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Article

Formation of Spessartine and CO2 via Rhodochrosite Decarbonation along a Hot Subduction P-T Path

by
Yuliya V. Bataleva
1,*,
Aleksei N. Kruk
1,
Ivan D. Novoselov
1,2 and
Yuri N. Palyanov
1,2,*
1
Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences, 630090 Novosibirsk, Russia
2
Department of Geology and Geophysics, Novosibirsk State University, 630090 Novosibirsk, Russia
*
Authors to whom correspondence should be addressed.
Minerals 2020, 10(8), 703; https://doi.org/10.3390/min10080703
Submission received: 13 July 2020 / Revised: 1 August 2020 / Accepted: 5 August 2020 / Published: 7 August 2020
(This article belongs to the Special Issue Behaviour of Volatiles and Fluid-Mobile Elements in Subduction Zones)
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Abstract

:
Experimental simulation of rhodochrosite-involving decarbonation reactions resulting in the formation of spessartine and CO2-fluid was performed in a wide range of pressures (P) and temperatures (T) corresponding to a hot subduction P-T path. Experiments were carried out using a multi-anvil high-pressure apparatus of a “split-sphere” type (BARS) in an MnCO3–SiO2–Al2O3 system (3.0–7.5 GPa, 850–1250 °C and 40–100 h.) with a specially designed high-pressure hematite buffered cell. It was experimentally demonstrated that decarbonation in the MnCO3–SiO2–Al2O3 system occurred at 870 ± 20 °C (3.0 GPa), 1070 ± 20 °C (6.3 GPa), and 1170 ± 20 °C (7.5 GPa). Main Raman spectroscopic modes of the synthesized spessartine were 349–350 (R), 552(υ2), and 906–907 (υ1) cm−1. As evidenced by mass spectrometry (IRMS) analysis, the fluid composition corresponded to pure CO2. It has been experimentally shown that rhodochrosite consumption to form spessartine + CO2 can occur at conditions close to those of a hot subduction P-T path but are 300–350 °C lower than pyrope + CO2 formation parameters at constant pressures. We suppose that the presence of rhodocrosite in the subducting slab, even as solid solution with Mg,Ca-carbonates, would result in a decrease of the decarbonation temperatures. Rhodochrosite decarbonation is an important reaction to explain the relationship between Mn-rich garnets and diamonds with subduction/crustal isotopic signature.

