Compositional effect on the pressure derivatives of bulk modulus of silicate melts

Although the bulk moduli (KT0) of silicate melts have a relatively narrow range of values, the pressure derivatives of the isothermal bulk modulus (KT0′) can assume a broad range of values and have an important influence on the compositional dependence of the melt compressibility at high pressure. Based on the melt density data from sink/float experiments at high pressures in the literature, we calculate KT0′ using an isothermal equation of state (EOS) (e.g., Birch–Murnaghan EOS and Vinet EOS) with the previously determined values of room-pressure density (ρ0) and room-pressure bulk modulus (KT0). The results show that best estimates of KT0′ vary considerably from ~ 3 to ~ 7 for different compositions. KT0′ is nearly independent of Mg # (molar Mg/(Mg + Fe)), but decreases with SiO2 content. Hydrous melts have anomalously small KT0′ leading to a high degree of compression at high pressures. For anhydrous melts, KT0′ is ~ 7 for peridotitic melts, ~ 6 for picritic melts, ~ 5 for komatiitic melts, and ~ 4 for basaltic melts.