Experimental Study of Crystal-Liquid Relationships at High Pressures in Olivine Nephelinite and Basanite Compositions

The crystallization sequences in olivine-rich nephelinitic and basanitic compositions have been experimentally studied under dry conditions at pressures up to 36 kb. Electron microprobe analyses of olivines, clinopyroxenes, garnets, and orthopyroxenes enable calculation of possible crystal fractionation trends for these magmas at various pressures. Low-pressure fractionation is dominated by olivine and yields derivative liquids of higher silica content and showing rapid iron enrichment. At pressures of 18–27 kb, fractionation is controlled by aluminous clinopyroxene with minor olivine or garnet. Derivative liquids show marked depletion in calcium accompanying silica depletion and increasing degree of undersaturation. .At pressures greater than 27 kb, crystal fractionation is controlled by garnet+clinopyroxene separation. Chemical analyses of these phases allow quantitative calculations of possible fractionation which show that large degrees of crystallization are required to produce quite small changes in silica content and in degree of undersaturation. In addition, fractionation by garnet and clinopyroxene separation is accompanied by depletion in calcium content in the more undersaturated rocks and high degrees of crystallization are necessarily accompanied by enrichment in iron relative to magnesium. These effects are inconsistent with the characteristics of natural magmas of mantle derivation in the range from alkali olivine basalts to olivine melilitites. It is concluded that separation of garnet and clinopyroxene under upper mantle conditions does not produce the natural magma series from olivine-rich tholeiite to olivine nephelinite and olivine melilitite. The transient role of orthopyroxene over a very small P, T range in the melting interval of two of the experimental compositions suggests that an olivine-rich basanite may be developed by small degrees of partial melting of a source pyrolite under dry conditions at 60–80 km depth. This liquid, which would form in equilibrium with residual olivine, aluminous orthopyroxene, and aluminous clinopyroxene, would contain approximately 5 per cent normative orthoclase, 5 per cent albite, 12 per cent nepheline, 20 per cent anorthite, 22 per cent diopside, and 31 per cent olivine.