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 0Co), enstatite (en), spinel (sp), anorthite (an), sapphirine (sa), and corundum (cor). Increasing pressure causes (1) the f o and an primary phase fields to contract, (2) the en, q, and cor fields to expand, (3) t h e f o e n 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 phases Liquid composition (wt %) Temperature (~ C) s p + s a + c o r AnstFotTQ~ 1575 f o + en + sp AnszF%gQ9 t 540 e n + s p + s a An59Fo31Qlo 1490 s a + a n + c o r AnToFOlsQls 1430 an+ q + cor An66FovQ27 1410 sa + an + en An62Fot sQ2o t400 an + en + q AnsgFol 5Q26 1380 It is concluded that (J) the shift of the f o e n 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.