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Volume 41 Issue 4
Aug.  2020
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Yage Zhao, Yanfei Zhang, Chao Wang, Zhenmin Jin, Qijin Xu. Experimental Constraints on Formation of Low-Cr# Chromitite: Effect of Variable H2O and Cr2O3 on Boninitic-Magma and Harzburgite Reactions. Journal of Earth Science, 2020, 31(4): 709-722. doi: 10.1007/s12583-020-1291-0
Citation: Yage Zhao, Yanfei Zhang, Chao Wang, Zhenmin Jin, Qijin Xu. Experimental Constraints on Formation of Low-Cr# Chromitite: Effect of Variable H2O and Cr2O3 on Boninitic-Magma and Harzburgite Reactions. Journal of Earth Science, 2020, 31(4): 709-722. doi: 10.1007/s12583-020-1291-0

Experimental Constraints on Formation of Low-Cr# Chromitite: Effect of Variable H2O and Cr2O3 on Boninitic-Magma and Harzburgite Reactions

doi: 10.1007/s12583-020-1291-0
More Information
  • Reactions between a boninitic or basaltic magma and harzburgite at shallow mantle depths are thought to be closely related to the formation of podiform chromitites, but little experimental data is available on these reactions. In this study, a series of experiments were conducted at 1.5 GPa and 1 000-1 400 oC to investigate the interactions between boninitic magma and harzburgite in homogenous mixed systems with varied bulk concentrations of water (~0.7 wt.%-10 wt.%) and Cr2O3 (~0.2 wt.%-4 wt.%). In the experimental charges, chromite grains can be observed coexisting with orthopyroxene, clinopyroxene±olivine, and quenched melt in the Cr-bearing systems. The bulk concentration of Cr2O3 in the starting material has a slight effect on compositional changes in the chromites generated. However, the Cr# (Cr#=100×Cr/(Cr+Al)) and Mg# (Mg#=100×Mg/(Mg+Fe)) values for the chromites exhibit positive and negative correlations, respectively, with the bulk H2O concentrations. At 1 100 oC, chromite Cr# values range from ~33-35 to ~58-65, and chromite Mg# values range from ~70-73 to~55-58 when bulk H2O contents in the starting material are increased from ~0.7 wt.% to ~10 wt.%. The experimentally produced chromites have compositions (as expressed by Cr#, Mg#, and NiO and MnO contents) similar to natural chromites from low-Cr# chromitite bodies. We suggest that the interactions between boninitic magmas with varied H2O contents and harzburgite in a shallow mantle wedge could be a possible mechanism that forms the low-Cr# chromitites found in ophiolites. We emphasize here that H2O may play an important role in the compositional evolutions of natural chromitites.
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Experimental Constraints on Formation of Low-Cr# Chromitite: Effect of Variable H2O and Cr2O3 on Boninitic-Magma and Harzburgite Reactions

doi: 10.1007/s12583-020-1291-0

Abstract: Reactions between a boninitic or basaltic magma and harzburgite at shallow mantle depths are thought to be closely related to the formation of podiform chromitites, but little experimental data is available on these reactions. In this study, a series of experiments were conducted at 1.5 GPa and 1 000-1 400 oC to investigate the interactions between boninitic magma and harzburgite in homogenous mixed systems with varied bulk concentrations of water (~0.7 wt.%-10 wt.%) and Cr2O3 (~0.2 wt.%-4 wt.%). In the experimental charges, chromite grains can be observed coexisting with orthopyroxene, clinopyroxene±olivine, and quenched melt in the Cr-bearing systems. The bulk concentration of Cr2O3 in the starting material has a slight effect on compositional changes in the chromites generated. However, the Cr# (Cr#=100×Cr/(Cr+Al)) and Mg# (Mg#=100×Mg/(Mg+Fe)) values for the chromites exhibit positive and negative correlations, respectively, with the bulk H2O concentrations. At 1 100 oC, chromite Cr# values range from ~33-35 to ~58-65, and chromite Mg# values range from ~70-73 to~55-58 when bulk H2O contents in the starting material are increased from ~0.7 wt.% to ~10 wt.%. The experimentally produced chromites have compositions (as expressed by Cr#, Mg#, and NiO and MnO contents) similar to natural chromites from low-Cr# chromitite bodies. We suggest that the interactions between boninitic magmas with varied H2O contents and harzburgite in a shallow mantle wedge could be a possible mechanism that forms the low-Cr# chromitites found in ophiolites. We emphasize here that H2O may play an important role in the compositional evolutions of natural chromitites.

Yage Zhao, Yanfei Zhang, Chao Wang, Zhenmin Jin, Qijin Xu. Experimental Constraints on Formation of Low-Cr# Chromitite: Effect of Variable H2O and Cr2O3 on Boninitic-Magma and Harzburgite Reactions. Journal of Earth Science, 2020, 31(4): 709-722. doi: 10.1007/s12583-020-1291-0
Citation: Yage Zhao, Yanfei Zhang, Chao Wang, Zhenmin Jin, Qijin Xu. Experimental Constraints on Formation of Low-Cr# Chromitite: Effect of Variable H2O and Cr2O3 on Boninitic-Magma and Harzburgite Reactions. Journal of Earth Science, 2020, 31(4): 709-722. doi: 10.1007/s12583-020-1291-0
  • Two Cr-free boninite samples were prepared for the experiments, a natural boninite sample (QL-Bon) from the Qilian Mountains with an H2O content of ~1.5 wt.%. The hydrous boninite sample Hybon was synthesized by the following steps: high-purity oxides (SiO2, TiO2, Fe2O3, MnO) and carbonates (CaCO3, Na2CO3, K2CO3) powers were milled together in alcohol with an agate mortar to produce a homogeneous mix; then the obtained mix was fired at 1 300 ℃ for 24 h under controlled fO2 (~10-10 to 10-11 MPa) to obtain a glass without CO2 and Fe3+; finally, certain proportions of Mg(OH)2 and Al(OH)3 were mixed with the glass and ground into fine powders (~15 µm) to obtain the required sample Hybon with a water content of ~9 wt.%. Three Cr-bearing boninite samples (Cr-Bon, Cr-Hybon, Cr-R-Hybon) were also prepared for the experiments. The Cr-Bon sample was synthesized by adding Cr2O3 to the natural boninite sample (QL-Bon) to yield a bulk Cr2O3 content of ~3 wt.% and H2O content of 1.47 wt.%. The other two Cr-bearing samples (Cr-Hybon, Cr-R-Hybon) were synthesized by adding of Cr2O3 to synthesized boninite sample (Hybon) yielding the bulk Cr2O3 contents of about 3 wt.% and 8 wt.%, and H2O contents of about 8.8 wt.% and 8.3 wt.%, respectively. One harzburgite sample (Harz) was synthesized by mixing together approximately 77 wt.% olivine +20 wt.% orhopyroxene +3 wt.% spinel ground into a very fine powder (~5–15 μm) under alcohol in an agate mortar. Two groups of starting materials (Cr-bearing and Cr-poor) were synthesized by mixing 1 : 1 weight proportions of the above boninite and harzburgite powders homogenously. The bulk Cr2O3 contents in these mixed powders ranged from 0.21 wt.% to 4.22 wt.% and the bulk H2O contents ranged from 0.73 wt.% to 10.02 wt.%. The chemical compositions of the boninite samples and the starting materials are listed in Table 1.

