Compressional behavior of end-member and aluminous iron-bearing diopside at high pressure from single-crystal X-ray diffraction and first principles calculations

Diopside (\(CaMgSi_{2}O_{6}\)), the Ca- and Mg-rich clinopyroxene is an important mineral in the Earth’s upper mantle and subducted lithospheric plate. Here, we report the results of high-pressure single-crystal X-ray diffraction experiments conducted on a natural aluminous iron-bearing diopside and a natural, nearly end-member diopside, up to 50 GPa in diamond anvil cell. Density functional theory calculation results on end-member diopside are also reported. Unit cell parameters a, b, c, \(\beta\), V, as well as bond lengths of diopside are reported and compared with other clinopyroxenes. Bulk modulus and its pressure derivative of the two diopside samples are determined using third-order Birch–Murnaghan equation of state. The density of the two diopside samples is calculated under cold subducting slab conditions and is compared with the seismic models. Along the cold slab geotherm, aluminous iron-bearing diopside has higher density than end-member diopside. In the upper mantle, eclogite with aluminous iron-bearing diopside is denser than eclogite with end-member diopside, and, therefore, provides larger slab pulling force. At the bottom of the transition zone and the top of the lower mantle, eclogite with aluminous iron-bearing diopside, though higher in density than the end-member diopside, is still less dense than the surrounding mantle and could contribute to the slab stagnation.