite, coesite, kyanite, and corundum (N 1744-A); (b) polycrystalline aggregate of ankerite, coesite, kyanite, and kyanite,Figure and 7. corundum SEM micrographs (N 1744-A); (BSE regime) (b) polycrystalline of polished sample aggregate fragments of ankerite, after experiments coesite, kyanite,in corundum (N 2135-A); (c) section of Pt capsule with a sample and fluid cavities therein (N 2113-A); and corundumCa(Mg,Fe)(CO (N 2135-A);3)2-SiO2-Al (c2)O section3 system: of ( Pta) capsule polycrystalline with a sampleaggregate and of fluid ankerite, cavities aragonite, therein coesite, (N 2113-A); (d) zoned aggregates of corundum, kyanite, and garnet in ankerite polycrystalline matrix (run N (d) zonedkyanite, aggregates and corundum of corundum, (N 1744-A); kyanite, (b) polycrystalline and garnet in ankeriteaggregate polycrystalline of ankerite, coesite, matrix kyanite, (run Nand 1738-A); 1738-A); (e) zoned aggregates of corundum, kyanite, and garnet as well as CO2 fluid cavities in corundum (N 2135-A); (c) section of Pt capsule with a sample and fluid cavities therein (N 2113-A); (e) zoned aggregates of corundum, kyanite, and garnet as well as CO2 fluid cavities in ankerite ankerite(d) zoned polycrystalline aggregates of matrix corundum, (N 2113-A); kyanite, ( fand) isometric garnet in garnet ankerite crystals polycrystalline and CO2 fluidmatrix cavities (run N in polycrystallineankerite-kyanite matrix (N polycrystalline 2113-A); (f) isometric aggregate garnet (N crystals 2145-A); and Ank—ankerite,CO2 fluid cavities Arg—aragonite, in ankerite-kyanite 1738-A); (e) zoned aggregates of corundum, kyanite, and garnet as well as CO2 fluid cavities in polycrystallineCoe—coesite, aggregate Ky—kyanite, (N Crn—corundum, 2145-A); Ank—ankerite, Grt—garnet. Arg—aragonite, Coe—coesite, Ky—kyanite, ankerite polycrystalline matrix (N 2113-A); (f) isometric garnet crystals and CO2 fluid cavities in Crn—corundum,ankerite-kyanite Grt—garnet. polycrystalline aggregate (N 2145-A); Ank—ankerite, Arg—aragonite, Coe—coesite, Ky—kyanite, Crn—corundum, Grt—garnet.

Figure 8. P,T diagrams with experimental parameters and results (this study): (a)

Ca(Mg,Fe)(CO3)2-SiO2-Al2O3 system, (b) CaMg(CO3) 2-SiO2-Al2O3 system; dashed lines—calculated Figure 8. P,T diagrams with experimental parameters and results (this study): (a) Figureposition 8. P,T of diagrams the reactions with of experimentalcomplete decarbonation—Grs parameters and50Prp results25Alm40 (this (a) and study): Grs50Prp (a)50 Ca(Mg,Fe)(CO (b). 3)2- Ca(Mg,Fe)(CO3)2-SiO2-Al2O3 system, (b) CaMg(CO3) 2-SiO2-Al2O3 system; dashed lines—calculated SiO2-Al2O3 system, (b) CaMg(CO3) 2-SiO2-Al2O3 system; dashed lines— calculated position of the position of the reactions of complete decarbonation—Grs50Prp25Alm40 (a) and Grs50Prp50 (b). reactions of complete decarbonation—Grs50Prp25Alm40 (a) and Grs50Prp50 (b).

