Experimental study of the system diopside-nepheline-sanidine at 0.1, 1 and 2 GPa [P(H₂O)= P(total)]: its significance in the genesis of alkali-rich basic and ultrabasic rocks

Abstract

Bulk compositions of nephelinite, melanephelinite, trachyte and phonolite can be represented in the join diopside-nepheline-sanidine, which is a part of the quinary system, nepheline-kalsilite-CaO-MgO-SiO₂. Thirty-two starting compositions were made in this join. Experiments were conducted on these compositions at 0.1, 1 and 2 GPa [P(H₂O) = P(Total)] and variable temperatures to investigate phase equilibria relations. In the present study, we determined the compositions and temperatures of equilibrium of the four-phase points in the diopside-nepheline-sanidine join at [0.1, 1 and 2 GPa P(H₂O) = P(Total)]. All phases are found to be solid solutions. At 0.1 GPa, the join intersects the primary phase volume of forsterite solid solution. Two four-phase points were established, one at Diopside₂₆Nepheline₆₆Sanidine₈and 990 °C where liquid co-exists with forsterite solid solution + diopside solid solution + nepheline solid solution and the other point at Diopside₁₁Nepheline₃₁Sanidine₅₈ and 865 °C at which diopside solid solution + nepheline solid solution + leucite solid solution is a stable suprasolidus phase assemblage. At 1 GPa liquid co-exists with diopside solid solution + nepheline solid solution + sanidine solid solution at Diopside₄Sanidine₆₅Nepheline₃₁ and 670 °C. The four-phase point (diopside solid solution + nepheline solid solution + sanidine solid solution + liquid) at 2 GPa was located at Diopside₃Sanidine₇₃Nepheline₂₄. The results at 0.1 GPa show that a nepheline-bearing leucitite can be derived from a leucitite, a nepheline-bearing italite or an olivine-bearing nephelinite by crustal differentiation processes. After reaction with liquid, leucite disappears at 815 °C near the solidus, and the final assemblage corresponds to a pyroxene-bearing phonolite. The present experimental results at 1 and 2 GPa suggest that a pyroxene-bearing phonolite can be generated either from a pyroxene trachyte or a nephelinite or a pyroxene-poor phonolite magma, and a phonolite melt itself can be derived at depths 30-35 (1 GPa) or 60-65 km (~2 GPa).