Equations of state of Na–K–Al host phases and implications for MORB density in the lower mantle

Abstract

We report equations of state (EoS) of two important silicate phases known to store Na, K, and Al, at pressures and temperatures prevailing in the Earth’s lower mantle. Using laser heated diamond anvil cell, we produced the Ca-ferrite type (CF) and the new aluminum rich (NAL) phases using different synthetic and natural starting materials, at pressures ranging from 23 to 65 GPa. Unit cell volumes were investigated by means of angle dispersive X-ray diffraction at the ID30 beamline of the ESRF synchrotron ring (Grenoble, France). CF-phases were synthesized in NaAlSiO 4–MgAl 2O 4 and NaAlSiO 4–(Mg,Fe) 2SiO 4 systems, and for a natural mid-ocean ridge basalt (MORB). The insertion of MgAl 2O 4 in the NaAlSiO 4-based lattice has reduced effects on the CF-phase EoS, whereas that of (Mg,Fe) 2SiO 4 is mainly to increase the room pressure volume. CF-phases most relevant to the Earth’s mantle show compression behavior comparable to the pure NaAlSiO 4-CF-phase, with bulk moduli values close to K 0=190 GPa and K 0′=4. We also synthesized (NAL) phases in NaAlSiO 4–MgAl 2O 4 system, and using CaNa- and K-rich silicate glasses as starting materials. All NAL-compounds investigated show similar compression behavior, except for one anomalously NaAlSiO 4-rich composition. For NAL-phase with the most “natural” conditions relevant composition, we refine bulk moduli of K 0=182 GPa and K 0′=6. We then model the density profiles expected for the CF and NAL-phases, using our newly determined EoS and the most accurate chemical compositions reported in the literature. These two phases appear significantly less dense than a pyrolitic type material at all lower mantle pressures and 300 K. However, the density profiles calculated for mineral assemblages corresponding to typical MORB compositions point out to denser MORBs compared to the pyrolite. This is due to the high Fe-content in MORB, yielding a densest Fe-rich perovskite that counterbalances the lower densities of the CF and NAL-phases. The density gap is calculated between 0.4 and 4.0% depending on the starting MORB composition and pressure.