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

摘要

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 SiO(2)content at 55 wt.%, MgO at about 12 wt.% and Cr(2)O(3)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 degrees C, and the mineral was joined by plagioclase and clinopyroxene at and below 1166 degrees 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 degrees 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 offO(2)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 increasedfO(2)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 degrees C) and increased the proportion of magnetite component in spinel solid solution. The addition of 4 wt.% H2O to the starting composition at 1200 degrees C resulted in complete melting of orthopyroxene, which left Cr-spinel as the only liquidus phase. Equal amount of H2O at 1125 degrees 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.