The join forsterite-anorthite-diopside with 0.3% M_??_Cr₂O₄ at 1 atm and its petrolo_??_ic si_??_nificance: A preliminary report

Abstract

Phase relations in the join forsterite (Mg₂SiO₄, Fo)-anorthite (CaAl₂Si₂O₈, An)-diopside (CaMgSi₂O₆, Di) with 0.3 wt% MgCr₂O₄ was determined in air at 1 atm. Spinel (sp) primary field is enlarged compared with that in the Cr-free limiting join Fo-An-Di at the expense of anorthite (an). The piercing points, fo (forsterite)+an+di (diopside)+liquid and fo+an+sp+liquid in the Cr-free join disappear, and two new “piercing points” are present; one is di_ss (ss: solid solution)+sp_ss+fo+liquid at 1279°±3°C and the other at 1277°±3°C di_ss+sp_ss+an+liquid. The “univariant lines” passing through these points meet at 1270°±5°C where spinel is dissolved and another “univariant line” di+fo+an+liquid is generated, indicating that the presence of a minor amount of Cr gives significant effect on the phase relations in the silicate system, and that the join Fo-An-Di with 0.3% MgCr₂O₄ (0.24% Cr₂O₃) cannot be thermal divide in the basalt tetrahedron. Therefore, alkali basalt magma can produce tholeiitic basalt by fractional crystallization at lower pressure if a minor amount of Cr₂O₃ (more than 0.2%) is present.