Minerals 2020, 10, 715 8 of 17

Minerals 2020, 10, x FOR PEER REVIEW 8 of 17 It was found that decarbonation in the Ca(Mg,Fe)(CO3)2-SiO2-Al2O3 system occurred at 1000 20 ◦C (3.0 GPa), 1250 20 ◦C (6.3 GPa), and 1400 20 ◦C (7.5 GPa) (Figure8a). When studying ± It was found that decarbonation± in the Ca(Mg,Fe)(СО± 3)2-SiO2-Al2O3 system occurred at 1000 ± 20 crystals°С of(3.0 synthesized GPa), 1250 ± garnet20 °С (6.3 by GPa), Raman and spectroscopy, 1400 ± 20 °C (7.5 it was GPa) found (Figure that 8a). the When main studying modes crystals for them of were 1 4 1 1 at 349–350synthesized cm− (librationalgarnet by Raman R(SiO spectroscopy,4) −), 552 cm −it was(internal found bending that the (Si-O)main modesbend, υ 2for), andthem 906–907 were at cm − 1 (stretching349–350 (Si-O) cm−1 str (librational, υ1) (Figure R(SiO9a,4) Table4−), 5523). cm Modes−1 (internal at 403–404, bending 415, (Si-O) 437,bend, 468, υ2), and486, 906–907 954 cm cm− −1were noted(stretching as secondary (Si-O) modesstr, υ1) (Figure characteristic 9a, Table of 3). grossular–pyrope–almandine Modes at 403–404, 415, 437, 468,garnet 486, 954 formed cm–1 were as noted a result of decarbonationas secondary reactions. modes characteristicThe composition of grossular–pyrope–almandine of the obtained garnet in all garnet experiments formed as corresponded a result of to decarbonation reactions. The composition of the obtained garnet in all experiments corresponded to the Ca0.70–0.75Mg0.94–1.15Fe1.10–1.38Al1.92–1.96Si3O12 (Table4). the Ca0.70–0.75Mg0.94–1.15Fe1.10–1.38Al1.92–1.96Si3O12 (Table 4).

Figure 9. Representative Raman spectra of the obtained garnets: (a) (1)—run N 2145-A, 7.5 GPa, 1450 Figure 9. Representative Raman spectra of the obtained garnets: (a) (1)—run N 2145-A, 7.5 GPa, 1450 ◦C, °C, (2)—run N 2115-A, 6.3 GPa, 1300 °C; (b) (1)—run N 2157-D, 7.5 GPa, 1550 °C, (2)—run N 2155-D, (2)—run N 2115-A, 6.3 GPa, 1300 ◦C; (b) (1)—run N 2157-D, 7.5 GPa, 1550 ◦C, (2)—run N 2155-D, 6.3 GPa, 1250 °C, (3)—run N 2122-D, 3.0 GPa, 1150 °C. 6.3 GPa, 1250 ◦C, (3)—run N 2122-D, 3.0 GPa, 1150 ◦C.

Minerals 2020, 10, 715 9 of 17

Table 3. Raman characterization of synthesized garnets.

Run N 2115-A 2145-A 2122-D 2155-D 2157-D

Phase Alm36Pp38Grs26 Alm43Prp34Grs23 Pp74Grs26 Pp70Grs30 Prp84Grs16 1 Raman Shift, cm− - 173 - - - - 267 - - 270 - 293 299 301 297 - - 324 325 - 358 354 368 369 368 4 R(SiO4) − - - - 390 - - - 402 403 - - - - 436 - 485 - 485 485 - 505 505 - 509 - - 520 - - 520 557 556 561 559 562 638 638 647 640 646 (Si-O)bend, υ2 - 720 ------827 853 853 853 851 860 911 911 915 912 920 (Si-O)str, υ1 - - - 949 - Prp—pyrope, Alm—almandine, Grs—grossular.

Table 4. Averaged compositions of garnets, kyanite, and carbonates after experiments in

Ca(Mg,Fe)(CO3)2-SiO2-Al2O3 system.

Mass Concentrations, wt. % Run N P, GPa T, ◦C Phase SiO2 Al2O3 FeO MnO MgO CaO CO2 * Total Ky 34 66.1 0.6 - - - - 100.5 1744-A 950 3.0 (1) (4) (1) Ank - - 17(3) 0.5(1) 9(1) 28(4) 45(1) 100.0

Grt 39.3(3) 21.8(1) 21.7(5) 0.5(0) 7.3(5) 9.0(3) - 100.5 1738-A 1050 3.0 Ky 34.5(4) 64(1) 0.9(1) - - - - 99.3 Ank - - 15(2) 0.4(1) 12.8(4) 28(2) 43.9(7) 100.0 Ky 35.8 63.6 0.5 - - - - 100.0 2117-A 1100 6.3 (8) (7) (1) Ank - - 13.4(4) 0.5(0) 9.6(3) 32.6(4) 44.0(4) 100.0 Ky 35.6 64.1 1.0 - - - - 100.3 2119-A 1200 6.3 (3) (9) (0) Ank - - 17.8(9) 0.6(1) 10(1) 28(1) 45(1) 100.0

