4:45 PM - 5:00 PM
[PPS06-06] Hydrothermal experiments of anorthite with implications for Na-metasomatism on carbonaceous chondrite parent body
Keywords:nepheline, sodalite, chondrite, aqueous alteration
We used natural plagioclase (An95) as a starting material. The reaction solution of pH 0, 7, 8.5, 10, 11.5, 13 and 14 were prepared for the hydrothermal experiments with different water/rock (W/R) ratios (46.7 ml/g). Na+ concentration in solutions is maintained at 1, 3 and 5.5 mol/l by addition of NaCl (thereby high Na+ concentration means high Cl- concentration). Hydrothermal alteration experiments were performed with a PTFE reaction vessel loaded into steel autoclave at 200 °C and 1.5 MPa for run duration time of 168 hours. The recovered samples from the hydrothermal experiments were analyzed by XRD, SEM, TEM. As results of experiments using Na+ = 1 mol/L solutions, no secondary phase was observed under pH 11.5, while sodalite and hydroxycancrinite were formed as secondary phases at pH 13. Moreover, fabriesite and hydroxycancrinite were formed at pH 14. In the case of Na+ = 3 and 5.5 mol/L, large amounts of secondary phases (mainly sodalite) were formed over pH 11.5 condition. Considering of the previous (Ichimura et al., 2017) and present results, we can conclude the following three point: (1) Anorthite-rich plagioclase (low Si/Al ratio) are favorably altered into zeolites with low Si/Al ratio (e.g., sodalite, fabriesite and hydroxycancrinite) rather those with high Si/Al ratio (e.g., analcime). (2) With increasing solution pH, Si/Al ratio in the secondary phase becomes low. (3) Using Na+ and Cl- concentration in solution, significant amounts of secondary phases were formed even under moderate alkaline condition.