Nepheline formation in chondrite parent bodies: Verification through experiments

Abstract

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 ∼15bar for 168h using solutions of pH 0, 7, 13, and 14 with a uniform Na concentration. In the gehlenite experiments, various amounts of SiO2 were added. The results revealed that a Na zeolite, analcime, was produced from 10/3 and 10/6 mixtures of gehlenite/SiO2 at pH 7, 13, and 14, and from a 10/10 mixture of gehlenite/SiO2 and 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 24h 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 >1yr. 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.