Grt 39.9(4) 22.0(5) 18(3) 0.6(1) 10.3(9) 9(1) - 100.2 2115-A 1300 6.3 Ank - - 14.9(1) - 8.4(2) 30.2(1) 46.3(3) 100.0 Ky 35.5(7) 64(1) 1.0(1) - - - - 100.3

Grt 39.4(5) 21.4(5) 20(1) 0.7(0) 10.1(3) 9(1) - 99.7 2113-A 1400 6.3 Ank - - 18(2) 0.3(0) 7.2(8) 29.2(7) 45.3(6) 100.0 Ky 36(1) 62(1) 2.0(2) - - - - 99.8 Ky 36.5 62.9 0.7 - - - - 100.2 2137-A 1150 7.5 (4) (4) (2) Ank - - 19(1) 0.5(1) 11(1) 26(2) 43.4(8) 100.0 Ky 36.1 62.4 0.5 - - 0.4 - 99.6 2135-A 1250 7.5 (1) (3) (9) (2) Ank - 0.2(0) 15(2) 0.5(1) 12(2) 28(3) 44.9(8) 100.0 Ky 36.1 62.9 1.7 - - - - 99.5 2141-A 1350 7.5 (2) (5) (2) Ank - - 17(1) 0.5(1) 11.2(6) 26.1(8) 45.3(9) 100.0

Grt 39.5(4) 21.7(1) 20.8(5) 0.7(0) 9.1(7) 8.6(5) - 100.4 2145-A 1450 7.5 Ky 35.3(3) 62.5(7) 1.7(4) - - 0.2(3) - 99.6 Ank - - 13.5(4) 0.5(1) 13.0(6) 28(1) 44.6(6) 100.0 Ank—ankerite, Coe—coesite, Ky—kyanite, Crn—corundum, Grt—garnet; *—calculated after the sum deficit; the values in parentheses are one sigma errors of the means based on replicate electron microprobe analyses reported as least units cited; 36.1 should be read as 36.1 0.1 wt. %. (1) ±

In a dolomite-bearing system (CaMg(CO3)2-SiO2-Al2O3), experiments were carried out in the temperature ranges of 1050–1150 ◦C (3.0 GPa), 1150–1450 ◦C (6.3 GPa), and 1450–1550 ◦C (7.5 GPa). Recrystallization of the initial dolomite and oxides, as well as a small amount of newly formed kyanite at Minerals 2020, 10, x FOR PEER REVIEW 10 of 17 Minerals 2020, 10, 715 10 of 17 In a dolomite-bearing system (CaMg(СО3)2-SiO2-Al2O3), experiments were carried out in the temperature ranges of 1050–1150 °С (3.0 GPa), 1150–1450 °С (6.3 GPa), and 1450–1550 °С (7.5 GPa). the contactsRecrystallization of corundum of the and initial coesite dolomite (Figure and 10 a), oxides, was establishedas well as a in small samples amount obtained of newly at temperatures formed belowkyanite the decarbonationat the contacts of reactions. corundum Atand higher coesite temperatures,(Figure 10a), was after established the onset in samples of the decarbonationobtained at reactiontemperatures (Figure 10 belowb–f), grossular–pyrope the decarbonation garnet reactions. and At CO higher2 fluid, temperatures, kyanite, and recrystallizedafter the onsetcarbonates of the anddecarbonation oxides are formed. reaction During (Figure the 10b–f), experiments, grossular–pyrope the resulting garnet fluid and segregated CO2 fluid, and kyanite, formed cavities and in therecrystallized samples (from carbonates 50 to 100 andµm) oxides (Figure are 10 formed.c,f). The During compositions the experiments, of the obtained the resulting mineral fluid phases are shownsegregated in Tableand formed5. The cavities composition in the samples of synthesized (from 50 garnetto 100 μm) in all(Figure experiments 10c,f). The corresponded compositions to of the obtained mineral phases are shown in Table 5. The composition of synthesized garnet in all Ca0.5–0.8Mg2.23–2.57Al1.92–1.98Si3O12. experiments corresponded to Ca0.5–0.8Mg2.23–2.57Al1.92–1.98Si3O12. It was found that decarbonation in the CaMg(CO3)2-SiO2-Al2O3 system occurs at 1100 20 ◦C It was found that decarbonation in the CaMg(СО3)2-SiO2-Al2O3 system occurs at 1100 ± 20± °С (3.0(3.0 GPa), GPa), 1320 1320 20 ± 20◦C °С (6.3 (6.3 GPa), GPa), and and1450 1450 ± 20 20 ◦ °СC (7.5 GPa) (Figure (Figure 8b).8b). The The main main Raman Raman ± ± 1 4 1 characteristicscharacteristics of the of the obtained obtained garnet garnet are are peaks peaks at at 368–369 368–369 cm cm− -1 (R(SiO(R(SiO44))4−−),), 559–562 559–562 cm cm−1 −((Si-O)((Si-O)bend,bend , 1 υ2), andυ2), and 912–920 912-920 cm cm− −1((Si-O) ((Si-O)strstr,, υυ1)) (Figure (Figure 9b,9b, Table Table 33),), and peaks peaks at at 270, 270, 297–301, 297–301, 402–403, 402–403, 485, 485, 520, 520, 1 640–647,640–647, 851–860 851–860 cm cm− −1are are noted noted as as secondary secondary modes.modes. In In most most experiments, experiments, fluid fluid composition composition was was monitoredmonitored by mass by mass spectrometry. spectrometry. At temperatures At temperatures below below decarbonation, decarbonation, no fluid no fluid was found was found in samples. in At temperaturessamples. At temperatures above decarbonation above decarbonation onset, scanning onset, the scanning massrange the mass from range 12 to from 46 amu 12 to revealed 46 amu the 2 presencerevealed of peaksthe presence at masses of peaks 44, 45, at andmasses 46, 44, which 45, and correspond 46, which exclusively correspond toexclusively CO2 (at otherto CO masses, (at the signalsother masses, did not the exceed signals the did background not exceed values). the background It was established values). It thatwas inestablished both relatively that in low-both and relatively low- and high-temperature experiments, the fluid composition corresponded to pure CO2, high-temperature experiments, the fluid composition corresponded to pure CO2, without impurities of without impurities of hydrogen or water. The obtained results of mass spectrometry are the best hydrogen or water. The obtained results of mass spectrometry are the best evidence of the effective evidence of the effective operation of the hematite buffer and, accordingly, adequate experimental operation of the hematite buffer and, accordingly, adequate experimental results. results.

