Density and structure of basaltic magma under high pressure and high temperature

The density of magma is one of the important properties for discussing evolution of magma ocean at the early history of the planets and magmatic activity in the planetary mantle. We have measured the density of basaltic melt at high temperature and high pressure by X-ray absorption method. The experiments were carried out using a DIA-type cubic press at BL22XU of the SPring-8. X-ray absorption method is accurate method for the density measurement under desired pressure and temperature compared to the other methods. This method for density measurements was originally developed by Katayama et al. (1993). It is based on the Lambert-Beer"fs law. The sample was placed in a diamond capsule to calibrate the sample thickness and the X-ray absorption profile of the sample was measured by ion chambers. We succeeded in measuring the density of basaltic melt up to 4.6 GPa and up to 2000 K. We obtained the compression curve of basaltic melt by using the Birch-Murnaghan equation of state with a negative pressure derivative of bulk modulus (dK/dP). A negative dK/dP might be caused by the structural change of the silicate melts, although it is unlikely in crystals. The structure of magma is based on continuous three-dimensional networks of corner-sharing SiO4 and AlO4 tetrahedra, as being derived from a network of tetrahedrally coordinated Si and Al atoms each linked to four others through a shared O atom. The principal mechanisms of compression for silicate melts involve continuous changes in T-O-T bond angles and bond lengths. Silicate melts might undergo continuous and gradual changes in topology and cation coordinations. In order to further understand these changes and how they are affected by the microscopic structure, we have conducted the energy-dispersive X-ray diffraction to determine the structure of the basaltic melt up to 5 GPa. High pressure and high temperature X-ray diffraction experiments on basaltic melts were carried out by the energy dispersive method using SPEED Mk-II apparatus installed at the BL04B1 of SPring-8. Diffracted X- rays were collected at 2theta of 4, 5, 7, 9, 11, 15, 18 and 22 using a solid state detector system. We obtained the structure factor, S(Q), from the raw X-ray diffraction data using an analytical program developed by Funakoshi (1997). The first peak of the S(Q) reflects the -T-O-T- intermediate-range silicate network. In this study, the first peak shifts to higher Q with increasing pressure, while the second peak does not change. This result indicates that the intermediate-range network in basaltic melt changes to denser structure (e.g., squeezing the open space and shrinking the network rings) without shortening the intra- tetrahedral atomic distance with increasing pressure.