    Harz QL-Bon Hybon Cr-Bon Cr-Hybon Cr-R-Hybon HB-1A HB-1B HB-1C HB-2B HB-2C HB-3A HB-3B
    Cr-rich systems Cr-poor systems
    SiO2 43.06 58.43 53.73 56.36 52.11 49.42 49.71 47.59 44.88 46.24 43.41 50.58 48.39
    TiO2 0.05 0.21 0.20 0.20 0.19 0.18 0.13 0.12 0.12 0.12 0.11 0.13 0.13
    Al2O3 2.48 12.36 11.32 11.92 10.98 10.41 7.20 6.73 6.35 6.45 6.05 7.38 6.90
    Cr2O3 0.42 0 - 3.01 3.01 8.02 1.71 1.71 1.62 4.22 3.96 0.21 0.21
    FeOT 9.33 8.58 7.86 8.28 7.62 7.23 8.80 8.48 8.00 8.28 7.77 8.93 8.59
    MnO 0.14 0.16 0.15 0.15 0.15 0.14 0.15 0.14 0.14 0.14 0.13 0.15 0.15
    NiO 0.26 0.01 - 0.01 - - 0.14 0.13 0.13 0.13 0.13 0.14 0.13
    MgO 44.4 8.57 7.77 8.27 7.54 7.15 26.33 25.97 24.49 25.77 24.20 26.46 26.09
    CaO 0.11 8.36 7.66 8.06 7.43 7.05 4.09 3.77 3.56 3.58 3.36 4.21 3.89
    Na2O 0.02 2.1 2.00 2.03 1.94 1.84 1.02 0.98 0.93 0.93 0.87 1.06 1.01
    K2O 0.01 0.25 0.24 0.24 0.23 0.22 0.13 0.12 0.12 0.12 0.11 0.13 0.13
    P2O5 - 0.02 - 0.02 - - 0.01 - - - - 0.01 -
    H2O - 1.52 9.07 1.47 8.80 8.34 0.73 4.40 9.81 4.17 10.02 0.76 4.54
    Total 100.28 100.57 100 100.01 100 100 100.15 100.14 100.13 100.14 100.13 100.15 100.16
    FeOT: Total Fe was calculated as FeO; Harz: harzburgite; QL-Bon, Hybon, Cr-Bon, Cr-Hybon, and Cr-R-Hybon: compositions of different boninite samples used in this study; HB-1A: 50 wt.% Harz+50 wt.% Cr-Bon; HB-1B: 50 wt.% Harz+50 wt.% Cr-Hybon; HB-1C: HB-1B+6 wt.% H2O; HB-2B: 50 wt.% Harz+50 wt.% Cr-R-Hybon; HB-2C: HB-2B+6.5 wt.% H2O; HB-3A: 50 wt.% Harz+50 wt.% QL-Bon; HB-3B: 50 wt.% Harz+50 wt.% Hybon.

    Table 1.  Compositions of starting materials used in this study

  • All high-pressure experiments were performed at 1.5 GPa using a 150-ton non-end-loaded piston cylinder apparatus at the Laboratory for the Study of the Earth's Deep Interior (SEDI-Lab), China University of Geosciences, Wuhan, China. The half-inch cell assembly used, comprising a NaCl pressure medium, a straight graphite furnace, and crushable MgO rods and spacers, has been described in detail by Wang et al. (2010). Pressure calibration was carried out against the albite=jadeite+ quartz phase transition and is accurate to ±0.1 GPa. An Au75Pd25 capsule was used for experiments conducted at 1 000–1 200 ℃ and Pt-graphite double capsules were used for experiments at 1 300–1 400 ℃ (Fig. 1). Before being loaded into the sample capsules, the homogeneous sample mixtures were kept at 120 ℃ for at least 12 h to remove water that may have adsorbed into the powdered samples. The capsules were then laser-sealed to prevent the loss of volatile components during the experiments. For all experiments, the load was first increased to the desired pressure (1.5 GPa) and then the temperature was increased to the target temperature and maintained for 10–40 h. The experimental temperature was monitored by a W95%Re5%-W74%Re26% (Type C) thermocouple located at the bottom of the sample capsule; the error was no more than ±10 ℃ (Wang et al., 2010).

    Figure 1.  Sketches of the capsules used in the high-pressure experiments. (a) Au75Pd25 capsule; (b) platinum-graphite double capsules.

  • The recovered charges were mounted in epoxy resin firstly and then polished with aluminum oxide paste to expose the center of the specimen for texture and phase composition analyses. A Quanta 2000-type scanning electron microscope (SEM) at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan), was used to study the microstructures of each recovered charge. Quantitative chemical analyses of the minerals and the quenched melt were obtained using a JXA-8100 type electron probe micro-analyzer (EPMA) at the Key Laboratory of Submarine Geosciences, State Oceanic Administration, Hangzhou, China. The EPMA accelerating voltage and beam current were 15 kV and 20 nA, respectively, and the beam size used for the quantitative chemical analyses was 1 μm for minerals and 5–15 μm for quenched melt glass. The counting times were 10 s for the measured elements and 5 s for the background. Standard materials were diopside for SiO2, MgO and CaO, hematite for FeO, jadeite for Na2O, K-feldspar for K2O, pyrope for Al2O3, eskolaite for Cr2O3, nickel oxide for NiO, rutile for TiO2 and Mn2O3 for MnO.

