Multi‐technique equation of state for Fe₂SiO₄melt and the density of Fe‐bearing silicate melts from 0 to 161 GPa

Abstract

We have conducted new equation of state measurements on liquid Fe 2 SiO 4 in a collaborative, multi‐technique study. The liquid density ( ρ ), the bulk modulus ( K ), and its pressure derivative ( K ′) were measured from 1 atm to 161 GPa using 1‐atm double‐bob Archimedean, multi‐anvil sink/float, and shock wave techniques. Shock compression results on initially molten Fe 2 SiO 4 (1573 K) fitted with previous work and the ultrasonically measured bulk sound speed ( C o ) in shock velocity ( U S )‐particle velocity ( u p ) space yields the Hugoniot: U S = 1.58(0.03) u p + 2.438(0.005) km/s. Sink/float results are in agreement with shock wave and ultrasonic data, consistent with an isothermal K T = 19.4 GPa and K ′ = 5.33 at 1500°C. Shock melting of initially solid Fe 2 SiO 4 (300 K) confirms that the Grüneisen parameter ( γ ) of this liquid increases upon compression where γ = γ o ( ρ o / ρ ) q yields a q value of –1.45. Constraints on the liquid fayalite EOS permit the calculation of isentropes for silicate liquids of general composition in the multicomponent system CaO‐MgO‐Al 2 O 3 ‐SiO 2 ‐FeO at elevated temperatures and pressures. In our model a whole mantle magma ocean would first crystallize in the mid‐lower mantle or at the base of the mantle were it composed of either peridotite or simplified “chondrite” liquid, respectively. In regards to the partial melt hypothesis to explain the occurrence and characteristics of ultra‐low velocity zones, neither of these candidate liquids would be dense enough to remain at the core mantle boundary on geologic timescales, but our model defines a compositional range of liquids that would be gravitationally stable.