An experimental study of the joins leucite-akermanite-albite and leucite-akermanite-albite_(50)anorthite_(50) at atmospheric pressure and 1 GPa (H_2O-saturated conditions): Its significance in the leucite-sodic plagioclase and melilite-plagioclase incomp

摘要

Leucite-sodic plagioclase and melilite-plagioclase do not coexist in natural rocks. However, leucite and calcium-rich plagioclase may remain in equilibrium in alkaline silica-deficient rocks. The join leucite-akerman-ite-albite studied at atmospheric pressure is characterized by two four-phase points: one point occurs at leucite_(31) akermanite_(47)albite_(22) and 1275 ℃, where leucite_(ss), melilite_(ss) and diopside_(ss) coexist in equilibrium with liquid, and the other point occurs at leucite_(38)akermanite22albite4o and 1250 ℃, where diopside_(ss), alkali feldspar, leucite_(ss) and liquid are in equilibrium. In the case of the join leucite-akermanite-albite_(50)anorthite_(50) investigated at atmospheric pressure, there are two four-phase points: one point occurs at leucite_(14)akermanite_(48)(albite5oanorthite_(50))_(38) and 1260 ℃, where melilite_(ss), leucite_(ss), diopside_(ss) and liquid coexist in equilibrium, and the other point occurs at Ieucite_(15)akermantite_(35)(albite_(50)anorthite_(50))_(50) and 1240 ℃, where diopside_(ss), leucite_(ss) and calcium-rich plagioclase are in equilibrium with liquid. When the join leucite-akermanite-albite_(50)anorthite_(50) was studied at 1 GPa (in H_2Osaturated conditions) to determine the effect of pressure, it was noted that the field of melilite_(ss) completely disappears, and this join is characterized by a single four-phase point occurring at Ieucite_(43)akermanite_(22)(albite_(50)anorthite_(50))_(35) and 1060 ℃, where diopside_(ss), calcium-rich plagioclase, and leucite_(ss) are in equilibrium with liquid. The present study indicates that: 1) leucite and sodic plagioclase do not coexist in equilibrium. When leucite is in equilibrium with feldspar, it is alkali feldspar having the compositional range, An_(3-4)Ab_(50-53)Or_(45-47). 2) In the presence of leucite, akermanite and albite react to form diopside_(ss), nepheline_(ss), wollastonite_(ss), and alkali feldspar. 3) In the join leucite -akermanite-albite_(50)anorthite_(50), plagioclase coexisting with leucite has the compositional range, An_(60-66)Ab_(17-26) Or_(12-17). 4) In the join leucite-akermanite-albite_(50)anorthite_(50) studied at 1 GPa (in H_2O-saturated conditions), melilite is completely eliminated.