The effects of temperature, oxygen fugacity and melt composition on the behaviour of chromium in basic and ultrabasic melts
The effects of temperature, oxygen fugacity and melt composition on the behaviour of chromium in basic and ultrabasic melts
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DOI：
10.1016/0016-7037(86)90245-0
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发表时间：
1986-09
影响因子：
5
通讯作者：
B. Murck;I. Campbell
中科院分区：
文献类型：
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作者：
B. Murck;I. Campbell
在适合地质条件的温度和氧逸度下，用两种天然碱性液体和一种天然超碱性液体平衡铬铁矿。这些实验旨在记录铁方铁矿和镍镍氧化物缓冲液之间矿物和玻璃成分的变化，特别强调沿石英-铁橄榄石-磁铁矿的条件。铬铁矿饱和状态下熔体的 Cr 含量随着温度的升高和氧逸度的降低而急剧增加。通过考虑结晶相和熔体相之间 FeCr2O4 组分的交换，描述了一种关系，该关系解释了尖晶石和硅酸盐熔体之间 Cr 分配的成分依赖性。根据这种交换对数据的解释表明，高铝熔体中的 Cr3+ 出现在八面体配位位点，并且它不依赖于单价阳离子的电荷平衡。在这个模型中，Cr3+被认为表现得像网络改性的Al3+和Fe3+，即过量的铝和三价铁，它们不参与四面体配位基体或网络形成络合物。该结果也可以应用于基性层状岩体中横向范围大的块状铬铁矿的形成问题。这些数据与一个模型一致，在该模型中，铬铁矿的结晶是通过岩浆混合引发的，并结合与岩浆周期性流入腔室相关的快速热损失。另一种模型是通过氧逸度的重复波动引发亚铬铁矿结晶，只有当岩浆 fO2 不受 QFM 等氧气缓冲剂控制时，才有可能实现。
Chromite was equilibrated with two natural basic liquids and one natural ultrabasic liquid at temperatures and oxygen fugacities appropriate to geological conditions. The experiments were designed to document changes in mineral and glass compositions between the iron-wüstite and nickel-nickel oxide buffers, with special emphasis on conditions along quartz-fayalite-magnetite. The Cr contents of the melts at chromite saturation increase strongly with increasing temperature and with decreasing oxygen fugacity.A relationship is described which accounts for the compositional dependence of the partitioning of Cr between spinels and silicate melts by considering the exchange of FeCr2O4component between the crystalline and melt phases. Interpretation of the data in terms of this exchange suggests that Cr3+in metaluminous melts occurs in octahedrally coordinated sites, and that it does not depend on charge-balancing by monovalent cations. In this model, Cr3+is proposed to behave like network-modifying Al3+and Fe3+,i.e., the excess aluminum and ferric iron which do not participate in tetrahedrally coordinated matrix or network-forming complexes.The results can also be applied to the problem of the formation of massive chromitites of great lateral extent in basic layered intrusions. The data are consistent with a model in which the crystallization of chromite is initiated through magma mixing, in combination with the rapid heat loss associated with periodic influxes of magma into a chamber. An alternative model, in which chromite crystallization is initiated by repeated fluctuations in oxygen fugacity, is possible only if the magmafO2is not controlled by an oxygen buffer such as QFM.