1. Introduction

Manganese is a 3d transition metal that can have various valences (1, 2, 3, 4, 6, 7) and spin states; this element is one of the most common in the Earth’s crust and mantle (12th and 11th in terms of abundance) and in the bulk Earth (12th in terms of abundance [1,2,3,4]). The greatest amount of Mn is contained in the oceanic crust in ferromanganese nodules as oxides and in marine sediments as Mn-carbonate (rhodochrosite) [5]. As shown in a number of modern experimental works on the behavior of rhodochrosite and Mn-oxides at high temperatures and pressures, these minerals can be thermodynamically stable to ultrahigh pressure (P) and temperature (T) conditions (Figure 1) [6,7,8,9,10,11,12]. However, during subduction of the oceanic crust, Mn-rich oxides and rhodochrosite not only are transported to the mantle but interact with mantle rocks, leaving characteristic chemical “traces” in mantle rocks, most pronounced in garnet-bearing assemblages. These “traces” consist of a sharp increase in the concentrations of the spessartine component (as well as almandine) in garnets, the presence of carbon phases (diamond, graphite, C-O-H fluid), as well as the “subduction carbon isotopic signature” or “crustal oxygen isotopic signature” [13,14,15,16,17]. Similar “traces” are also described in diamond-bearing rocks of ultra-high pressure (UHP) terranes [18,19,20].
Analysis of modern data shows evidence for a direct relationship between Mn-rich minerals and diamond. There are four described large occurrences: (1) diamonds of eclogite paragenesis with inclusions of Fe,Mn-rich garnet from the Finsch kimberlite pipe, South Africa [13,14,15]; (2) microdiamond inclusions in Mn-rich garnet from Western Alps UHP terrain [18,19] and (3) Kokchetav massif, Kazakhstan [21]; (4) diamond with Mn-rich ilmenite inclusions from kimberlitic rocks of Juina, Brazil [22]. A potential genetic relationship of diamond and rhodochrosite was experimentally demonstrated by Liu et al. [7]. They have shown that rhodochrosite can dissociate into Mn3O4 + graphite + O2 (P = 60–80 kbar; T > 2000 °C) and Mn3O4 + diamond + O2 (P > 120 kbar; T > 2000 °C).
The interconnection between rhodochrosite, spessartine, and carbon-bearing phases under subduction settings can be described in terms of the following decarbonation reactions:
3MnCO3 + 3SiO2 + Al2O3 → Mn3Al2Si3O12 + 3CO2
3MnCO3 + Al2SiO5 + 2SiO2→ Mn3Al2Si3O12 + 3CO2
However, there is a lack of experimental studies of rhodochrosite-involving decarbonation reactions. Previously, decarbonation reactions resulting in the formation of pyrope (or pyrope-almandine) were experimentally studied by [23,24,25], and theoretical positions of some decarbonation reactions were calculated by [26,27]. The main goal of this study is to experimentally simulate the rhodochrosite-involving decarbonation reactions, resulting in the formation of spessartine and CO2-fluid, in a wide range of pressures and temperatures, with implications to the Mn-carbonate stability and spessartine formation under hot subduction conditions.
Minerals 10 00703 g001 550
Figure 1. Pressure (P)-temperature (T) diagram for MnCO3 and Mn2O3 stability after [12,28]. MnCO3-I-rhombohedral structure (rhodochrosite), MnCO3-II-triclinic structure high-pressure polymorph.
Figure 1. Pressure (P)-temperature (T) diagram for MnCO3 and Mn2O3 stability after [12,28]. MnCO3-I-rhombohedral structure (rhodochrosite), MnCO3-II-triclinic structure high-pressure polymorph.
Minerals 10 00703 g001

2. Materials and Methods

2.1. High-Pressure High-Temperature Experimental Methods

Experimental simulation of decarbonation reactions in the system MnCO3-SiO2-Al2O3 was carried out using a multi-anvil high-pressure split-sphere apparatus (BARS) [29,30] at pressures of 3.0, 6.3, and 7.5 GPa, in the temperature range of 850–1250 °C and durations from 40 to 100 h. Natural rhodochrosite Mn0.97Fe0.02Mg0.01Ca0.01CO3 (China, Guangxi Zhuang Autonomous Region, Wuzhou Prefecture, Cangwu Co., Wutong Mine (Wudong Mine)) (Figure 2) as well as synthetic oxides SiO2 and Al2O3 (99.99% purity) were used as starting reagents. Before the experiments, the reagents were powdered and thoroughly mixed. The weight proportions of the starting MnCO3, SiO2, and Al2O3 were 55:29:16 (wt.%); they were selected stoichiometrically according to the decarbonation reaction (1). Weights of MnCO3, SiO2, and Al2O3 depended on capsule sizes and corresponded to 5.5, 2.9, and 1.6 mg (3.0 and 6.3 GPa) and 4.4, 2.3, and 1.3 mg (7.5 GPa). Methodological features of the assembly, the design of the high-pressure cell, as well as calibration data were published previously [31,32,33]. Based on previous experimental studies dealing with carbonate-oxide media [24] coupled with the generation of fluid in the initial solid-phase matrix as well as on recent data on the chemical reactivity between carbon dioxide and three transition metals [34], platinum was chosen as the material of the capsules. The volume of reaction capsules was selected to ensure that all necessary analytical methods could be employed, taking into account the size of the high pressure cell. The internal size of the Pt capsules for experiments at 3.0 and 6.3 GPa was 1.5 mm (diameter) with a length of 6 mm, and at 7.5 GPa, it was 1.5 mm with a length of 4 mm.
In high pressure (HP)-high temperature (HT) experiments, the problem of hydrogen diffusion through the capsules into the samples is well known [35,36]. This diffusion can result in a significant decrease in oxygen fugacity in the samples [35,36] and a corresponding temperature shift to the lower values of decarbonation curves. In this study, to prevent the influence of hydrogen influx on the course of the experiment, we used a specially designed high-pressure cell with a hematite container (buffer) [33]. The effective time of this buffer at temperatures <1200 °C was at least 150 h, and at 1500 °C, it was about 5 h. After the experiments, the chemical composition of the hematite buffer container was analyzed. In all cases, it was represented by hematite and magnetite (±wüstite), which indicates the effectiveness of the hematite buffer throughout the experiments. The duration of the experiments for each temperature was selected based on the effective time of the hematite buffer.

