Experimental study of the system diopside- albite -nepheline at P(H₂O)= P(total)= 2 and 10 kbar and at P(total)= 28 kbar

Abstract

Bulk compositions of many nephelinites, melanephelinites, urtites, phonolites and tephrites plot in the system diopside - albite - nepheline. A study of the system under various pressures and temperatures both in the presence and absence of H₂O thus has relevance in the genesis of these rock types. Sixty-five experiments were conducted on 20 starting compositions at 28 kbar under dry conditions. The primary phase volumes of nepheliness, clinopyroxene and albitess were encountered. The four-phase assemblage Cpx + Pl + Qtz + L occurs at 1200 ± 5° C toward the albite-rich side of the subsystem, whereas in the nepheline-rich portion, the three-phase assemblage of Cpx + Ness + L is obtained at 1249± 5° C. The clinopyroxene is a solid solution in the series CaMgSi₂O₆ (diopside) . NaAlSi₂O₆ (jadeite) at 28 kbar. This confirms the observation of Bell & Davis (1969), who suggested complete solid-solution between the two phases at this pressure and above. A nephelinitic or a melanephelinitic magma on further crystallization yields a phonolitic liquid. The same system was also studied at P(H₂O) = P(total) = 10 and 2 kbar. Eighteen experiments were done at 10 kbar, and the gisobaric invarianth point was observed at Di₄Ab_46.6Ne_49.4 and 835 ± 5°C, whereas this point lies at Di₅Ab₆₆Ne₂₉ (885 ± 5°C) at P(H₂O) = P(Total) = 2 kbar. The invariant point at 10 kbar is a reaction point, where the equilibrium assemblage comprises Cpx + Ne_ss + Ab_ss + L + V. The reaction point at 2 kbar corresponds to the composition of many nepheline-bearing mafic to felsic rocks. The present data at 10 and 2 kbar [P(H₂O) = P(Total)] indicate that a phonolitic liquid can be obtained by the following schemes of fractionation: i) melanephelinite→nephelinite→phonolite, and ii) "trachyte"→phonolite.