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.

中文翻译：

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水对布什维尔德杂岩体下部和临界区结晶影响的实验研究，重点是铬铁矿的形成

Bushveld 杂岩体下部和临界区的镁铁质-超镁铁质层状岩石被认为是由母体岩浆形成的，通常标记为 B1，其中 SiO2 含量为 55 wt.%，MgO 含量约为 12 wt.%，Cr2O3 浓度为 0.15 wt.%。 %。先前在 300 MPa 和名义干燥条件下的实验研究表明，该组合物在 1250 至 1300 °C 之间的液相线上具有斜方辉石，并且该矿物在 1166°C 及以下与斜长石和单斜辉石结合。本研究的目的是研究不同的 H2O 添加量（起始组合物中的 2、4 和 6 重量%）对液相线和亚液相线相平衡的影响。主要关注Cr-尖晶石的溶解度和熔体和结晶相之间的Cr分布。在 300 MPa 的内部加热压力容器 (IHPV) 和 1100–1300 °C 的温度区间内进行了六个实验。氧化还原条件不受控制，H2O 组分热解离，随后氢气通过容器壁扩散，导致运行产物明显氧化。实验中 fO2 的有效值使用 SPINMELT 2.0 软件根据实验数据对尖晶石-熔体平衡的回归进行计算。根据计算，在等温系列实验中添加 H2O 使 fO2 比名义上的空运行中的水平增加了 0.8-1.6 log-单位。总的来说，实验表明添加 H2O 抑制了硅酸盐的结晶，将橄榄石-斜方辉石包晶平衡向更高的橄榄石稳定性方向移动（橄榄石在 1175 °C 时首次出现在含 H2O 的组合物中），并增加了尖晶石固溶体中磁铁矿组分的比例。在 1200 °C 下向起始组合物中添加 4 wt.% H2O 导致斜方辉石完全熔化，留下 Cr-尖晶石作为唯一的液相线。在 1125 °C 下等量的 H2O 导致斜长石、斜方辉石和单斜辉石完全熔化，只留下橄榄石和尖晶石稳定，并将液体质量分数从名义干燥组合物中的 0.62 增加到 0.9。这项研究的结果似乎支持了布什维尔德杂岩体临界区的铬铁矿层是由硅酸盐堆积物的水合熔化事件形成的观点。