Density measurements of basaltic magma at high-pressure and high-temperature using X-ray absorption method

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Density of magma is one of the important physical properties to control the transportation of magma in the Earth and planetary interiors. We have established the new technique to measure the density of magma under pressure by using large volume high-pressure apparatus and synchrotron radiation, on which X-ray absorption is based. We have measured the density of mid-ocean ridge basalt (MORB) magma up to about 5 GPa and 2000K by suing high-pressure X-ray absorption technique, which is the most abundant magma on the Earth and the Earth-like planets. Experiments were carried out using the cubic press, SMAP1, installed at BL22XU of SPring-8, where a highly brilliant monochromatic X-ray is available. The basaltic magma was confined in the single crystal diamond capsule that is X-ray transparent rather than silicate, the hardest material, which is uniformly deformed under pressure, and chemically inert with magma. Absorption of X-ray from the basaltic magma was measured up to 4.6 GPa and 1900 K, and density was calculated from Lambert-Beer law, I = I_0exp(-μpt). X-ray absorption measurements give the density of basaltic magma along its melting curve. Fitting the Birch-Murnaghan equation of state to density data yields anomalous negative pressure derivative of the bulk modulus of the basaltic magma. This means the basaltic magma becomes more compressible at higher pressure, opposite to the normal crystalline solid. This result strongly indicates that the structure of basaltic magma is changing with increasing pressure, including the shrinkage of networks of SiO_4 and AIO_4 tetrahedra and the increase of the coordination number of aluminum ions.