Open AccessBehavior of the cotectic curve En‐Ol in the system leucite‐olivine‐quartz under dry conditions to 2.0 GPa
Author(s) -
Conceição R. V.,
Green D. H.
Publication year - 2000
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2000gc000071
Subject(s) - leucite , enstatite , sanidine , geology , olivine , quartz , mineralogy , analytical chemistry (journal) , plagioclase , materials science , chemistry , ceramic , chondrite , composite material , physics , chromatography , paleontology , meteorite , astronomy
The system leucite‐olivine (Fo 90 )‐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 (Liq 1 ) ↔ enstatite (En) + leucite (Lc) + liquid (Liq 2 ) peritectic occurs at 1205°C and Lc 31.7 Qz 66.5 Ol 1.8 , while at 1.0 GPa, at 1265°C, it has moved to Lc 38 Qz 58 Ol 4 as the leucite field contracts. A second peritectic reaction, Lc + Liq 3 ↔ Sa + En + Liq 4 , occurs at 1.0 GPa and ∼1260°C (Lc 37 Qz 60 Ol 3 ). 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 (Lc 46 Qz 47.5 Ol 6.5 ). At 2.0 GPa the leucite field contracts farther toward the leucite corner, and the peritectic reaction En + Liq 6 ↔ Ol + Sa + Liq 7 takes place at 1350°C (Lc 50 Qz 43 Ol 7 ). 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 Na 2 O 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 CO 3 = and/or fluorine.