2.2. Analytical Methods

All analytical methods were performed in the Sobolev Institute of Geology and Mineralogy SB RAS and at the Analytical Center for multi-elemental and isotope research SB RAS. The samples obtained after the experiments were impregnated with epoxy resin, cut, and polished. The phase and the chemical compositions of the samples as well as the phase relationships were studied by electron scanning microscopy (SEM) and energy dispersive spectroscopy (EDS) (Tescan MIRA3 LMU scanning electron microscope, TESCAN, Brno, Czech Republic). Silicate phases were measured at an accelerating voltage of 20 kV, a probe current of 20 nA, a counting time of 10 s on each analytical line, and probe diameter of 2–3 μm. The structural features of the obtained garnet crystals 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 the excitation source. To control the effectiveness of the hematite buffer, the composition of the fluid phase was qualitatively determined by mass spectrometry. For this, the platinum ampoule after the experiment was placed in a vacuum device connected to a sample injection system in a Delta V Advantage Isotope Ratio Mass Spectrometer (IRMS) (Thermo Fisher Scientific, Bremen, Germany) and equipped with a special mechanism for piercing samples [37,38]. After preliminary evacuation 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 ampoule was punctured, and the gas released at room temperature was let into the mass spectrometer analyzer.

3. Results

The parameters and the results of the experiments are presented in Table 1. Taking into account the previously developed approach and published results [23], the appearance of spessartine and CO2 fluid in the reaction volume was considered as the main criterion for the decarbonation reaction.
Accordingly, the formation of these phases was accompanied by a decrease in the amount of rhodochrosite. It must be emphasized that the partial conservation 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.
Experimental studies in the MnCO3-SiO2-Al2O3 system were carried out in the temperature ranges 850–1000 °C (3.0 GPa), 1050–1100 °C (6.3 GPa), and 1150–1200 °C (7.5 GPa). At temperatures below the decarbonation reactions, the samples obtained were represented by recrystallized rhodochrosite and oxides as well as a small amount of newly formed kyanite (Figure 3a) at the corundum and the coesite contacts. At temperatures above the decarbonation reaction (Figure 3b–d), spessartine, kyanite, and recrystallized starting materials were formed in the samples. Fluid cavities were commonly observed in the matrix of the samples (e.g., Figure 3d). The composition of the obtained spessartine in all experiments corresponded to the formula Mn2.84–2.96Ca0.05Fe0.05–0.09Al1.94–2.05Si2.95–3.02O12 (Figure 4, Table 2). It was found that decarbonation in the MnCO3-SiO2-Al2O3 system occurred at 870 ± 20 °C (3.0 GPa), 1070 ± 20 °C (6.3 GPa), and 1170 ± 20 °C (7.5 GPa) (Figure 5a,b). As shown above, in the MnCO3-SiO2-Al2O3 system at temperatures exceeding the onset of decarbonation, the formation of fluid cavities was established in the samples (Figure 3d) formed as a result of the separation of the CO2 fluid. The cavity size ranged from 10 to 200 µm. In a number of experiments, the composition of the fluid in the obtained samples was controlled by mass spectrometry. During the study, scanning the mass range from 12 to 46 amu revealed the presence of peaks at masses 44, 45, and 46, which corresponded exclusively to CO2 (signals at other masses did not exceed background values). Thus, it was found that, in all samples, both in relatively low and high temperatures, the composition of the fluid corresponded to pure CO2 without impurities of hydrogen or H2O.
These data indicate the effective operation of the hematite buffer and adequate experimental results. The obtained crystals of spessartine and recrystallized rhodochrosite were studied by Raman spectroscopy, and a comparative study of the characteristics of the synthesized garnets with the available literature data was carried out (Figure 4a–c, Table 3).
The main characteristics of the Raman spectra of natural spessartine were modes near 350, 552, and 905 cm−1, which were related to librational (R(SiO4)4−), internal bending ((Si-O)bend, υ2), and stretching ((Si-O)str, υ1) vibrations of the SiO4 tetrahedron, respectively [39]. In the Raman spectra of spessartine obtained as a result of decarbonation reactions, the main modes were 349–350, 552, and 906–907 cm−1 (Figure 4, Table 3). Peaks of 403–404, 415, 437, 468, 486, and 954 cm−1 were noted as secondary modes characteristic of spessartine resulting from decarbonation reactions and not found in the Raman spectra of natural Mn-rich garnet. In addition, in the Raman spectra of the obtained garnet, there were no peaks at 573, 592, 630, 879, or 913 cm–1, characteristic of natural spessartine [39]. These differences can be explained by two possible reasons: (1) grossular and almandine components (even minor) can affect the Raman spectra of spessartine and (2) synthesized spessartine can contain micro inclusions of carbonate, kyanite, corundum, or coesite, which also reflects in Raman spectra.

