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

岩浆密度是控制地球和行星内部岩浆运移的重要物理性质之一。我们建立了以x射线吸收为基础的大体积高压仪器和同步辐射测量压力下岩浆密度的新技术。利用高压x射线吸收技术测量了高达5gpa和2000K的洋中脊玄武岩（MORB）岩浆密度，它是地球和类地行星上最丰富的岩浆。实验使用安装在SPring-8的BL22XU上的立方压机SMAP1进行，在那里可以获得高亮度的单色x射线。玄武岩岩浆被限制在x射线透明的单晶金刚石胶囊中，而不是硅酸盐。硅酸盐是最坚硬的物质，在压力下均匀变形，与岩浆一起发生化学惰性。玄武岩岩浆对x射线的吸收高达4.6 GPa和1900 K，密度由Lambert-Beer定律计算，I = I_0exp（-μpt）。x射线吸收测量给出了玄武岩岩浆沿其熔化曲线的密度。将Birch-Murnaghan状态方程拟合密度数据，得到玄武岩岩浆体积模量的异常负压导数。这意味着玄武岩岩浆在更高的压力下变得更容易压缩，这与正常的结晶固体相反。这一结果强烈表明玄武岩岩浆的结构随着压力的增加而发生变化，包括SiO_4和AIO_4四面体网络的收缩和铝离子配位数的增加。

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.