FigureFigure 10. 10.SEM SEM micrographs micrographs (backscattered (backscattered electronselectrons (BSE) regime) regime) of of polished polished sample sample fragments fragments (CaMg(CO3)2-SiO2-Al2O3 system): (a) polycrystalline aggregate of dolomite, coesite and kyanite (N (CaMg(CO3)2-SiO2-Al2O3 system): (a) polycrystalline aggregate of dolomite, coesite and kyanite 1743-D); (b,c) sections of Pt-capsules with samples (N 2155-D and 2157-D); (d,e) zoned aggregates of (N 1743-D); (b,c) sections of Pt-capsules with samples (N 2155-D and 2157-D); (d,e) zoned aggregates corundum, kyanite, and garnet in dolomite–coesite polycrystalline matrix (N 2122-D and 2112-D); (f) of corundum, kyanite, and garnet in dolomite–coesite polycrystalline matrix (N 2122-D and 2112-D); zoned aggregates of kyanite and garnet and CO2 fluid cavities in dolomite–coesite polycrystalline f ( ) zonedmatrix aggregates (N 2120-D); of Dol—dolomite, kyanite and garnet Coe—coesite, and CO Ky—kyanite,2 fluid cavities Crn—corundum, in dolomite–coesite Grt—garnet. polycrystalline matrix (N 2120-D); Dol—dolomite, Coe—coesite, Ky—kyanite, Crn—corundum, Grt—garnet.

Minerals 2020, 10, 715 11 of 17

Table 5. Averaged compositions of garnets, kyanite, and carbonates after experiments in

CaMg(CO3)2-SiO2-Al2O3 system.