4. Discussion

4.1. Conditions of Rhodochrosite-Involving Decarbonation Reactions and Comparison to the Stability of Other Carbonates

Most existing thermal and thermodynamic models [48,49,50,51] predict that decarbonation involving Mg and Ca carbonates is rarely realized along most subduction P-T paths (Figure 5b). These models predict that most subducted carbon will be transported to great depths. Modern theoretical and experimental works on the simulation of decarbonation reactions under mantle pressures and temperatures [39,40,41,42,43,44] and findings of carbonate inclusions in diamonds [52,53,54,55,56,57,58,59,60] confirm these ideas. However, there is a lack of studies devoted to the simulation of decarbonation reactions involving other carbonates than magnesite, calcite, and dolomite. As it is shown in Figure 4b, decarbonation of Ca,Mg-carbonate + oxide or Ca,Mg-carbonate + silicate assemblages occurs at temperatures much higher than it is intended for hot subduction. As it was demonstrated in the present study, Mn-carbonate is the most probable candidate to undergo decarbonation under hot subduction P-T paths. We have to emphasize that siderite is also a potential carbonate to be involved in decarbonation immediately in the undergoing slab, since Fe and Mn have many common basic chemical characteristics (both 3d transition metals that can have various valence and similar spin states). In this study, we determined the position of the decarbonation curve, resulting in the formation of nearly pure spessartine and CO2 fluid. The spessartine crystallization and the CO2-fluid liberation at a depth of ~90 km would occur at T of 800–900 °C and at a depth of ~190–225 km at 1070–1170 °C. However, taking into account that rhodochrosite is not widely distributed in the Earth’s crust (compared to other carbonates), it is unlikely that pure spessartine would be widely distributed in the slab. More likely, carbonates in the subducting slab would form various solid solutions. For example, formation of (Mn,Fe)CO3 and its further decarbonation most probably would result in spessartine-almandine garnet crystallization, whereas subduction of carbonate solid solutions of (Mg,Mn)CO3 and (Ca,Mn)CO3 compositions would potentially result in the partial decarbonation with the crystallization of Mn-rich garnet and preservation of Mg or Ca carbonate end-members in the down-going slab. One of the main implications of our research is the fact that the presence of MnCO3 in subduction slab, even as solid solution with Mg,Ca-carbonates, would result in a decrease of the decarbonation temperatures. However, when reconstructing the possible mechanisms of the formation of Mn-rich garnet in terms of decarbonation reactions, one can notice that these reactions in nature can not only occur during slab subduction but also in other processes, such as kimberlite eruption [61,62].

