Nepheline formation in chondrite parent bodies: Verification through experiments
Nepheline formation in chondrite parent bodies: Verification through experiments
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球粒陨石母体中霞石的形成：通过实验验证
DOI：
10.1016/j.gca.2017.04.025
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发表时间：
2017
影响因子：
5
通讯作者：
Y. Seto and K. Tomeoka
中科院分区：
文献类型：
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作者：
S. Ichimura;Y. Seto and K. Tomeoka
霞石以细粒形式存在，主要赋存于CV、CO碳质球粒陨石中的难熔包裹体和球粒陨石中。霞石主要由难熔包裹体中的黄长石和斜长石以及球粒陨石中的斜长石和玻璃交代而成。霞石的形成被认为是在陨石母体的水蚀变和热变质过程中形成的。为了验证这一假设，我们进行了以下实验：在200℃、∼15℃、pH分别为0、7、13和14的水热实验中，钠离子浓度均一，对钙辉石(富铝黄长石)和斜长石(An48)进行水热实验。在硅灰石实验中，加入了不同量的二氧化硅。结果表明，在pH值为7、13和14时，钙镁石/二氧化硅10/3和10/6的混合物，在pH 13和14时，钙镁石/二氧化硅和斜长石的10/10的混合物生成方沸石。特别是在pH 14的条件下，除方解石外，还有两种沸石：方沸石和羟基沸石。差热分析表明，辉沸石和羟基沸石均有向霞石转变的放热峰，且随着升温速率的降低，放热峰的温度逐渐降低。利用这些数据进行的动力学分析表明，如果加热102yr，辉石在<；600℃转变为霞石；如果加热<；1yr，羟基橄榄石在<；430℃转变为霞石。方解石以1℃/min的非等温加热速率在高于等温实验所确定的温度下转变为霞石，这表明如果加热时间更长，其转变温度也会降低。结果表明，陨石中难熔包裹体中的霞石和陨石球粒中的霞石经历了两个阶段的蚀变过程：(1)黄长石、斜长石和玻璃在少量富钠和碱性水溶液存在下，在低温(可能为<；300℃)的水热作用下形成钠沸石；(2)钠沸石在溶液消失后，由于高温(300-700℃)加热很长时间而转变为霞石。
Nepheline is present as fine grains mainly in refractory inclusions and chondrules in CV and CO carbonaceous chondrites. The nepheline has been formed primarily by replacement of melilite and plagioclase in refractory inclusions and plagioclase and glass in chondrules. The nepheline formation is thought to have occurred during aqueous alteration and thermal metamorphism in the meteorite parent bodies. To verify this hypothesis, we performed the following experiments.Hydrothermal experiments of gehlenite (Al-rich melilite) and plagioclase (An48) were carried out at 200 °C and ∼15 bar for 168 h using solutions of pH 0, 7, 13, and 14 with a uniform Na concentration. In the gehlenite experiments, various amounts of SiO2were added. The results revealed that a Na zeolite, analcime, was produced from 10/3 and 10/6 mixtures of gehlenite/SiO2at pH 7, 13, and 14, and from a 10/10 mixture of gehlenite/SiO2and plagioclase at pH 13 and 14. In particular, at pH 14, in addition to analcime, significant amounts of two other zeolites, fabriesite and hydroxycancrinite, were produced from the 10/6 mixture of gehlenite/SiO2, and fabriesite from plagioclase.Isothermal heating experiments for 24 h showed that fabriesite, hydroxycancrinite, and analcime transform to nepheline at 600–650, 550–600, and 750–800 °C, respectively. Differential thermal analysis of these zeolites revealed that fabriesite and hydroxycancrinite exhibit exothermic peaks, which correspond to transformation to nepheline, and that the temperatures of those peaks decrease steadily with decreasing heating rate. Kinetic analysis using these data revealed that fabriesite transforms to nepheline at <600 °C if heated for durations >102yr and hydroxycancrinite transforms to nepheline at <430 °C if heated for durations >1 yr. Analcime heated non-isothermally at a rate of 1 °C/min transformed to nepheline at temperature higher than that determined by the isothermal experiments, suggesting that its transformation temperature also decreases if it is heated for a much longer duration. From these experiments and analyses, we conclude that fabriesite, hydroxycancrinite, and possibly analcime are capable of transforming to nepheline by heating in meteorite parent bodies.From our results, we propose that the nepheline in refractory inclusions and chondrules in meteorites formed by a two-stage alteration process: (1) formation of the Na zeolites from melilite, plagioclase, and glass by hydrothermal alteration at low temperature (probably <300 °C) in the presence of small amounts of Na-rich and alkaline aqueous solutions, and (2) transformation of the Na zeolites to nepheline due to heating at higher temperature (300–700 °C) for very long durations after the solutions were lost.