The system forsterite-diopside-akermanite-leucite and its significance in the origin of potassium-rich mafic and ultramafic volcanic rocks

Abstract

The phase relations of forsterite (Fo), diopside (Di), akermanite (Ak), and leucite (Lc), important end members of naturally-occurring potassium-rich mafic and ultramafic volcanic rockg were determined experimentally at atmospheric pressure. The following three joins were determined in the study: (l) diopside-akermanite- leucite, (2) forsterite--diopside-leucite, and (B) forsterite-akermaniteleucite. The phase assemblages, compositions, and temperatures of the piercing points in these joins are summarized below: Join (1) Di_ss + Ak_ss + Lc_ss + L Di₃₉Ak₂₉Lc₃₂ 1281° ± 3°C (2) Fo_ss + Di_ss + Lc_ss + L Fo₃Di₆₀Lc₂₇ 1296° ± 3°C (3) Fo_ss + Mo_ss + Ak_ss + L Fo₁₇Ak₇₈Lc₅ l428° ± 3°C (3) Fo_ss + Ak_ss + Lc_ss + L Fo₉Ak_43.5Lc_47.5 1286° ± 3°C. The join diopside-akermanite-leucite with 3 percent forsterite wa,s also investigated to determine the temperature of the 5-phase assemblagq forsterite forsterite_ss diopside_ss + akermanite_ss + leucite_ss + liquid. The fifth phase (forsterite_ss) in this assemblage eppears at 1274° ± 4°C and the solidus temperature occurs at 1255°C. Study of the system suggests that an olivine-melilite leucitite magma ean be generated from the following magma tyqes: katungite, melilite leucitite, olivine melilitite, or olivine leucitite. A katungite magma of composition lying within the tetrahedron forsterite-diopside-akermanite-leucite can be derived from a monticellite alnoite magma because of a reaction relationship of monticellite with liquid.