  • Table 2 summarizes all the experimental conditions for this study and the phases in the run products. Quantitative chemical compositions for the phases from typical runs are listed in Tables 34. Photomicrographs of representative experiments are shown in Fig. 2, and graphs showing Cr#, Mg# (Mg#=100×Mg/(Mg+Fe)), and weight percent of experimentally produced chromites versus Cr2O3 and H2O in the starting material are shown in Fig. 3. Water contents in the produced phases were not given, because the crystal sizes were too small (~ < 50 µm) to be measured reliably by FT-IR spectroscopy. The estimated bulk water contents in the run products were roughly consistent with those in the starting materials in most of the experiments (Table 2). Weight proportions of the observed phases in the run products were calculated based on mass balance calculations, using the bulk compositions of the starting materials and the analyzed compositions of the coexisting phases in the recovered charges. The estimated residuals between the calculated results based on the weight proportions of coexisting phases and the starting materials are in most cases no more than ~1%.

    Run No. P (GPa) T (℃) D (h) SM Capsule Bulk Cr2O3 Bulk H2O Run products Estimated H2O*
    Cr-rich experiments
    PC569-A 1.5 1 100 19 HB-1A Au75Pd25 1.71 wt.% 0.73 wt.% Opx (71), Cpx (16), Chr (7), melt (6) 0.8 wt.%
    PC572 1.5 1 200 15 HB-1A Au75Pd25 1.71 wt.% 0.73 wt.% Ol (16), Opx (50), Cpx (18), Chr (5), melt (11) 2.2 wt.%
    PC569-B 1.5 1 100 19 HB-1B Au75Pd25 1.71 wt.% 4.40 wt.% Opx (74), Chr (6), melt (20) 2.8 wt.%
    PC567 1.5 1 200 15 HB-1B Au75Pd25 1.71 wt.% 4.40 wt.% Ol (36), Opx (31), Chr (3), melt (30) 4.6 wt.%
    PC590 1.5 1 100 10 HB-1C Au75Pd25 1.62 wt.% 9.81 wt.% Ol (20), Opx (17), Chr (5), melt (54) 7.6 wt.%
    PC569-C 1.5 1 100 19 HB-2B Au75Pd25 4.22 wt.% 4.17 wt.% Opx (50), Cpx (12), Chr (10), melt (28) 3.6 wt.%
    PC592-B 1.5 1 000 10 HB-2C Au75Pd25 3.96 wt.% 10.02 wt.% Opx (52), Cpx (39), Chr (9), tr.melt
    Cr-poor experiments
    PC468 1.5 1 100 40 HB-3A Au75Pd25 0.21 wt.% 0.76 wt.% Ol (5), Opx (80), Cpx (15)
    PC508 1.5 1 200 40 HB-3A Au75Pd25 0.21 wt.% 0.76 wt.% Ol (5), Opx (75), Cpx (13), melt (7 1.3 wt.%
    PC532 1.5 1 300 14 HB-3A Pt-Graph 0.21 wt.% 0.76 wt.% Ol(12), Opx (49), Cpx(4), melt (18) 2.7 wt.%
    PC534 & 1.5 1 400 13 HB-3A Pt-Graph 0.21 wt.% 0.76 wt.% Ol (7), Opx(63), melt (30) 5.4 wt.%
    PC516 1.5 1 100 25 HB-3B Au75Pd25 0.21 wt.% 4.54 wt.% Ol (25), Opx (36), Cpx (8), melt (31) 4.0 wt.%
    PC518 1.5 1 200 10 HB-3B Au75Pd25 0.21 wt.% 4.54 wt.% Ol (27), Opx (28), Cpx (5), melt (40) 4.13 wt.%
    PC531 1.5 1 300 14 HB-3B Pt-Graph 0.21 wt.% 4.54 wt.% Ol (21), Opx (49), melt (30) 7.2 wt.%
    Mass balance calculations were performed to estimate the phase proportions (wt.%) in mixed experiments.*. Water contents in the run products were estimated based on mass balance calculations and EPMA totals in quench melts. & . The estimated bulk water content in this experiment was inconsistent with that in the starting material. SM. Starting materials; Graph. graphite; Ol. olivine; Opx. orthopyroxene; Cpx. clinopyroxene; Sp. spinel; Chr. chromite.

    Table 2.  Summary of experimental conditions and run products.