4.2. Spessartine + CO2 Formation and Possible Implications for Diamond or Graphite Genesis

In the Earth’s mantle, Mn predominantly enters silicates, oxides, carbonates, and sulfides. Concentrations of MnO in mantle silicates are relatively low and amount, on average, to 0.05–0.35 wt.% in olivine [63], 0.06–0.20 wt.% in pyroxenes [64], and 0.2–0.4 wt.% in garnets [59,65,66]. Detailed investigation of the Mn content in these minerals was performed for geothermometry [64,65], and as a result, a wealth of data is available that let one notice mantle silicates with increased Mn concentrations.
The most interesting problem to discuss in these terms is a possible genesis of unusually Fe- and Mn-enriched garnets in assemblage with diamonds in several large localities worldwide: (1) Finsch kimberlite pipe, (2) West Greenland and Western Alps UHP terranes, and (3) Dachine, French Guiana. Diamonds of eclogitic paragenesis at Finsch kimberlitic pipe contain Fe- and Mn-enriched garnets as inclusions [13,14,15,16,17]. These garnets contain 15–27 wt.% FeO and up to 1.6 wt.% MnO [13]; they also demonstrate a positive correlation between MnO contents and Sr isotopic compositions. Isotopic characteristics showed that the carbon source for these diamonds was a protolith consisting of subducted oceanic crust enriched in Mn and Fe [16]. Formation conditions of these garnet inclusions are estimated to be 150–200 km and 900–1200 °C [13]. Diamond-bearing ultra-high pressure rocks from West Greenland and Western Alps demonstrate protoliths characterized by higher Fe and Mn contents that resulted from ocean-floor hydrothermal alteration compared to protoliths of both continental and oceanic affinity from other UHP terranes [18,19,20]. Diamonds in these rocks occurred within spessartine-rich garnet as inclusions in syngenetic association with C–O–H fluid inclusions, SiO2, and magnesite [18]. Diamonds from Dachine, French Guiana have unique eclogitic inclusions of Mn-rich garnet with MnO content of 12.8–18.8 wt.% [17]. The carbon source for these diamonds was established to be of crustal (seafloor) origin. All studies described above imply the participation of ocean crust materials as a source for both Mn-rich garnet formation and diamond crystallization. These sources, on the one hand, can be oceanic Mn-rich nodules and organic carbon and, on the other hand, Mn-carbonates, e.g., rhodochrosite. The data obtained in the present study demonstrate that rhodochrosite-involving decarbonation reactions can be a viable source for the spessartine component in diamond-assembling garnets and CO2 fluid, acting as a carbon source for diamond crystallization.

5. Conclusions

  • Our HP-HT experiments with a specially designed hematite-buffered high-pressure cell showed that decarbonation in the MnCO3-SiO2-Al2O3 system resulted in the formation of a CO2-fluid and spessartine at 870 ± 20 °C (3.0 GPa), 1070 ± 20 °C (6.3 GPa), and 1170 ± 20 °C (7.5 GPa) along a hot subduction P-T path.
  • Using the mass spectrometry method (IRMS), the effectiveness of the hematite buffer was demonstrated, and it was shown that the composition of the fluid, liberated as a result of decarbonation, corresponded to pure CO2.
  • An experimental reconstruction of the position of the decarbonation curve leading to the formation of a CO2-fluid in assemblage with spessartine was carried out in P-T space. It was found that the experimentally determined decarbonation curve for the formation of spessartine + CO2 was located 300–350 °C lower than that for pyrope + CO2. Our results indicate that the formation of spessartine + CO2 from the decarbonation of rhodochrosite under hot subduction settings would occur at a depth of ~90 km and 850–900 °C and at 190–225 km depth and 1070–1170 °C.
  • We experimentally demonstrated that (1) the presence of rhodochrosite as solid solution with Mg,Ca-carbonates in the subducting slab can result in a significant decrease of the decarbonation temperatures, and (2) rhodochrosite decarbonation is an important reaction to explain the relationship between Mn-rich garnets and diamonds with subduction/crustal isotopic signature.

Author Contributions

Conceptualization, Y.V.B. and Y.N.P.; Data curation, Y.V.B. and Y.N.P.; Formal analysis, Y.V.B. and I.D.N.; Funding acquisition, Y.V.B.; Investigation, Y.V.B., A.N.K., and I.D.N.; Methodology, A.N.K. and Y.N.P.; Project administration, Y.V.B.; Visualization, Y.V.B. and I.D.N.; Writing—Original draft, Y.V.B.; Writing—Review & editing, Y.N.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by 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 the help in implementation of mass spectrometry analyses and to Yuri M. Borzdov and Alexander G. Sokol for scientific consultation.