Mass Concentrations, wt. % Run N P, GPa T, ◦C Phase SiO2 Al2O3 MgO CaO CO2 Total Ky 36.0 64.2 - - - 100.2 1743-D 3.0 1050 (1) (6) Dol - - 21.6(8) 29.8(4) 48.6(9) 100.0

Grt 43.3(3) 24.2(1) 21.6(4) 10.7(8) - 99.7 2122-D 3.0 1150 Ky 33(2) 67(2) - - - 100.1 Dol - - 18(1) 34(1) 47.7(5) 100.0 Ky 36.2 63.8 - - - 100.1 2160-D 6.3 1200 (3) (7) Dol - - 25.3(5) 24.9(5) 49.8(4) 100.0 Ky 35.3 64.8 - - - 100.1 2155-D 6.3 1250 (6) (5) Dol - - 20.7(8) 30.4(9) 48.9(3) 100.0

Grt 43.3(3) 24.3(1) 22.8(3) 9.1(3) - 99.5 2128-D 6.3 1350 Ky 36.7(1) 62.5(2) - - - 99.1 Dol - - 26.2(6) 24.6(0) 49.0(4) 100.0

Grt 44.1(3) 22.9(3) 23.2(2) 9.3(4) - 100.6 2120-D 6.3 1450 Ky 34.1(4) 66.2(4) - - - 100.3 Dol - - 16.3(8) 38.7(7) 45.0(3) 100.0 Ky 36.5 63.6 - - - 100.1 2161-D 7.5 1450 (5) (4) Dol - - 25.2(4) 25.6(0) 49.2(3) 100.0

Grt 42.1(3) 23.5(1) 21.5(2) 11.5(5) - 99.5 2157-D 7.5 1550 Ky 33.7(2) 66.5(0) - - - 100.2 Dol - - 19.8(1) 31.7(2) 48.5(5) 100.0 Grt—garnet, Dol—dolomite, Ky—kyanite; the values in parentheses are one sigma errors of the means based on replicate electron microprobe analyses reported as least units cited; 6.1 should be read as 6.1 0.1 wt. %. (1) ±

4. Discussion A detailed study of the phase and chemical composition of the obtained samples, as well as the characteristic structures of the zonal aggregates, makes it possible to reconstruct decarbonation processes in the carbonate–oxide systems CaMg(CO3)2-Al2O3-SiO2 and Ca(Mg,Fe)(CO3)2-Al2O3-SiO2. It was experimentally demonstrated that at temperatures below the onset of decarbonation reactions in the reaction volume, an interaction between coesite and corundum occurs, with the formation of kyanite at the contact of these phases. A similar process was previously established in the carbonate–oxide–sulfide system (MgCO3–SiO2–Al2O3–FeS system, P = 6.3 GPa, T = 1250–1450 ◦C) [11]. At the same time, it must be emphasized that both ankerite and dolomite under P,T parameters below the decarbonation reactions in the samples are stable and do not undergo breakdown or phase transitions, which is confirmed by the results of Raman spectroscopy and mass spectrometry and is also consistent with modern experimental data (Figure2)[7]. At temperatures above the onset of decarbonation, a number of processes occur in the reaction volume: (1) crystallization of kyanite, (2) interactions of kyanite + coesite + carbonate and corundum + coesite + carbonate, leading to crystallization of garnet and CO2 fluid release, and (3) recrystallization of the starting carbonates and change of their compositions. It was established that the proportions of Ca, Mg, and Fe in the synthesized garnets differ from the initial carbonates. It should be noted that the methodical approach with hematite buffer limits the duration of the experiments; therefore, only partial rather than complete decarbonation is realized in the samples. In the case of complete decarbonation, the molar ratio of divalent cations in the initial carbonate and newly formed garnet would be completely identical. However, it is the established features of partially realized decarbonation reactions that make it possible to reconstruct natural processes. The main regularities were as follows: a decrease in Ca# and an increase in Fe# in garnets relative to the proportions in the initial carbonates; a corresponding increase in Ca# and a decrease in Fe# in carbonates after the experiments, relative to the initial ones (Figure 11). Minerals 2020, 10, x FOR PEER REVIEW 12 of 17

proportions in the initial carbonates; a corresponding increase in Ca# and a decrease in Fe# in carbonates after the experiments, relative to the initial ones (Figure 11). According to the existing theoretical and experimental data on the decarbonation parameters of Fe,Ca,Mg carbonate–oxide systems, the lowest temperatures are required for the formation of ferrous garnets and the highest for ones [20,58,59]. Specifically, it was demonstrated that diluting pyrope with the almandine component causes temperature downshift of decarbonation reactions, while adding a grossular component, on the contrary, increases the reaction temperature (Figure 4). Correspondingly, CO2 fluid and garnets with decreased Ca# and increased Fe# obtained in this work as a result of partial decarbonation of dolomite or ankerite-bearing assemblages are formed at temperatures lower than those necessary for complete decarbonation. This process is accompanied by a specific change in the composition of dolomite or ankerite (an increase in Ca# and Minerals 2020, 10, 715 12 of 17 a decrease in Fe# relative to the initial ones), expanding the stability field of these carbonates without significantly changing their structures.

