Thermal equation of state of majoritic knorringite and its significance for continental upper mantle

摘要

The P-V-T equation of state (EoS) for majoritic knorringite Mg3.19Cr1.60Si3.19O12 at pressures to 17GPa and temperatures to 1673K was obtained from in situ X-ray diffraction experiments using a Kawai-type multi-anvil apparatus. Fitting of the room-temperature P-V data to a third-order Birch-Murnaghan EoS yielded an isothermal bulk modulus, K-0,K-300=154 (4) GPa, and a pressure derivative, K-0,K-300=5.4 (1.2). When fitting a high-temperature Birch-Murnaghan EoS using all of the P-V-T data at a fixed V-0=1549.08 angstrom(3), we find that K-0,K-300=157 (2) GPa, K-0,K-T=4.5 (6), (K-0,K-T/T)(P)=-0.019 (4) (GPaK(-1)), a=3.00 (14)x10(-5)K(-1), and b=0.65 (24)x10(-8)K(-2), where =a+bT is the volumetric thermal expansion coefficient. Fitting the Mie-Gruneisen-Debye EoS with the present data with a Debye temperature fixed at (0)=731K yielded a Gruneisen parameter, (0)=1.34 at q=1.0 (fixed). The thermoelastic parameters of pure knorringite were estimated and were compared with the previous data on other garnet compositions. The presence of Cr2O3 in pyrope garnets in the upper mantle decreases P- and S-velocities by 1.6% and the density increases by 1.7% (for 20mol.% knorringite end member) compared to pure pyrope. The results show the importance of accounting knorringite end-member for accurate estimation of mantle garnet acoustic velocities.