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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Minerals 10 00703 g002 550
Figure 2. Raman spectrum of natural rhodochrosite Mn0.97Fe0.02Mg0.01Ca0.01CO3 (China, Guangxi Zhuang Autonomous Region, Wuzhou Prefecture, Cangwu Co., Wutong Mine (Wudong Mine)). υ1, υ4—the high-frequency vibrational modes representing internal modes of the CO32− group.
Figure 2. Raman spectrum of natural rhodochrosite Mn0.97Fe0.02Mg0.01Ca0.01CO3 (China, Guangxi Zhuang Autonomous Region, Wuzhou Prefecture, Cangwu Co., Wutong Mine (Wudong Mine)). υ1, υ4—the high-frequency vibrational modes representing internal modes of the CO32− group.
Minerals 10 00703 g002
Minerals 10 00703 g003 550
Figure 3. SEM micrographs of polished fragments of samples obtained in the MnCO3-SiO2-Al2O3 system: (a) polycrystalline aggregate of newly formed kyanite and recrystallized coesite and rhodochrosite (run N 2129-R, 6.3 GPa, 1050 °C); (b) a platinum ampoule with a sample (run N 2117-R, 6.3 GPa, 1100 °C); (c) polycrystalline aggregate of newly formed spessartine and kyanite and recrystallized initial oxides and rhodochrosite (run N 2117-R, 6.3 GPa, 1100 °C); (d) polycrystalline aggregate of spessartine, mullite, and recrystallized coesite, with CO2-fluid cavities (run N 2130-R, 6.3 GPa, 1100 °C); Grt—spessartine garnet, Coe—coesite, Crn—corundum, Ky—kyanite, Rds—rhodochrosite.
Figure 3. SEM micrographs of polished fragments of samples obtained in the MnCO3-SiO2-Al2O3 system: (a) polycrystalline aggregate of newly formed kyanite and recrystallized coesite and rhodochrosite (run N 2129-R, 6.3 GPa, 1050 °C); (b) a platinum ampoule with a sample (run N 2117-R, 6.3 GPa, 1100 °C); (c) polycrystalline aggregate of newly formed spessartine and kyanite and recrystallized initial oxides and rhodochrosite (run N 2117-R, 6.3 GPa, 1100 °C); (d) polycrystalline aggregate of spessartine, mullite, and recrystallized coesite, with CO2-fluid cavities (run N 2130-R, 6.3 GPa, 1100 °C); Grt—spessartine garnet, Coe—coesite, Crn—corundum, Ky—kyanite, Rds—rhodochrosite.
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Minerals 10 00703 g004 550
Figure 4. Representative Raman spectra of the obtained rhodochrosite ((a), run N 2129-R, 6.3 GPa, 1050 °C) and spessartine ((b) run N 1744-R, 3.0 GPa, 950 °C; (c) run N 2117-R, 6.3 GPa, 1100 °C). R, υ1, υ2—Modes, related to librational (R(SiO4)4−), internal bending ((Si-O)bend, υ2), and stretching ((Si-O)str, υ1) vibrations of the SiO4 tetrahedron, respectively.
Figure 4. Representative Raman spectra of the obtained rhodochrosite ((a), run N 2129-R, 6.3 GPa, 1050 °C) and spessartine ((b) run N 1744-R, 3.0 GPa, 950 °C; (c) run N 2117-R, 6.3 GPa, 1100 °C). R, υ1, υ2—Modes, related to librational (R(SiO4)4−), internal bending ((Si-O)bend, υ2), and stretching ((Si-O)str, υ1) vibrations of the SiO4 tetrahedron, respectively.
