An Experimental Study of the Influence of Oxygen Fugacity on Fe-Ti Oxide Stability, Phase Relations, and Mineral—Melt Equilibria in Ferro-Basaltic Systems
An Experimental Study of the Influence of Oxygen Fugacity on Fe-Ti Oxide Stability, Phase Relations, and Mineral—Melt Equilibria in Ferro-Basaltic Systems
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DOI：
10.1093/petrology/36.5.1137
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发表时间：
1995-10
影响因子：
3.9
通讯作者：
M. Toplis;M. Carroll
中科院分区：
文献类型：
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作者：
M. Toplis;M. Carroll
平衡结晶实验在大气压下，并在一定范围内的氧逸度（fO 2）已进行了铁玄武组合物类似的液体建议已父母的大部分Skaergaard入侵的暴露部分。在Fe-Ti氧化物饱和之前，fO 2对液相下降线的影响很小。然而，磁铁矿-尖晶石固溶体（Mt）和钛铁矿-赤铁矿固溶体（Ilm）的出现温度强烈依赖于fO 2。在铁橄榄石-磁铁矿-石英（FMQ）缓冲层Mt上方是出现在液相线上的第一氧化物相，但在FMQ缓冲层Ilm下方是结晶的第一氧化物。Mt的出现温度在FMQ时为1100°C，Mt液相线斜率在FMQ−;2和FMQJ+1之间为130 °C/logfO 2单位。Ilm的液相线在FMQ和FMQ−2之间的1100°C处，但在较高的fO 2处移动到较低的温度，其中Mt是第一氧化物相。结果表明，熔体中三价铁含量与温度呈线性关系，熔体中二价铁饱和度与熔体中TiO 2含量密切相关。Mt饱和在熔体相中产生SiO2的立即富集和FeO* 的耗尽，而Ilm饱和产生类似的SiO2富集，但Inn富集可在钛铁矿液相线以下持续约10°C。实验液体达到最大值的18wt%FeO *，在48wt%SiO2的钛铁矿饱和熔体在低fO 2，更分化的熔体具有较低的铁和较高的二氧化硅。来自质量平衡计算的CO2比例与天然样品和其他实验研究的数据吻合良好。橄榄石的再吸收被推断为在所有fO 2下发生，再吸收的开始发生温度比磁铁矿的出现温度高10°C。O2对硅酸盐矿物成分的影响，以及共存矿物-熔体对之间的元素分配，是小的。热力学的考虑表明，在这项研究中产生的富铁橄榄石，辉石和熔体之间的铁-镁分配的变化可以解释由已知的非理想的铁-镁混合的结晶相，而不是在共存的熔体的非理想性。这些实验还提供了对天然火山岩和深积岩拉斑系列的许多共同特征的见解，并提供了模拟分离结晶和接近氧气的结晶所需的实验数据，这些过程不容易通过实验进行研究。
Equilibrium crystallization experiments at atmospheric pressure and over a range of oxygen fugacity (fO2) have been carried out on a ferro-basaltic composition similar to liquids proposed to have been parental to much of the exposed portion of the Skaergaard intrusion. Before Fe-Ti oxide saturation the liquid line of descent is little affected byfO2. However, the appearance temperatures of the magnetite-ulvöspinel solid solution (Mt) and the ilmenite-haematite solid solution (Ilm) depend strongly onfO2. Above the fayalite-magnetite-quartz (FMQ) buffer Mt is the first oxide phase to appear on the liquidus, but below the FMQ buffer Ilm is the first oxide to crystallize. The appearance temperature of Mt is ∼1100°C at FMQ and the Mt liquidus slope is ∼30°C/logfO2unit between FMQ−;2 and FMQJ+1. The Ilm liquidus is at ∼1100°C between FMQ and FMQ−2, but moves to lower temperature at higherfO2where Mt is the first oxide phase. The results indicate that the ferric iron content of Mt-saturated melts varies linearly with inverse temperature, and that Ilm saturation is closely related to melt TiO2content. Mt saturation produces an immediate enrichment of SiO2and depletion in FeO*in the melt phase, whereas Ilm saturation produces similar enrichment in SiO2, but inn enrichment may continue for ∼10°C below the ilmenite liquidus. The experimental liquids reach a maximum of ∼18 wt% FeO*, at ∼48 wt% SiO2for ilmenite-saturated melts at low fO2, more differentiated melts having lower iron and higher silica. Cotectic proportions, derived from mass balance calculations, are in good agreement with data from natural samples and other experimental studies. Olivine resorption is inferred at allfO2, with the onset of resorption occurring ∼10°C higher than the appearance of magnetite. The effect offO2on silicate mineral compositions, and partitioning of elements between coexisting mineral-melt pairs, is small. Thermodynamic considerations suggest that variations of Fe-Mg partitioning between the iron-rich olivines, pyroxenes and melts produced in this study may be explained by known non-idealities of Fe-Mg mixing in the crystalline phases, rather than nonidealities in the coexisting melts. These experiments also provide insights into many features common to natural tholeiitic series of volcanic and plutonic rocks, and provide experimental data required for modelling of fractional crystallization and crystallization closed to oxygen, processes which are not easily investigated experimentally.