Multi-anvil re-equilibration experiments of a Dabie Shan ultrahigh-pressure eclogite within the diamond-stability fields

An eclogite (UHP303) formed at ~700°C and P >4 GPa from the Dabie Shan ultrahigh-pressure (UHP) terrane and the same sample +1.1 wt% K₂O (UHK303) were used for re-equilibrium experimental studies at P = 2.6-3.2 GPa and T = 900-1000°C, employing a piston cylinder apparatus, and at P = 3-7 GPa and T = 900-1200°C, employing a 1000 ton uniaxial multi-anvil apparatus (SPI-1000). Run products of UHP303 are composed of garnet, sodic augite, with or without a SiO₂ phase and phengite. Textural evidence indicates that the solidus lies between 900 and 1000°C at 5 GPa, and above 1000°C at 3 and 7 GPa for UHP303. The solidus temperature decreases to 1000°C at 3-5 GPa and below 900°C at 5 GPa in UHK303. Composition of rock-forming minerals of all run products were determined and compared with those of starting materials. Garnets of all run products have a relatively higher almandine + spessartine component (alm + sps 24-30 mol%) and lower grossular component (12-16) with constant pyrope component (58-61), compared to the starting garent (grs 16-26, alm 21-26, pyp 42-61). The stable Ca-Na pyroxenes are sodic augite rather than omphacite at T higher than 900°C in the P = 3-7 GPa pressure range, and the Cpx compositional gap is completely closed. The K₂O content of Ca-Na pyroxenes is a function of P and T, and serves as a potential barometer in the diamont-stability field. Sodium content in garnet is negligible. Using the proposed K₂O content in Ca-Na pyroxene as a function of pressure, the maximum Kokchetav UHP condition is suggested to have been 7 GPa at T = 900°C.