    Run No. P (GPa) T (oC) n Phase SiO2 TiO2 Al2O3 Cr2O3 FeOT MnO NiO MgO CaO Na2O K2O Total Cr# Mg#
    PC569-A 1.5 1 100 4 Chr 1.60 0.19 38.19 29.02 13.06 0.19 0.07 18.25 0.19 0.06 0.02 100.82 33.77 71.36
    σ 1.57 0.03 1.24 0.77 0.45 0.04 0.01 0.26 0.06 0.04 0 0.61 0.82 0.98
    3 Opx 54.71 0.09 3.65 0.71 6.65 0.17 0.07 32.53 1.02 0.08 0.01 99.68 11.26 89.70
    σ 0.69 0.02 0.25 0.20 0.41 0.02 0.04 0.67 0.33 0.02 0 0.26 2.41 0.74
    3 Cpx 52.55 0.17 4.44 1.18 3.43 0.11 0.02 17.31 19.24 0.89 0.01 99.33 15.00 89.99
    σ 0.11 0.02 0.46 0.20 0.12 0.01 0 0.34 0.12 0.04 0 0.48 1.03 0.39
    3 melt 54.34 0.17 21.24 0.12 2.33 0.06 - 3.84 3.23 1.14 0.72 87.20 0.38 74.62
    σ 1.51 0.10 1.09 0.07 0.83 0.03 - 2.09 1.30 0.73 0.03 2.21 0.25 10.84
    PC572 1.5 1 200 5 Chr 0.83 0.14 33.67 34.22 13.71 0.15 0.09 17.93 0.15 0.01 0.01 100.91 40.54 69.97
    σ 1.29 0.02 0.63 0.88 0.23 0.05 0.02 0.29 0.05 0 0 0.67 0.56 0.69
    3 Opx 55.79 0.04 3.63 1.00 6.81 0.13 0.08 32.24 1.66 0.11 0.01 101.49 15.86 89.43
    σ 0.51 0 0.83 0.16 0.08 0.01 0.01 0.56 0.02 0.02 0 0.19 1.38 0.1
    4 Cpx 53.21 0.14 4.72 0.82 4.02 0.07 0.05 18.39 17.66 0.89 - 100.72 10.06 89.07
    σ 0.36 0.06 0.40 0.64 0.4 0.02 0.04 0.64 0.34 0.06 - 0.13 0.32 0.17
    3 Ol 42.39 - 0.14 0.17 9.85 0.12 0.15 49.91 0.21 - - 102.96 53.86 90.12
    σ 0.26 - 0.01 0.01 0.11 0.02 0.02 0.06 0.01 - - 0.19 6.83 0.01
    5 melt 51.44 0.10 21.78 0.06 1.76 0.03 0.02 0.45 3.33 0.68 0.65 80.36 0.18 31.35
    σ 2.38 0.05 1.36 0.04 0.41 0.02 0.02 0.20 0.66 0.20 0.06 4.04 0.12 5.28
    PC569-B 1.5 1 100 5 Chr 0.14 1.22 21.92 46.43 14.71 0.23 0.04 16.23 0.14 0.01 0.02 101.07 58.69 66.29
    σ 0.03 0.03 0.40 0.82 0.14 0.03 0.03 0.18 0.03 0 0 0.36 0.83 0.22
    4 Opx 56.32 0.20 2.55 0.80 5.80 0.13 0.06 33.90 0.81 0.04 0.01 100.59 17.57 91.26
    σ 0.11 0.05 0.30 0.24 0.37 0.02 0.03 0.19 0.18 0.01 0 0.46 5.71 0.48
    3 melt 54.17 1.20 18.88 0.07 1.96 0.12 0.01 1.07 7.89 0.36 0.26 85.98 0.24 49.29
    σ 0.16 0.55 0.58 0.01 0.44 0.04 0.01 0.93 0.58 0.12 0.07 1.76 0.04 18.00
    PC567 1.5 1 200 6 Chr 0.15 0.96 17.18 52.77 10.00 0.22 0.04 17.86 0.06 - 0.02 99.23 67.32 76.10
    σ 0.02 0.04 0.41 0.68 0.17 0.05 0.01 0.34 0.01 - 0.01 0.52 0.79 0.64
    4 Opx 57.01 0.16 1.78 0.93 3.97 0.14 0.03 35.79 0.68 0.03 0.01 100.50 26.29 94.15
    σ 0.53 0.02 0.43 0.19 0.12 0.01 0.01 0.19 0.03 0.01 0 0.33 1.52 0.19
    4 Ol 40.94 0.01 0.06 0.18 6.99 0.12 0.08 51.84 0.06 0.01 - 100.26 65.23 93.03
    σ 0.56 0.08 0.01 0 0.69 0.01 0.08 0.68 0.01 0.01 - 0.99 0.10 0.26
    5 melt 53.16 1.34 17.74 0.05 1.70 0.11 0.01 0.89 9.00 0.24 0.18 84.41 0.20 48.17
    σ 1.00 0.36 0.33 0.02 0.23 0.02 0.01 0.12 0.34 0.04 0.03 0.95 0.08 3.76
    PC569-C 1.5 1 100 5 Chr 0.24 1.29 20.39 46.97 13.45 0.21 0.04 16.48 0.11 0.03 0.02 99.19 60.71 68.60
    σ 0.08 0.11 0.27 0.66 0.15 0.03 0.03 0.06 0.01 0.01 0 0.42 0.64 0.27
    6 Opx 56.90 0.24 2.45 0.96 5.23 0.13 0.04 34.46 1.28 0.07 0.02 101.76 21.30 92.14
    σ 0.36 0.03 0.47 0.07 0.14 0.03 0.02 0.62 0.12 0.03 0.01 0.63 3.25 0.31
    2 Cpx 50.19 2.72 9.54 0.64 4.74 0.11 0.06 20.57 12.07 1.08 0.13 101.81 4.45 88.44
    σ 1.34 0.71 1.43 0.05 0.03 0.04 0.03 2.25 1.91 0.10 0.02 0.43 0.98 1.06
    3 melt 51.12 0.67 14.18 0.20 3.43 0.08 0.03 11.86 4.69 0.45 0.02 86.86 0.92 86.06
    σ 3.87 0.07 0.29 0.12 0.18 0.01 0.02 1.07 0.80 0.25 0.02 4.02 0.56 1.74
    PC590 1.5 1 100 5 Chr 0.12 1.46 18.77 43.93 19.87 0.25 0.14 14.53 0.13 0.01 0.01 99.20 61.09 56.58
    σ 0.01 0.03 0.14 0.73 0.22 0.03 0.03 0.10 0.02 0 0 0.72 0.49 0.27
    4 Opx 56.24 0.17 3.00 0.55 6.34 0.13 0.10 34.60 0.72 0.04 0.01 101.88 11.89 90.68
    σ 0.23 0.07 0.67 0.18 0.34 0.03 0.03 0.38 0.29 0.01 0 0.24 5.95 0.54
    3 Ol 41.23 0.04 0.03 0.10 9.97 0.15 0.29 51.21 0.07 0.01 0.01 103.10 68.48 90.15
    σ 0.22 0.03 0.01 0.05 0.26 0.01 0.01 0.39 0.02 0 0 0.24 5.85 0.29
    2 melt 54.97 0.99 19.13 0.06 2.03 0.10 0.01 0.49 7.87 0.23 0.31 86.20 0.20 30.38
    σ 0.26 0.05 0.26 0 0.24 0.05 0.01 0.01 0.31 0.02 0.02 0.12 0.01 2.05
    PC592-B 1.5 1 000 4 Chr 2.92 1.49 18.90 44.31 17.57 0.23 0.08 13.22 0.22 0.06 0.04 99.03 61.14 57.26
    σ 0.66 0.01 1.09 1.41 0.64 0.04 0.03 0.68 0.03 0.01 0.01 1.36 2.11 1.58
    4 Opx 55.45 0.19 3.34 0.83 6.33 0.17 0.08 33.61 0.60 0.06 0.01 100.51 14.41 90.43
    σ 0.45 0.08 0.43 0.55 0.43 0.04 0.04 0.71 0.16 0.01 0 0.52 9.69 0.78
    4 Cpx 45.99 0.95 10.42 1.93 4.40 0.11 0.06 19.81 9.66 2.33 0.30 96.86 11.03 89.01
    σ 0.48 0.03 0.18 0.10 0.03 0.03 0 0.42 0.42 0.08 0.04 0.18 0.5 0.80
    FeOT. Total Fe was calculated as FeO. Mineral abbreviations: Ol. Olivine; Opx. orthopyroxene; Cpx. clinopyroxene; Chr. chromite.

