Liquidus Phase Relations in the System CaO–MgO–Al2O3–SiO2 at 2·0 GPa: Applications to Basalt Fractionation, Eclogites, and Igneous Sapphirine

with suggestions that fractionation of tholeiitic basalt in this pressure To model magmatic crystallization processes for mafic to intermediate range yields an alkalic basalt. Both trends may occur for natural compositions at high pressure, liquidus phase relations in the basalts depending on the Na2O content of the parental magma. forsterite–anorthite–diopside–silica (FADS) tetrahedron within the Also, the data show that the minimum pressure for the formation CaO–MgO–Al2O3–SiO2 system have been determined at 2·0 GPa. of cumulate eclogites and garnet pyroxenites is about 1·8–1·9 GPa. Compositions of five liquidus invariant points have been determined The lower limit of pressure at which sapphirine can crystallize and the approximate compositions of five others have been inferred. from a liquid in the FADS tetrahedron is estimated to be 1·1–1·5 These involve primary phase volumes for forsterite (fo), enstatite GPa and the upper limit is >3 GPa. Sapphirine crystallizes from (en), diopside (di), high quartz (qz), spinel (sp), sapphirine (sa), magmas intermediate in composition between basalt and andesite. garnet (gt), anorthite (an), and corundum (cor). The determined Probable igneous sapphirine in mafic associations is rare, but it (with wt % coefficients) and inferred reactions (without coefficients) occurs as part of a pyroxenite xenolith from Delegate, Australia, that define each isobaric invariant point are as follows: that we suggest is a cumulate assemblage and in a sapphirine norite 23 en + 68 di + 9 sp = 84 liq + 16 fo at Wilson Lake, Labrador, Canada. 37 di + 63 sa = 47 liq + 40 sp + 13 en 100 gt = 21 liq + 27 sa + 55 en + 18 di 1 di + 59 en + 41 an = 43 liq + 57 gt 18 di + 21 qz + 15 en + 47 an = 100 liq