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Minerals 10 00703 g005 550
Figure 5. P-T diagrams with (a) experimentally determined decarbonation reaction, associated with the formation of spessartine and CO2-fluid; and (b) previously constrained decarbonation curves after [39] (1), [40,41] (2), [23] (3), [42,43] (4), [44] (5) and graphite-diamond transition line (6) [45]. Ms—magnesite, Pc—periclase, Opx—enstatite, Ol—forsterite, Coe—coesite, Ky—kyanite, Prp—pyrope, Dm—diamond, Gr—graphite, Rds—rhodochrosite, Crn—corundum, Sps—spessartine, Ox—oxides.
Figure 5. P-T diagrams with (a) experimentally determined decarbonation reaction, associated with the formation of spessartine and CO2-fluid; and (b) previously constrained decarbonation curves after [39] (1), [40,41] (2), [23] (3), [42,43] (4), [44] (5) and graphite-diamond transition line (6) [45]. Ms—magnesite, Pc—periclase, Opx—enstatite, Ol—forsterite, Coe—coesite, Ky—kyanite, Prp—pyrope, Dm—diamond, Gr—graphite, Rds—rhodochrosite, Crn—corundum, Sps—spessartine, Ox—oxides.
Minerals 10 00703 g005
Table
Table 1. Experimental parameters and results of experiments in the MnCO3-SiO2-Al2O3 system.
Table 1. Experimental parameters and results of experiments in the MnCO3-SiO2-Al2O3 system.
Run NP, GPaT, °Ct, HoursFinal Mineral Phases
2131-R3.0850100Ky, Crn, Rds, Coe
2130-R3.0900100Sps, Ky, Crn, Rds
1744-R3.095060Sps, Ky, Crn, Rds, Coe
1215-R3.0100060Sps, Ky, Rds, Crn, Coe
2129-R6.3105060Ky, Rds, Coe
2117-R6.3110040Sps, Ky, Crn, Rds, Coe
2143-R7.5115060Ky, Rds, Coe
2144-R7.5120060Sps, Ky, Rds, Coe
Sps—spessartine, Rds—rhodochrosite, Coe—coesite, Crn—corundum, Ky—kyanite.
Table
Table 2. Averaged compositions of spessartine, kyanite, rhodochrosite, and oxides after experiments.
Table 2. Averaged compositions of spessartine, kyanite, rhodochrosite, and oxides after experiments.
Run NP, GPaT, °CPhaseMass Concentrations, wt.%n(O)Cations Per Formula Unit
SiO2Al2O3FeOMnOCaOCO2 *TotalSiAlFeMnCaC *Σ Cat
2131-R3.0850Crn-99.5(6)-0.5(5)--100.03-1.99(1)-0.01(1)--2.00
Rds--2.0(4)58.0(6)-40(1)100.03--0.03(1)0.92(2)-1.02(1)1.97
Ky36.1(4)63.2(5)-0.8(1)--100.150.97(2)2.02(2)-0.02(1)--3.01
Coe99.9(1)--0.1(1)--100.021.00(1)--0.01(1)--1.00
2130-R3.0900Sps35.9(4)21.1(2)1.6(0)41.2(8)0.6(3)-100.5122.95(2)2.05(2)0.11(0)2.87(5)0.05(3)-8.03
Crn-99.3(8)-0.6(3)--100.13-1.99(1)-0.01(1)--2.00
Rds--2.0(6)58.0(8)-40(1)100.03--0.03(1)0.92(2)-1.02(1)1.98
Ky36.2(1)63.8(4)-0.6(2)--100.650.97(2)2.02(2)-0.02(1)--3.01
Coe99.8(7)--0.9(5)--100.721.00(1)--0.01(1)--1.00
1743-R3.0950Sps36.1(6)21(1)1.4(2)41(1)0.5(2)-100.2122.96(6)2.1(1)0.09(1)2.84(9)0.04(2)-8.01
Crn-98.2(3)-0.9(1)--99.13-1.99(0)-0.01(0)--2.00
Ky36.5(1)62.7(1)-1.4(1)--100.650.99(0)1.99(0)-0.04(0)--3.02
Rds--2.0(5)57.8(6)0.3(1)39.5(8)100.03--0.03(1)0.92(1)-1.01(2)1.98
Coe99.4(2)--0.9(1)--100.321.00(0)--0.01(0)--1.01
1215-R3.01000Sps36.4(2)20.6(1)1.6(2)41.3(6)0.5(1)-100.2122.99(2)1.99(1)0.11(2)2.88(3)0.04(1)-8.01
Rds--1.9(1)58(1)0.1(3)40(1)100.03--0.03(0)0.93(4)-1.02(1)1.98
