Behavior of the cotectic curve En-Ol in the system leucite-olivine-quartz under dry conditions to 2.0 GPa

The system leucite-olivine (Fo₉₀)-quartz has been experimentally studied from 0.5 to 2.0 GPa under dry conditions, and the position of the invariant point involving olivine, enstatite, K-rich phase (leucite or sanidine), and liquid has been determined. We find that at 0.5 GPa the olivine (Ol) + liquid (Liq₁) 虠 enstatite (En) + leucite (Lc) + liquid (Liq₂) peritectic occurs at 1205°C and Lc_31.7Qz_66.5Ol_1.8, while at 1.0 GPa, at 1265°C, it has moved to Lc₃₈Qz₅₈Ol₄ as the leucite field contracts. A second peritectic reaction, Lc + Liq₃ 虠 Sa + En + Liq₄, occurs at 1.0 GPa and 1260°C (Lc₃₇Qz₆₀Ol₃). With increasing pressure from 1.0 GPa the stability field of sanidine increases such that at 1.5 GPa the subsolidus reaction En + Lc = Ol + San intersects the solidus, defining an invariant point at 1290°C (Lc₄₆Qz_47.5Ol_6.5). At 2.0 GPa the leucite field contracts farther toward the leucite corner, and the peritectic reaction En + Liq₆ 虠 Ol + Sa + Liq₇ takes place at 1350°C (Lc₅₀Qz₄₃Ol₇). The Ol + En + Liq boundary moves from the silica-oversaturated field at 0.5 and 1.0 GPa into the silica-saturated field at 1.5 GPa and to the silica-undersaturated field at 2.0 GPa. Our new data, together with earlier studies of anhydrous and hydrous Lc-Ol-Qz systems for 2.8 GPa, suggest that highly potassic (i.e., with very low Na₂O contents), silica-oversaturated magmas with minor contents of water could be direct partial melts of a potassic-harzburgite or phlogopite-bearing lherzolite mantle at pressures of nearly 1.5 GPa and temperatures of about 1200°-1300°C. Generation of strongly undersaturated K-rich magmas requires higher pressures (>2.0 GPa) and is favored by the presence of dissolved carbon as CO₃⁼ and/or fluorine.