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

• Published in: Journal of geophysical research. Solid earth 2014-11, Vol.119 (11), p.8034-8046
• Main Authors: Dymshits, Anna M., Litasov, Konstantin D., Sharygin, Igor S., Shatskiy, Anton, Ohtani, Eiji, Suzuki, Akio, Funakoshi, Kenichi
• Format: Article
• Language: English
• Subjects: acoustic velocity; Density; Derivatives; Diffraction; equation of state; Equations of state; experiment; Fittings; garnet; Garnets; Geophysics; High temperature; knorringite; Magnesium aluminum silicates; Mantle; Mineralogy; peridotite; Thermal expansion; Upper mantle; X-ray diffraction
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1002/2014JB011194

The P‐V‐T equation of state (EoS) for majoritic knorringite Mg3.19Cr1.60Si3.19O12 at pressures to 17 GPa and temperatures to 1673 K 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, K0,300 = 154 (4) GPa, and a pressure derivative, K′0,300 = 5.4 (1.2). When fitting a high‐temperature Birch‐Murnaghan EoS using all of the P‐V‐T data at a fixed V0 = 1549.08 Å3, we find that K0,300 = 157 (2) GPa, K′0,T = 4.5 (6), (∂K0,T/∂T)P = −0.019 (4) (GPa K−1), a = 3.00 (14) × 10−5 K−1, and b = 0.65 (24) × 10−8 K−2, where α = a + bT is the volumetric thermal expansion coefficient. Fitting the Mie‐Grüneisen‐Debye EoS with the present data with a Debye temperature fixed at θ0 = 731 K yielded a Grüneisen parameter, γ0 = 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 20 mol.% 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. Key Points We have conducted in situ X‐ray diffraction experiments on knorringiteThe P‐V‐T relations of knorringite garnet to 17 GPa and 1673 K was developedKnorringite admixture decreases P‐ and S‐velocities of pyrope garnets