铬铁矿和镁铬铁矿在硅酸盐熔体中溶解度的新模型
New model of chromite and magnesiochromite solubility in silicate melts
摘要：
铬铁矿是一种重要的岩浆矿物，常被用来指示镁铁质岩浆结晶的物理化学条件。在这项工作中，在1440°C和大气压力下，在受控的CO-CO2气体混合物（对应于低于铁橄榄石-磁铁矿-石英缓冲层的两个对数单位到高于两个对数单位的范围）下，研究了镁铬铁矿和铬铁矿在天然玄武岩和不含铁的单玄武岩中的溶解度。铬的来源是天然铬铁矿或合成Cr2 O3，后者与玄武岩液体反应形成镁铬铁矿。铬的最高浓度的单玄武岩熔体中发现在平衡与镁铬铁矿，取决于氧化还原条件。详细地，在低fO 2下，液体具有高Cr含量，但log [Cr，ppm]的变化不是log fO 2的线性函数。利用我们的新数据和文献中的数据，铬浓度在铬铁矿/镁铬铁矿饱和硅酸盐熔体的模型已开发的基础上的热力学形式主义。我们的模型可以用来评估熔体组成的影响，在硅酸盐熔体中的铬铁矿和镁铬铁矿溶解度橄榄岩熔融和同化。在中等和高氧逸度，铬在铬铁矿饱和度的浓度水平较高的超镁铁岩比长英质岩石。我们的数据表明，镁铬铁矿蛇纹岩岩石圈的同化作用可能导致镁铁质熔体中的高Cr含量，触发铬铁矿的大量结晶，特别是在系统的冷却，氧化和岩浆脱气。该模型可用于定量预测铬铁矿结晶和以铬铁矿和纯橄榄岩为代表的陆壳幔过渡带和地幔的形成。
英文摘要：
Chromite is a key magmatic mineral frequently used as petrogenetic indicator of physico-chemical conditions of mafic magma crystallization. In this work, magnesiochromite and chromite solubility in a natural basalt and an iron-free haplobasalt was investigated at 1440°C and atmospheric pressure under controlled CO-CO2 gas mixtures corresponding to the range two log units below to two log units above the fayalite-magnetite-quartz buffer. The source of chromium was either natural chromite or synthetic Cr2O3, the latter reacting with the basaltic liquids to form a magnesiochromite. The highest concentrations of Cr in haplobasaltic melts are found in equilibrium with magnesiochromite, depending on redox conditions. In detail, at low fO2, liquids have high Cr contents, but the variation of log [Cr, ppm] is not a linear function of log fO2. Using our new data and data from the literature a model for Cr concentrations at chromite/magnesiochromite saturation in silicate melts has been developed based upon a thermodynamic formalism. Our model may be used to assess the effect of melt composition on chromite and magnesiochromite solubility in silicate melts during peridotite melting and assimilation. At moderate and high oxygen fugacities, concentration levels of Cr at chromite saturation are higher for ultramafic than for felsic rocks. Our data imply that assimilation of magnesiochromite-bearing serpentinite lithosphere could result in high Cr contents in mafic melts, triggering massive crystallization of chromite, especially upon the system hydridization, cooling, oxidation and magma degassing. Our model may be applied for quantitative prediction of chromite crystallization and formation of the terrestrial mantle-crust transition zones and mantle represented by chromitites and dunites.
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