Experimental study of the joins forsterite–diopside–leucite– and forsterite–leucite–åkermanite up to 2.3 GPa [P(H2O) = P(Total)] and variable temperatures: Its petrological significance

We conducted a series of melting experiments in the join forsterite–diopside–leucite under 0.1 and 2.3 GPa and in the join forsterite–leucite–åkermanite under 2.3 GPa to understand paragenetic relationships amongst different types of lamproitic and lamprophyric magmas with K-rich mafic and ultramafic volcanic (kamafugitic) rocks. Both the joins were studied in the presence of excess water. The experimental results of the join forsterite–diopside–leucite at 0.1 GPa show that the five-phase point of forsterite (Fo)_ss + diopside (Di)_ss + leucite (Lc)_ss + liquid (Liq) + vapour (V) (equivalent to ugandite lava) occurs at Fo₂Di₅₀Lc₄₈ at 880 ± 5 °C. Phlogopite appears as the last phase at 830 ± 15 °C. The final crystalline assemblage of forsterite_ss + diopside_ss + leucite_ss + phlogopite is similar to the phenocryst assemblage of missourite lava. Present study suggests that an olivine leucitite (ugandite) can be derived from an olivine italite, a slightly potassic peridotite and a leucitite magma.

A study of the join Fo–Di–Lc [P(H₂O) = P(Total)] at 2.3 GPa shows that liquid compositions penetrate the primary phase volumes of forsterite_ss, phlogopite_ss, kalsilitess, K-feldspar_ss and diopside_ss. It has the following three five-phase points: 1) one occurring at Fo₉Di₄₉Lc₄₂ and 1005 ± 5 °C, where liquid and vapour coexists with forsterite_ss, phlogopite_ss and diopside_ss (phlogopite-bearing madupite), 2) the second one at Fo₄Di₅₀Lc₄₆ and 990 ± 10 °C, where diopside_ss, K-feldspar_ss and phlogopite_ss coexist with liquid and vapour (pyroxene-bearing minette), and 3) the third one at Fo₃Di₂₁Lc₇₆ and 775 ± 5 °C, where phlogopite_ss, kalsilite_ss and K-feldspar_ss are in equilibrium with liquid plus vapour (kalsilite-bearing minette).

The experimental results of the join Fo–Lc–åkermanite (Ak) show that the join 40 penetrates the primary phase volumes of forsterite_ss, phlogopite_ss, kalsilite, K-feldspar_ss, diopside_ss and merwinite_ss. The data indicate the presence of four five-phase points: 1) one occurring at Fo₇Lc₄₂Ak₅₁ and 1165 ± 5 °C, where phlogopite_ss, forsterite_ss, diopside_ss coexists with liquid and vapour (olivine-bearing madupite), 2) the second one at Fo₃Lc₄₉Ak₄₈ and 1140 ± 10 °C, where a liquid is in equilibrium with phlogopite_ss, K-feldspar_ss, diopside_ss and vapour (pyroxene-bearing minette), 3) the third one at Fo₁₈Lc₂₁Ak₆₁ and 1255 ± 10 °C, where merwinite_ss, forsterite_ss and diopside_ss are in equilibrium with liquid and vapour (merwinite-bearing wherlite), and 4) the fourth one at Fo₅Lc_73.5Ak_21.5 and 770 ± 5 °C, where kalsilite_ss, phlogopite_ss and K-feldspar coexist with liquid and vapour (kalsilite-bearing minette). The present data suggest that high pressure heteromorphic equivalent of a katungite magma is represented by a kalsilite-bearing minette, a pyroxene-bearing minette, or an olivine-bearing madupite.