    Table 3.  Major element compositions (wt.%) of representative phases in Cr-rich experiments

    Run No. P (GPa) T (℃) n Phase SiO2 TiO2 Al2O3 Cr2O3 FeOT MnO NiO MgO CaO Na2O K2O Total
    PC468 1.5 1 100 3 Ol 42.2 0.04 1.25 0.02 8.93 0.22 0.38 47.97 0.85 0.21 0.04 102.1
    σ 0.4 0.03 0.26 0.02 0.77 0.03 0.05 1.17 0.26 0.04 0.02 1.48
    4 Cpx 48.66 0.34 10.29 0.31 3.48 0.12 0.09 17.86 15.12 2.04 0.23 98.53
    σ 4.47 0.24 5.61 0.04 0.54 0.02 0.06 0.69 5.31 1.27 0.22 1.24
    4 Opx 54.97 0.07 4.34 0.25 6.03 0.22 0.08 32.06 1.63 0.24 0.03 99.9
    σ 0.67 0.03 1.01 0.07 0.39 0.02 0.01 0.87 0.66 0.23 0.04 0.14
    PC508 1.5 1 200 4 Ol 42.32 0 0.3 0.02 3.83 0.12 0.15 52.77 0.23 0.02 - 99.76
    σ 0.7 0.01 0.21 0.01 0.32 0.03 0 1.23 0.08 0.01 - 0.92
    3 Cpx 53.17 0.13 5.45 0.38 2.1 0.11 0.01 18.86 18.93 0.77 - 99.91
    σ 0.17 0.02 0.13 0.01 0.37 0.03 0.01 0.43 0.13 0.06 - 0.37
    3 Opx 55.68 0.06 5.18 0.39 3.72 0.2 0.02 34 1.3 0.08 - 100.63
    σ 0.7 0.04 0.54 0.1 1.02 0.03 0.02 1.06 0.18 0.01 - 0.42
    4 Melt 48.96 0.15 22.53 0.02 0.56 0.06 0.03 1.36 5.97 0.56 0.62 80.82
    σ 0.26 0.02 0.32 0.02 0.06 0.02 0.03 0.76 0.09 0.1 0.02 0.89
    PC532 1.5 1 300 3 Ol 42.97 0.01 0.33 0.13 8.38 0.1 0.05 50.06 0.27 0.01 - 102.31
    σ 2.65 - 0.34 0.02 0.43 0.02 0.05 2.03 0.12 0.01 - 0.86
    3 Cpx 49.6 0.26 11.03 0.26 4.55 0.19 - 18.36 14.71 0.68 0.01 99.65
    σ 0.5 0.05 0.84 0.07 0.25 0.03 - 0.86 0.25 0.03 0.01 0.12
    4 Opx 57.21 0.04 2.56 0.41 4.71 0.11 0.02 34.86 1.02 0.05 - 100.99
    σ 0.21 0.02 0.24 0.02 0.42 0.03 - 0.21 0.1 0.01 - 0.32
    5 Melt 40.16 0.2 19.37 0.05 5.02 0.11 - 5.94 9.19 2.41 0.5 82.97
    σ 0.92 0.01 0.33 0.01 0.19 0.01 - 0.66 0.17 0.67 0.01 1.04
    PC534 1.5 1 400 4 Ol 41.47 - 0.07 0.05 4.7 0.05 0.01 53.61 0.12 0.02 0.01 100.11
    σ 0.37 - 0.03 0.03 0.28 0.04 0.01 0.29 0.01 0.01 0 0.49
    4 Opx 56.9 0.01 1.98 0.24 2.34 0.06 - 36.99 0.75 0.02 - 99.3
    σ 1.1 0.02 0.36 0.09 1.25 0.03 - 1.32 0.08 0.01 - 1.05
    6 Melt 39.51 0.2 15.96 0.03 3.92 0.09 0.04 9.6 9.76 2.58 0.31 82
    σ 1.46 1.15 0.58 0.03 0.16 0.01 0.03 0.63 0.54 0.17 0.03 2.47
    PC516 1.5 1 100 1 Ol 41.64 0.04 0.04 0.06 7.51 0.27 0.2 53.11 0.12 - - 102.98
    6 Cpx 45.72 2.12 11.66 0.07 3.41 0.14 0.02 16.92 15.59 1.66 0.17 97.48
    σ 3.06 0.33 3.68 0.06 0.79 0.03 0.01 1.1 5.14 1.06 0.13 1.26
    5 Opx 55.69 0.25 3.65 0.39 5.59 0.16 0.05 33.01 1.05 0.05 0.02 99.9
    σ 0.48 0.02 0.72 0.05 0.44 0.04 0.04 0.76 0.1 0.02 0.02 0.64
    2 Melt 53.76 0.64 19.24 0.03 1.41 0.1 0.02 3.57 7.01 0.46 0.6 86.84
    σ 0.81 0.09 1.45 0.05 0.75 0.01 0.01 2.41 0.72 0.22 0.11 1.07
    C518 1.5 1 200 3 Ol 41.64 0.04 0.04 0.06 7.37 0.27 0.2 53.11 0.12 - - 102.85
    σ 0.1 0.02 0.01 0.02 0.22 0.05 0.05 0.27 0.02 - - 0.41
    3 Cpx 45.78 3.99 14.95 0.04 4.65 0.2 0.03 13.88 13.73 1.54 0.19 98.98
    σ 1.42 0.32 2.77 0.04 0.32 0.03 0.02 1.47 2.64 0.4 0.14 1.71
    5 Opx 55.52 0.24 4.16 0.34 4.54 0.18 0.03 34.06 1.3 0.08 0.01 100.44
    σ 0.48 0.08 0.79 0.21 1.01 0.04 0.04 0.54 0.15 0.01 0 0.6
    3 Melt 51.73 1.79 18.22 0.07 2.9 0.11 - 7.89 7.84 0.99 0.4 91.94
    σ 1.18 0.37 1.21 0.03 0.43 0.02 - 2.94 0.42 0.06 0.13 1.49
    PC531 1.5 1 300 3 Ol 41.74 0.01 0.05 0.15 4.19 0.08 0.04 53.8 0.07 - - 100.12
    σ 0.29 0 0 0.06 0.84 0.02 0.02 1.11 0 - - 0.48
    3 Opx 58.42 0.13 1.07 0.22 1.55 0.08 0.04 38.85 0.48 0.01 - 100.85
    σ 3.05 0.62 3.03 0.04 0.52 0.03 0.02 3.89 2.3 0.11 - 0.64
    7 Melt 43.35 1.35 16.58 0.03 1.32 0.11 - 3.06 8.57 0.87 0.35 75.63
    σ 1.32 0.21 1.38 0.02 0.47 0.02 - 0.96 0.99 1.32 0 0.96
    FeOT. Total Fe was calculated as FeO. Mineral abbreviations: Ol. Olivine; Opx. orthopyroxene; Cpx. clinopyroxene; Chr. chromite.