Figure 11. Triangle diagram of the averaged chemical compositions (mol.%) of garnets and Figure 11. Triangle diagram of the averaged chemical compositions (mol.%) of garnets and carbonates. carbonates.

AccordingDespite to the the existingfact that, theoreticalin the present and study, experimental we performed data experiments on the decarbonationon the elucidating parameters of of Fe, Ca,positions Mg carbonate–oxide of decarbonation systems, curves in the drylowest systems, temperatures we find it useful are required to discuss for the the principal formation of ferrous garnetsdifferences and between the highest our results for calciumand existing ones ideas [20 on,58 realistic,59]. Specifically, mantle conditions it was where demonstrated aqueous that diluting pyropefluids are withpresent. the In almandineour previous research component [16,21] causeson the formation temperature of diamond downshift and graphite of decarbonationvia the carbonate–silicate interaction in MgCO3-SiO2, CaMg(CO3)2-SiO2 and MgCO3-SiO2-Al2O3 systems reactions,under while mantle adding P,T conditions, a grossular the component, source of hydrogen on the was contrary, used. According increases to gas the chromatographic reaction temperature (Figure4).data, Correspondingly, fluid composition CO during2 fluid diamond and garnets crystallization with decreasedvaried from Ca#almost and pure increased CO2 to aqueous, Fe# obtained in this workand as ait resultstrongly of affected partial the decarbonation decarbonation ofparameters. dolomite For or example, ankerite-bearing in the MgCO assemblages3-SiO2 system areat formed at temperatures1500 °C lowerand 6 GPa, than in those the case necessary of H2O-CO for2 fluid complete presence decarbonation. in the reaction volume, This process the formation is accompanied of by a specificenstatite change was in recorded, the composition i.e., decarbonation of dolomite occurred; or ankerite with the same(an increase parameters in Ca# but in and a “dry” a decrease in system, the final assemblage was magnesite and coesite, i.e., no decarbonation processes took place. Fe# relative to the initial ones), expanding the stability field of these carbonates without significantly In the MgCO3-Al2O3-SiO2 system at 6 GPa and 1500 °C, the formation of garnet occurred regardless changingof their the fluid structures. composition. However, when H2O-component dominated in fluid, the amount of pyrope Despiteincreases the fact sharply, that, which in the can present be explained study, weboth performed by the additional experiments decomposition on the of elucidating magnesite, not ofpositions of decarbonationonly due to curves the thermodynamic in dry systems, factor we but findalso due it useful to the redox to discuss factor, theand principalby an increase diff inerences kinetics between due to the formation of an aqueous fluid. Another important result obtained is evidence of poor our results and existing ideas on realistic mantle conditions where aqueous fluids are present. In our wettability of carbon dioxide under mantle P,T parameters, obtained from the specific structural previous researchfeatures of [16 experimental,21] on the samples. formation As ofshown diamond in Figures and 7c,e,f graphite and 10c,f, via the fluid carbonate–silicate segregates and forms interaction in MgCOrounded3-SiO2, bubbles, CaMg(CO which3)2 allows-SiO2 usand to determine MgCO3-SiO the high2-Al wetting2O3 systems angle; this under is in mantlegood agreement P,T conditions, the source of hydrogen was used. According to gas chromatographic data, fluid composition during diamond crystallization varied from almost pure CO2 to aqueous, and it strongly affected the decarbonation parameters. For example, in the MgCO3-SiO2 system at 1500 ◦C and 6 GPa, in the case of H2O-CO2 fluid presence in the reaction volume, the formation of enstatite was recorded, i.e., decarbonation occurred; with the same parameters but in a “dry” system, the final assemblage was magnesite and coesite, i.e., no decarbonation processes took place. In the MgCO3-Al2O3-SiO2 system at 6 GPa and 1500 ◦C, the formation of garnet occurred regardless of the fluid composition. However, when H2O-component dominated in fluid, the amount of pyrope increases sharply, which can be explained both by the additional decomposition of magnesite, not only due to the thermodynamic factor but also due to the redox factor, and by an increase in kinetics due to the formation of an aqueous fluid. Another important result obtained is evidence of poor wettability of carbon dioxide under mantle P,T parameters, obtained from the specific structural features of experimental samples. As shown in Figure7c,e,f and Figure 10c,f, fluid segregates and forms rounded bubbles, which allows us to determine the high wetting angle; this is in good agreement with studies of CO2 fluid wetting properties under crustal conditions [63,64]. Thus, implicating the results both of our present and previous [16,21] studies on the mantle processes, we can state that the presence of water-bearing fluids will reduce the temperatures of garnet + CO2 fluid formation and overall increase the intensity of the decarbonation process. The formation of H2O-CO2 fluid under these conditions results in the better wettability of mantle rocks with fluid, its better migration, and wider distribution of this mantle metasomatic agent. Minerals 2020, 10, 715 13 of 17

