K. Okamoto et S. Maruyama, MULTI-ANVIL RE-EQUILIBRATION EXPERIMENTS OF A DABIE SHAN ULTRAHIGH-PRESSURE ECLOGITE WITHIN THE DIAMOND-STABILITY FIELDS, Island arc, 7(1-2), 1998, pp. 52-69
An eclogite (UHP303) formed at similar to 700 degrees C and P > 4 GPa
fi om the Dabie Shall ultrahigh-pressure (UHP) terrane and the same sa
mple + 1.1 wt% K2O (UHK303) were used for re-equilibrium experimental
studies at P = 2.6-3.2 GPa and T = 900-1000 degrees C, employing a pis
ton cylinder apparatus, and at P = 3-7 GPa and T = 900-1200 degrees C,
employing a 1000 toll uniaxial multi-anvil apparatus (SPI-1000). Run
products of UHP303 are composed of garnet, sodic augite, with or witho
ut a SiO2 phase and phengite. Textural evidence indicates that the sol
idus lies between 900 and 1000 degrees C at 5 GPa, and above 1000 degr
ees C at 3 and 7 GPa for UHP303. The solidus temperature decreases to
1000 degrees C at 3-5 GPa and below 900 degrees C at 5 GPa in UHK303.
Composition of rock-forming minerals of all run products have determin
ed and compared with these of stal ting materials. Gal nets of all run
products have a relatively higher almandine + spessartine component (
alm + sps 24-30 mol%) and lower grossular component (12-16) with const
ant pyrope component (58-61), compared to the stal ting gal net (grs 1
6-26, alm 21-26, pyp, 42-61). The stable Ca-Na pyroxenes are sodic aug
ite rather than omphacite at T higher than 1000 degrees C in the P = 3
-7 GPa pressure range, and the Cpx compositional gap is completely clo
sed. The K2O content of Ca-Na pyroxenes is a function of P and T, and
serves as a potential barometer in the diamond-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 cond
ition is suggested to have been 7 GPa at T = 900 degrees C.