LIQUIDUS PHASE-RELATIONSHIPS ON THE JOIN ANORTHITE-FORSTERITE-QUARTZ AT 20 KBAR WITH APPLICATIONS TO BASALT PETROGENESIS AND IGNEOUS SAPPHIRINE

Liquidus phase relationships determined on the join anorthite-forsterite-quartz at 20 kbar show primary phase fields for quartz (q), forsterite (fo), enstatite (en), spinel (sp), anorthite (an), sapphirine (sa), and corundum (cor). Increasing pressure causes (1) the fo and an primary phase fields to contract, (2) the en, q, and cor fields to expand, (3) the fo-en boundary line to move away from the Q apex, (4) the en-q boundary line to move also away from the Q apex but by a smaller amount, and (5) a primary phase field for sa to appear at a pressure between 10 and 20 kbar. Seven liquidus piercing points at 20 kbar have been located as follows:Crystalline phasesLiquid composition (wt %)Temperature (°C)sp+sa+corAn81Fo17Q21575 fo+en+spAn52Fo39Q91540 en+sp+saAn59Fo31Q101490 sa+an+corAn70Fo15Q151430 an+q+corAn66Fo7Q271410 sa+an+enAn62Fo18Q201400 an+en+qAn59Fo15Q261380 It is concluded that (1) the shift of the fo-en boundary line away from the quartz apex with pressure confirms many other studies showing that melts generated from peridotite source rocks decrease in silica content as pressure increases; (2) even though plagioclase is not a stable phase in peridotite source rocks at pressures above about 10 kbar, it can crystallize from mafic magmas at pressures at least up to 20 kbar; (3) sapphirine crystallizes from mafic magmas at high pressures and may be important in the construction of petrogenetic models based on trace element considerations. © 1990 Springer-Verlag.