As a natural media simulated in this paper, one can consider carbonated eclogite. The stability field of garnet + clinopyroxene + kyanite + coesite + carbonate assemblage relatively to garnet + clinopyroxene + CO2 depends on garnet and clinopyroxene carbonation/decarbonation P,T parameters. Processes of carbonation/decarbonation also influence the mantle fluid regime and global cycle of carbon. Results obtained in the present study show that an increase in the Ca component of carbonate expands its stability field, while increasing the Fe component has the opposite effect, or, in other words, Fe-rich garnets have a larger stability field than Ca-rich ones. Thus, it can be suggested that the association of group I eclogites is more stable in processes of CO2 metasomatism than that of group II, as group I garnets are richer in iron and lower in calcium [65]. Carbonation of garnet, however, occurs under higher pressures than that of pyroxenes (reactions (2–4)), which should be taken into account (Knoche et al., 1999) [20]. Another important aspect of this work is the use of a hematite buffer. This allows us to study a non-ultrareduced dry system and observe the decarbonation reaction without formation of -like melts, leaving, however, a possibility for further research into water-bearing systems. The currently available information on inclusions of Ca, Mg, Fe carbonates in natural diamonds, as well as on the composition of garnets from carbonated eclogites, indicates that the data obtained in this work can be fully applicable in the reconstruction of decarbonation processes in the Earth’s mantle. In particular, diamonds of pipes of Mwadui, Tansania [31], Juina, Brasil [66], and Cancan, Guinea [67] contain dolomite inclusions (Ca# ~0.5) and siderite (63.5 wt. % FeO). Ankerite microinclusions are also described in a number of works [68,69]. The composition of garnets from eclogite xenoliths varies over a wide range of Mg# 0.46–0.89, Ca# 0.02–0.42, with an average content of Fe# 0.30 [66,70]. Thus, the results of the present experimental study on the estimation of CO2 and double carbonates’ stability fields in the Earth’s mantle can be used in terms of constraints of diamond-forming processes, with a CO2 as a carbon source for diamond crystallization [71–73]. When comparing the obtained data on the position of decarbonation curves with experimental and calculated results [20], it was found that decarbonation of the CaMg(CO3)2-Al2O3-SiO2 (Ca# 49) system begins at 170 ◦C (3.0 GPa), 330 ◦C (6.3 GPa), and 400 ◦C (7.5 GPa) lower than CaCO3-Al2O3-SiO2 and 100 ◦C (3.0 GPa) and 50 ◦C (6.3 GPa) higher than MgCO3-Al2O3-SiO2. When comparing the positions of decarbonation curves with the participation of ankerite and dolomite obtained in this work, it was demonstrated that the formation of garnet in the system with ankerite occurs at temperatures of 100 ◦C (3.0 GPa), 70 ◦C (6.3 GPa), and 50 ◦C (7.5 GPa) lower than in a system with dolomite.

Author Contributions: Conceptualization, Y.V.B. and Y.N.P.; data curation, Y.V.B. and Y.N.P.; formal analysis, Y.V.B., I.D.N., and O.V.F.; funding acquisition, Y.V.B.; investigation, Y.V.B., A.N.K., and I.D.N.; methodology, A.N.K.; project administration, Y.V.B.; visualization, Y.V.B. and O.V.F.; writing—original draft, Y.V.B.; writing—review and editing, Y.N.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Russian Foundation for Basic Research, grant number 18-35-20016, and by state assignment of IGM SB RAS. Acknowledgments: The authors express their sincere thanks to Vadim N. Reutsky for helping with the implementation of mass spectrometry analyses and to Yuri M. Borzdov and Alexander G. Sokol for scientific discussions at various stages of the work. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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