    Table 4.  Major element compositions (wt.%) of representative phases in mixed experiments for Cr-poor experiments

    Figure 2.  Photomicrographs showing the textures of representative run products in HB-1A (a), (b), HB-1B (c), (d), HB-1C (e) and HB-2C (f). (a) Experiment carried out at 1.5 GPa and 1 100 ℃ (PC569A, HB-1A), showing chromite coexisting with orthopyroxene, clinopyroxene and quenched melt in the run products. (b) Experiment conducted at 1.5 GPa and 1 200 ℃ (PC572, HB-1A), demonstrating that quenched melt coexists with chromite, olivine, orthopyroxene, and clinopyroxene. (c) Run products at 1.5 GPa and 1 100 ℃ (PC569B, HB-1B), in which the results are mainly orthopyroxene, chromite, and quenched melt. (d) Experiment conducted at 1.5 GPa and 1 200 ℃ (PC567, HB-1B), in which olivine, orthopyroxene, chromite, and quenched melt can be observed in the products. (e) Experiment at 1.5 GPa and 1 100 ℃ (PC590, HB-1C), the products are composed of olivine+orthopyroxene+clinopyroxene+chromite+quenched melt. (f) In HB-2C system, orthopyroxene, clinopyroxene, and chromite coexist with a trace of quenched melt at 1.5 GPa and 1 000 ℃ (PC592B).

    Figure 3.  Values for Cr# (a), (b), Mg# (c), (d), and weight proportions of experimentally produced chromite (e), (f) plotted vs. Cr2O3 and H2O concentrations in the starting materials for the Cr-bearing experiments.

  • The bulk Cr2O3 concentrations were about 1.6 wt.%–1.7 wt.% in most of the samples used in the Cr-bearing experiments (starting materials HB-1A/B/C). For the sample with a bulk H2O content of 0.73 wt.% (HB-1A), the starting material was transformed into an assemblage composed mainly of orthopyroxene, clinopyroxene, chromite, and quenched melt glass in experiment carried out at 1 100 ℃ (PC569A, Fig. 2a). The same starting material produced a run product containing olivine when the experiment was run at 1 200 ℃ (PC572, Fig. 2b). With a bulk H2O content of 4.4 wt.% (HB-1B), small amounts of chromite coexisted with orthopyroxene and quench melt in the recovered charge from the experiment conducted at 1 100 ℃ (PC569B, Fig. 2c), but the mineral assemblage changed to olivine+orthopyroxene+chromite+quench melt when the run temperature was increased to 1 200 ℃ (PC567, Fig. 2d). In the H2O oversaturated experiments (HB-1C, H2O=9.81 wt.%), chromite can be observed coexisting with olivine, orthopyroxene, clinopyroxene, and quench melt in the run products for experiments carried out at 1 100 ℃ (PC590, Fig. 2e). In addition, two experiments with bulk Cr2O3 contents of ~3.9 wt.%–4.2 wt.% (HB-2B/C) were conducted for comparison and in these experiments chromite was observed coexisting with orthopyroxene and clinopyroxene at temperatures of 1 000– 1 100 ℃ (PC592B, Fig. 2f). It is noteworthy that there was no olivine in these run products. This may be caused by the relatively high Cr2O3 in the starting materials compared with HB-1B/C as starting material, because high Cr2O3 contents preferentially stabilize pyroxene and chromite over olivine.

  • A series of experiments were also conducted in Cr-poor systems (Cr2O3=0.21 wt.%) at temperatures ranging from 1 100 to 1 400 ℃. For a bulk H2O content of 0.76 wt.% (HB-3A), the run products at 1 100 ℃ were composed mainly of orthopyroxene, clinopyroxene, and a small amount of olivine (~5 wt.%). For experiments conducted at 1 200 and 1 400 ℃, the weight proportion of quenched melt is greater and both orthopyroxene and clinopyroxene were consumed with the increase of temperatures (Table 2). When the bulk H2O content was increased to 4.54 wt.% (HB-3B), the starting material, at 1 100–1 200 ℃, was first transformed to an orthopyroxene, clinopyroxene, olivine, and quench melt assemblage. When the experiment was conducted at 1 300 ℃, clinopyroxene disappeared and was replaced by greater proportions of olivine and quenched melt. The amount of olivine (~12 wt.%–21 wt.%) in the HB-3B system was greater than that in the HB-3A system. This may be related to the relatively higher bulk H2O content in the starting material, because H2O can enhance partial melting and favor the formation of olivine. To summarize, chromite was not observed in the recovered charges in Cr-poor experiments, indicating that Cr-bearing melt is probably a prerequisite for the formation of chromite by boninitic magma-harzburgite interaction.

  • In Cr-poor experiments, Cr2O3 was mainly incorporated into orthopyroxene and clinopyroxene because chromite was not produced during the experiments. In Cr-bearing experiments, chromite was the main Cr-bearing phase. The variations of Cr# and Mg# in chromite versus the concentrations of Cr2O3 and H2O in the starting materials are shown in Fig. 3. The Cr2O3 abundances in the experimental chromites range widely in Cr2O3 (~28 wt.%–53 wt.%) as do the Al2O3 contents (~17 wt.%–40 wt.%). This results in the Cr# values ranging from about 32 to approximately 68. The chromites have relatively narrow FeO and MgO ranges (~10 wt.%–20 wt.% and ~12 wt.%–19 wt.%, respectively), with Mg# values ranging from about 55 to 77. The Cr# and Mg# values of chromites show weak correlations with the bulk Cr2O3 contents when considering the uncertainties (Figs. 3a and 3c). The weight proportion of chromite exhibits a positive correlation with the increase of Cr2O3 contents at a given temperature and similar bulk H2O content (e.g., at 1 100 ℃ and a bulk H2O content of ~4.2 wt.%–4.4 wt.%) (Fig. 3e).