Crn-99.8(1)-0.4(1)--100.13-2.00(0)-0.00(1)--2.00
Ky36.4(9)62.5(4)-1.1(5)--100.050.99(0)1.99(0)-0.04(0)--3.02
Coe99.7(1)--0.7(4)--100.421.00(0)--0.01(1)--1.01
2129-R6.31050Ky36.3(9)62.7(1)-1.0(7)--100.050.99(1)2.00(4)-0.03(1)--3.02
Rds--1.7(5)58.8(6)-39.2(4)100.03--0.03(1)0.94(1)-1.01(1)1.99
Coe100.0(4)-----100.521.00(0)-----1.00
2117-R6.31100Sps36.1(3)20(1)1.3(1)41.9(4)0.5(3)-100.4123.01(5)1.92(8)0.09(1)2.96(5)0.05(3)-8.04
Ky36.0(5)63.4(8)-0.8(3)--100.250.97(2)2.02(2)-0.02(1)--3.01
Crn-100.0(7)-0.4(1)--100.33-2.00(0)-0.01(1)--2.01
Rds-0.5(2)1.9(3)58.2(7)-39.8(6)100.03--0.03(1)0.93(2)-1.02(1)1.98
Coe99.8(4)--0.8(3)--100.621.00(0)--0.01(1)--1.01
2143-R7.51150Ky36(1)63.7(2)-0.7(1)--100.050.96(3)2.04(4)-0.02(1)--3.02
Rds--1.9(1)57.8(5)-40.2(6)100.03--0.03(0)0.91(2)-1.03(1)1.98
Coe99.9(6)-----100.321.00(0)-----1.00
2144-R7.51200Sps36.6(2)21(1)1.0(9)41.7(4)0.5(1)-100.8123.02(2)1.94(8)0.05(1)2.96(1)0.05(1)-8.04
Ky36.1(1)63.2(3)-0.8(3)--100.150.97(3)2.03(1)-0.02(1)--3.02
Rds--2.0(1)58.1(3)-39.9(5)100.03--0.03(0)0.91(1)-1.03(1)1.98
Coe99.9(3)-----99.921.00(0)-----1.00
Sps—spessartine, Coe—coesite, Crn—corundum, Ky—kyanite, Rds—rhodochrosite; *—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(1) should be read as 36.1 ± 0.1 wt.%.
Table
Table 3. Raman characterization of spessartine garnet, synthesized in MnCO3-SiO2-Al2O3 system and previously published data.
Table 3. Raman characterization of spessartine garnet, synthesized in MnCO3-SiO2-Al2O3 system and previously published data.
Run N1744-R2117-RSps1Sps2Sps2cal
P, GPa3.06.3
T, °C9501100
Raman Shift, cm−1
106---105
163164162162163
174174175175-
-195196196195
221221221221221
270-269269-
300300302302298
----314
325321321321320
R(SiO4)4−349---347
R(SiO4)4−-350350350365
376371372372375
404403---
415----
437468---
-473475475475
486487---
-500500500505
522-522522530
(Si-O)bend, υ2553553552552560
--573573-
--592592587
--630630639
-848849849845
852---852
--879879876
(Si-O)str, υ1907906905905910
(Si-O)str, υ1--913913913
954----
-1031102910291033
1—According to [46]; 2—according to [47]; Sps—spessartine.

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Bataleva, Y.V.; Kruk, A.N.; Novoselov, I.D.; Palyanov, Y.N. Formation of Spessartine and CO2 via Rhodochrosite Decarbonation along a Hot Subduction P-T Path. Minerals 2020, 10, 703. https://doi.org/10.3390/min10080703

AMA Style

Bataleva YV, Kruk AN, Novoselov ID, Palyanov YN. Formation of Spessartine and CO2 via Rhodochrosite Decarbonation along a Hot Subduction P-T Path. Minerals. 2020; 10(8):703. https://doi.org/10.3390/min10080703

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Bataleva, Yuliya V., Aleksei N. Kruk, Ivan D. Novoselov, and Yuri N. Palyanov. 2020. "Formation of Spessartine and CO2 via Rhodochrosite Decarbonation along a Hot Subduction P-T Path" Minerals 10, no. 8: 703. https://doi.org/10.3390/min10080703

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