Compositional and temperature effects on sulfur speciation and solubility in silicate melts
Compositional and temperature effects on sulfur speciation and solubility in silicate melts
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DOI：
10.1016/j.epsl.2018.12.006
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发表时间：
2019-02
影响因子：
5.3
通讯作者：
William M. Nash;D. Smythe;B. Wood
中科院分区：
文献类型：
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作者：
William M. Nash;D. Smythe;B. Wood
我们已经确定了溶解的硫的化学形态和硫浓度在固定的氧和硫逸度为广泛的硅酸盐熔体组合物（从富铁玄武岩英安岩）。每种熔体在1300° C和1个大气压下平衡，氧逸度（fO 2）相对于铁橄榄石-磁铁矿-石英（FMQ）缓冲液在− 1.67和+ 1.6 log单位之间，绝对硫逸度在− 5.1和− 1.2 log单位之间。实验采用COCO 2 SO2混合气体控制fO 2和fS 2。采用X射线吸收近边谱（XANES）、二次离子质谱（西姆斯）和电子探针（SEM）测量了玻璃中硫的平衡浓度与fS 2/fO 2比值的关系，并对淬火玻璃中硫的形态进行了分析。随着fO 2的增加，各熔体中溶解态硫的形态发生了从S2-到S6+的突变，并且当熔体FeO浓度从1.5wt%增加到1.18wt%时，这种突变的fO 2值增加了0.5log单位。由于在fO 2和fS 2恒定的情况下，对于更多的富FeO熔体，硫化物浓度始终更高，因此组成对形态形成的影响可以通过熔体的硫化物容量（CS 2−）对FeO浓度的众所周知的敏感性来解释。玻璃的S6 +/S2 −比值与Fe 3+/Fe 2+呈线性关系，表明铁和硫的氧化还原对可以直接相互关联。我们使用热力学数据来模拟Fe和S氧化态之间的相互关系，根据平衡FeS+ 8 FeO 1.5= 8 FeO+ FeSO 4将数据拟合到我们在1300° C的实验中，我们获得了以下的温度依赖性物种形成的表达式：log ψ（S 6+ S 2−）= 8 log ψ（Fe 3+ Fe 2+）+ 8.7436× 10 6 T 2− 27703 T+ 20.273该方程适用于我们所有成分的数据，并且与1050° C和950° C时的早期结果一致。我们使用S和Fe氧化态的相互依赖性来推断来自夏威夷莫纳克亚山的玻璃在淬火过程中Fe 2+和S6+之间的电子转移。这一效应足以导致天然玻璃中平衡Fe 3+/Fe 2 O3 Fe的显著高估和fO 2的相应高估约0.8log单位。在最氧化条件下平衡的玻璃（仅含S6+）具有与四面体（Si+ Ti）阳离子的摩尔分数呈负相关的平衡S浓度。
We have determined the chemical speciation of dissolved sulfur and the sulfur concentration at fixed oxygen and sulfur fugacities for a wide range of silicate melt compositions (from Fe-rich basalt to dacite). Each melt was equilibrated at 1300° C and 1-atmosphere pressure at oxygen fugacities (fO 2) between− 1.67 and+ 1.6 log units relative to the Fayalite–Magnetite–Quartz (FMQ) buffer and absolute sulfur fugacities between− 5.1 and− 1.2 log units. The fO 2 and fS 2 of the experiments were controlled by using gas mixtures of CO–CO 2–SO 2. The speciation of sulfur in the quenched glasses was determined using both X-ray Absorption Near-Edge Spectroscopy (XANES), and from the dependence of equilibrium sulfur concentration on the fS 2/fO 2 ratio measured by secondary-ion mass spectrometry (SIMS) and electron microprobe. The speciation of dissolved sulfur in each melt undergoes an abrupt transformation from S 2− to S 6+ with increasing fO 2, and this transition is shifted∼ 0.5 log units higher in fO 2 as melt FeO concentration increases from∼ 5 wt% to∼ 18 wt%. Since sulfide concentrations at constant fO 2 and fS 2 are consistently greater for more FeO-rich melts, the compositional effect on speciation may be explained by the well-known sensitivity of the sulfide capacity (C S 2−) of the melt to FeO concentration. S 6+/S 2− ratios for the glasses exhibit a linear relationship with Fe 3+/Fe 2+, indicating that the redox couples for iron and sulfur can be directly related to one another. We used thermodynamic data to model the interrelationship between Fe and S oxidation states in terms of the equilibrium FeS+ 8 FeO 1.5= 8 FeO+ FeSO 4 Fitting the data to our experiments at 1300° C we obtained the following expression for the temperature-dependence of speciation: log(S 6+ S 2−)= 8 log(Fe 3+ Fe 2+)+ 8.7436× 10 6 T 2− 27703 T+ 20.273 This equation fits the data for all our compositions and is also consistent with earlier results at 1050° C and 950° C. We used the interdependence of S and Fe oxidation states to infer electron transfer between Fe 2+ and S 6+ during quenching of glasses from Mauna Kea, Hawaii. The effect is sufficient to cause significant overestimation of equilibrium Fe 3+/ΣFe in natural glasses and corresponding overestimate of fO 2 by about 0.8 log units. Glasses equilibrated under the most oxidizing conditions (containing S 6+ only) have equilibrium S concentrations that are negatively correlated with their mole fractions of tetrahedral (Si+ Ti) cations.