Stability of phengite and biotite in eclogites and characteristics of biotite- or orthopyroxene-bearing eclogites

Stability of phengite and biotite in eclogite is discussed using petrological data of natural eclogites, and the observational data are examined by thermodynamic calculations. Generally, phengite is a major K phase in natural eclogite and is stable in wide range of bulk composition. However, in eclogites from several localities of the Caledonides, biotite occurs as a stable eclogite-facies mineral, and is often associated with orthopyroxene. Bulk compositions of such biotite- or orthopyroxene-bearing eclogites are compared with those of eclogites from the Dabie-Sulu region, China, where phengite is a major K phase in eclogite. The biotite- or orthopyroxene-bearing eclogites from the Western Gneiss Region of the Caledonides are rich in MgO (10-15 wt%) and relatively poor in CaO (7-8 wt%) and Al₂O₃ (12-16 wt%). The CaO/MgO ratios of the biotite- or orthopyroxene-bearing eclogites are clearly lower than those of eclogites from the Dabie-Sulu region, indicating that MgO-rich and CaO-poor environments should be important for stabilizing of biotite and orthopyroxene in eclogite. Biotite-bearing eclogite from the North-East Greenland Eclogite Province is rich in MgO (≈ 16 wt%) and CaO (≈ 15.5 wt%) and extremely poor in Al₂O₃ (≈ 8 wt%). To stabilize biotite in eclogite, Al₂O₃-poor environments are also important. Bulk compositions of these biotite- or orthopyroxene-bearing eclogites are similar to picrite basaltic compositions. To examine these observational data, thermodynamic calculations were carried out in a seven-component system Firstly, calculations were performed on the average bulk composition of eclogites from the Dabie-Sulu region to lherzolite (KLB-1). The calculation results confirmed that phengite should be stable in eclogite with 'ordinary' basaltic composition, whereas biotite and orthopyroxene should be stable in picrite basaltic compositions (e.g. MgO > 11.0 wt%, CaO < 9.8 wt%, Al₂O₃ < 15.2 wt% at 700 °C, 2.5 GPa). Further calculations in basaltic system confirmed that increase of MgO content and decrease of CaO and Al₂O3 contents were important to stabilize biotite and orthopyroxene in eclogite. Thus, mineral assemblage in picrite basalt system should be completely different from that in normal basaltic system.