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Experimental study on the effects of H2O upon crystallization in the Lower and Critical Zones of the Bushveld Complex with an emphasis on chromitite formation

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Abstract

Mafic–ultramafic layered rocks of the Lower and Critical Zones of the Bushveld Complex are believed to have formed from a parental magma usually labelled B1 with SiO2 content at 55 wt.%, MgO at about 12 wt.% and Cr2O3 concentration at 0.15 wt.%. Previous experimental study at 300 MPa and nominally dry conditions showed that this composition had orthopyroxene on liquidus between 1250 and 1300 °C, and the mineral was joined by plagioclase and clinopyroxene at and below 1166 °C. The objective of this study was to investigate the effects of variable additions of H2O (2, 4 and 6 wt.% in starting compositions) on liquidus and sub-liquidus phase equilibria. Main attention was given to the solubility of Cr-spinel and Cr distribution between melt and crystalline phases. Six experiments were carried out in the internally heated pressure vessel (IHPV) at 300 MPa and in the temperature interval 1100–1300 °C. Redox conditions were not controlled and thermal dissociation of the H2O component followed by hydrogen diffusion through container walls resulted in a noticeable oxidation of run products. The effective values of fO2 in experiments were calculated using SPINMELT 2.0 software bases on the regression of experimental data on the spinel-melt equilibrium. According to the calculations, H2O additions in isothermal series of experiments increased fO2 by 0.8–1.6 log-units from the level in a nominally dry run. Overall, the experiments showed that H2O additions suppressed crystallization of silicates, shifted olivine-orthopyroxene peritectic equilibrium towards higher olivine stability (olivine first appears in H2O-bearing compositions at 1175 °C) and increased the proportion of magnetite component in spinel solid solution. The addition of 4 wt.% H2O to the starting composition at 1200 °C resulted in complete melting of orthopyroxene, which left Cr-spinel as the only liquidus phase. Equal amount of H2O at 1125 °C leads to complete melting of plagioclase, orthopyroxene and clinopyroxene, which leaves only olivine and spinel stable and increases the liquid mass fraction from 0.62 in the nominally dry compositions to 0.9. The results of this study appear to support the idea that chromitite layers in the Critical Zone of the Bushveld Complex were formed by episodes of hydration melting of silicate cumulates.

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Acknowledgements

We are grateful to Oona Appelt and Cora Wohlgemuth-Ueberwasser for their help with electron microprobe and LA ICP-MS analyses of the run products. Thoughtful reviews by Alan Boudreau and Ed Mathez helped us to improve the original version of the paper. This work has been supported by DFG grant VE 619-6-1. T.H. acknowledges support by China Nature Foundation of Sciences (41922912 and 91962102) and Sino-German NFSC-DFG project (41761134086; HO1337/39-1), MOST Special Fund from the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (MSFGPMR201804) and 111 Project (B18048).

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Correspondence to Tong Hou.

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Communicated by Timothy L. Grove.

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Veksler, I.V., Hou, T. Experimental study on the effects of H2O upon crystallization in the Lower and Critical Zones of the Bushveld Complex with an emphasis on chromitite formation. Contrib Mineral Petrol 175, 85 (2020). https://doi.org/10.1007/s00410-020-01733-w

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