    For a bulk Cr2O3 content of ~1.6 wt.%–1.7 wt.%, the Cr# in chromite exhibit positive correlations with the bulk H2O content at a constant temperature. For instance, the Cr# values increase from ~32–35 to ~60 with the bulk H2O contents ranging from ~0.7 wt.% to 10 wt.% at 1 100 ℃ (Fig. 3b), and the corresponding chromite Mg# values decrease from ~70–73 to ~56 (Fig. 3d). For experiments at 1 200 ℃, the Cr# values in chromite were slightly higher than they were at 1 100 ℃ (Fig. 3b). The weight proportions of experimentally produced chromites show slightly negative correlations with temperature and the bulk H2O content (with Cr2O3 ~1.6 wt.%–1.7 wt.%) (Fig. 3f).

  • Natural chromitites have a wide range of chemical compositions, and they can be divided into low-Cr# and high-Cr# groups based on the variations of Cr# of the chromite. Two types of magmas (basaltic magma and boninitic magma) derived from different tectonic settings have been proposed as being involved in the formation of the two groups of chromitites (Xiong Q et al., 2017; Xiong F H et al., 2015; Moghadam et al., 2015; Zhou et al., 1996). To constrain the genesis and tectonic evolutions of podiform chromitites, it is instructive to compare the compositions of the experimentally produced chromites with the compositions of chromites from natural chromitites. As shown in Fig. 4, a notable feature of our experimentally produced chromites is that their Cr# values all fall below the Cr#=70 line, the value generally used to divide natural chromitites into the low-Cr# and high-Cr# groups (Fig. 4a). Both the Cr# and Mg# values for the experimental chromites are in good agreement with the values for those indexes reported by other investigators for chromites from low-Cr# chromitites (Xiong F H et al., 2018, 2017a, b, 2015; Xiong Q et al., 2017; Zhou et al., 2014). The TiO2 concentration in chromite has also been used to constrain the origins of podiform chromitites (Arai and Matsukage, 1998; Bonavia et al., 1993). Our experimentally produced chromites have TiO2 and Al2O3 concentrations similar to those in chromites from MORB, but have slightly higher TiO2 contents than chromites from podiform chromitites and more Al2O3 than chromites from boninites (Fig. 4b). In addition, the NiO and MnO contents in the experimental chromites are in good agreement with the abundances of those elements in low-Cr# chromitites, except for the natural chromites with Cr# values lower than ~30 (Figs. 4c, 4d). These comparisons suggest that the chemical compositions of chromites (as expressed by Mg#, Cr#, and NiO and MnO contents) in low-Cr# chromitites can be well explained by the reaction between H2O-bearing boninitic magma and harzburgite. However, the compositions of the chromites in high-Cr# chromitites are inconsistent with our experimentally produced chromites.

    Figure 4.  Graphs comparing the compositions of the experimental chromites produced by this study with natural chromites. The grey solid circles represent natural chromites from podiform chromitites from the Mayari-Cristal massif and the Moa-Baracoa massif, Cuba, the Acoje and Coto ophiolites, Philippines, the Tibet and Sartohay ophiolites, China, and the Köycegiz ophiolite, Turkey (Xiong F H et al., 2018, 2017a, b; Xiong Q et al., 2017; Zhou et al., 2014). The "MORB" and "Boninite" fields represent the compositions of chromites from mid-oceanic ridge basalts and boninites, respectively.

    In chromitites, orthopyroxene is generally observed as matrix grains or as inclusions within the chromite grains; the orthopyroxene's chemical compositions is also thought to contain information related to chromitite formation (Xiong F H et al., 2017a, 2015; Xiong Q et al., 2017). Figure 5 compares the compositions of the orthopyroxene produced in our experiments with the compositions of orthopyroxene in natural podiform chromitites, and orthopyroxene in the surrounding harzburgites and lherzolites. The Mg# values for orthopyroxenes from low-Cr# and high-Cr# groups range from ~90–90.5, and ~93–94.5, respectively; whereas the Mg# values for orthopyroxenes from harzburgites and lherzolites are around ~90–95, and 90.5–91, respectively. The CaO and Cr2O3 contents of the orthopyroxenes in harzburgites are slightly lower than the orthopyroxenes in podiform chromitites and lherzolites. It is obvious that the compositions of the orthopyroxene from low-Cr# chromitites fall well within the compositional range for the experimentally produced orthopyroxene, when considering the variations of bulk H2O contents in the experimental studies. The compositions of orthopyroxenes from high-Cr# chromitites fall outside the range of Mg#-CaO and Mg#-Cr2O3 values covered by the experimentally produced orthopyroxene, except for one experiment conducted at 1 200 ℃ and with a bulk H2O content of ~4 wt.%. Therefore, the orthopyroxene in low-Cr# chromitites can be interpreted as having been produced by reactions between H2O-bearing boninitic magma and harzburgite, but the orthopyroxene in high-Cr# chromitites probably cannot be produced by those same reactions.

    Figure 5.  Graphs comparing CaO and Cr2O3 vs. Mg# for orthopyroxene produced in this study with those values for orthopyroxene from low-Cr# and high-Cr# chromitites, and orthopyroxene from the surrounding harzburgites and lherzolites. Low-Cr# and high-Cr#, compositions of orthopyroxene from low-Cr# and high-Cr# chromitites, respectively (Xiong F H et al., 2017a, 2015; Xiong Q et al., 2017). Lherzolite and Harzburgite, compositions of orthopyroxene from lherzolites and harzburgites, respectively (Xiong F H et al., 2017a, 2015; Xiong Q et al., 2017).

    The chromites in podiform chromitites typically contain a variety of mineral inclusions. The inclusions may represent the crystallization products of liquids/melts trapped during chromite crystallization. The presence of hydrous mineral inclusions, such as amphibole and phlogopite, indicates that the magmas from which the chromites crystallized were hydrous (Xiong Q et al., 2017; Griffin et al., 2016; Robinson et al., 2015; Zhou et al., 2014). Other mineral inclusions of orthopyroxene, clinopyroxene and olivine are also commonly observed within chromite grains, and these minerals are also thought to have crystallized from a hydrous magma during chromite grain precipitation. Mineral assemblages like those also exist in the run products from both the Cr-bearing and the Cr-poor experiments performed in this study. Orthopyroxene was observed in the products of all the Cr-bearing experiments, but olivine was only observed in the relatively high temperature experiments and clinopyroxene was only seen in the experiments at lower temperatures. This suggests that the experimental conditions adopted in this study were appropriate for constraining the temperatures that generate podiform chromitites.

  • Several different mechanisms have been proposed for podiform chromitite formation at shallow depths in the upper mantle, including magma/rock reaction model, and magma/ rock reaction followed by magmas mixing model (Payot et al., 2013; Rollinson and Adetunji, 2013; Shi et al., 2012; Uysal et al., 2009; Proenza et al., 2008, 1999; Shi et al., 2007; Gervilla et al., 2005; Arai, 1997; Arai and Yurimoto, 1994). In magma/ rock reaction model, researchers have suggested that in the first stage, primitive magma reacted with harzburgite and dissolved orthopyroxene in the first step, through the following reaction.

    As a result of this reaction, the primitive magma would become silica-rich and move into the chromite stability field, where chromite grains could be precipitated to form podiform chromitites and the dunites with which they are commonly associated. In the magma/rock reaction+magmas mixing model, a hypothesized mantle-derived olivine-chromite saturated melt would react with the orthopyroxene in harzburgite to form a secondary silica-rich melt and a dunite layer. Then the silica- rich melt could mix with the unaltered mantle-derived melt to produce a chromite oversaturated melt that precipitates chromite to form chromitites. For these models, an important issue concerns the chemical composition of the proposed mantle-derived magma related to the formation of podiform chromitites.

    Researchers have suggested low-Cr# chromitite can be formed from the interaction between basaltic magma and harzburgite (Xiong Q et al., 2017; González-Jiménez et al., 2011; Rollinson, 2008; Zhou et al., 1998; Leblanc, 1997). However, there are also some studies that have pointed out that the chromites in low-Cr# chromitites that plot in the field of MORB-like chromites are not likely to have been derived from MORB-like magmas (Zhou et al., 2014), because the magma/ rock interaction process may have changed the chemical compositions of the original magma and the precipitated chromites. In the present study, the experimental results have demonstrated that a reaction between H2O-bearing boninitic magma and harzburgite could produce chromites that match the compositions of natural chromites (such as Cr#, Mg#, NiO, and MnO) and associated orthopyroxenes (such as Mg#, CaO and Cr2O3) in low-Cr# chromitites. The importance of H2O/fluid in the formation of podiform chromitites in different tectonic settings has been demonstrated by many studies (Johan et al., 2017; Arai and Miura, 2016; Matveev and Ballhaus, 2002; Edwards et al., 2000; Gaetani et al., 1994), because H2O/fluid can help dissolve and precipitate chromites and is thought to be of secondary importance in addition to pure melt. We speculated here that H2O-bearing boninitic magma should be a good candidate for the formation of low-Cr# chromitites in a magma/rock interaction model. We emphasize that variable H2O contents in the original boninitic magma should significantly affect the chemical compositions of the precipitated chromites during magma/ harzburgite reactions in a shallow mantle wedge, whereas the variations in Cr2O3 content may have only a slight effect on the compositions of the precipitated chromites. In addition, we also note that the experimental formed chromites coexist with peridotitic minerals, and the proportions of orthopyroxene and clinopyroxene are higher than those observed in natural rocks. We suggest that this may have relation with the harzburgite: boninite ratios, as boninite magma is silica-oversaturated, thus higher content of boninite could result in the formation of more pyroxene and less olivine.

  • Early studies have proposed several models to explain the formation and tectonic evolution of podiform chromitites. (a) Boninitic magma reacted with lithospheric mantle in subduction zones (Zaccarini et al., 2011; Graham et al., 1996). (b) Basaltic magma reacted with depleted harzburgite in a mid-ocean-ridge setting and fertile lherzolite in transform-fault controlled mantle to form high-Cr# and low-Cr# chromitites, respectively (Leblanc, 1997). (c) Basaltic magma reacted with depleted harzburgite in a mid-ocean-ridge setting (or backarc spreading center) to form low-Cr# chromitites, and the interaction between boninitic magma and mantle peridotite in the later slab subduction stage to form high-Cr# chromitites (Xiong F H et al., 2017a; González-Jiménez et al., 2011; Uysal et al., 2009; Rollinson, 2008; Ahmed and Arai, 2002; Zhou et al., 1998, 1996; Melcher et al., 1997; Schiano et al., 1997). (d) Low-Cr# chromitites were formed by the interaction between basaltic magma and harzburgite at the initial stage of subduction, and high-Cr# chromitites were formed by the interaction between bonnitic magma and harzburgite in a shallow mantle wedge (Xiong Q et al., 2017; Zhou et al., 2014; Rollinson and Adetunji, 2013; Morishita et al., 2011). In this study, new experimental results show that the interaction between H2O- and Cr-bearing (Cr2O3≤8 wt.%) boninitic magma and harzburgite can produce chromite and orthopyroxene with compositions consistent with the compositions of those materials in low-Cr# chromitite bodies. This indicates that the low-Cr# chromitites should be formed in a shallow mantle wedge during slab subduction. Here we note that none of the experiments produced chromites with a Cr# value higher than ~70. This may imply that the mechanism that forms high-Cr# chromitites is different from the mechanism that forms chromites in the low-Cr# group. A third magma/fluid other than boninitic or basaltic magmas, or some other geological processes like recycled from the deep upper mantle or modified by melt/fluid from the upper mantle or a mantle wedge, might be involved in high-Cr# chromitite formation. Such fluids or processes could change the compositions of the precipitated chromites and may contribute to the formation and evolution of high-Cr# chromitites. On the other hand, the bulk Cr contents in the starting materials are limited (within ~4.2 wt.%), thus whether high-Cr# chromites can be formed due to the reaction between Cr-rich (Cr2O3 > 8 wt.%) boninitic magma and harzburgite is also unknown, as Cr is mainly incorporated in chromite (the solely Cr-rich phase) in the phase assemblages. Therefore, further studies are still needed to investigate the reactions between Cr-rich (Cr2O3 > 8 wt.%) boninitic magma and harzburgite, to provide more evidences for full understanding the formation mechanism chromites.

  • This study was supported by the National Programme on Global Change and Air-Sea Interaction (No. GASI-GEOGE- 02), the National Nature Science Foundation of China (Nos. 41772040, 91858104) and the Fundamental Research Funds for the Central Universities, Hohai University (No. 2013/B18020030). We thank Zhong Gao and Biji Luo for providing the natural boninite sample (QL-Bon), and Jihao Zhu and Jianggu Lu for technical support during EPMA analysis. We acknowledge the use of EPMA in the Key Laboratory of Submarine Geosciences, State Oceanic Administration. We thank Junlong Yang, Xiangfa Wang and Xingdong Zhou for technical support during high-pressure experiments. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1291-0.

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