2015 年 124 巻 3 号 p. 429-443
Mantle xenoliths provide snapshots of lithospheric mantle processes at the time of an eruption. Their fluid inclusions have the potential to enhance the resolution of images. Identifying major volatile compositions of fluid inclusions provides accurate figures for mantle fluid and various aspects of the lithospheric mantle. Fluid inclusions in mantle xenoliths are generally composed of CO2 coexisting occasionally with smaller amounts of CO. We can calculate the oxygen fugacity of a host xenolith from the relative abundance of CO. Isotopic characteristics of elements in fluid inclusions are useful when considering the origins of fluid inclusions. The fluid density of a fluid inclusion is also a useful depth probe for mantle xenoliths. The internal pressure of fluid inclusions was equilibrated with ambient pressure before the mantle xenoliths were trapped by magma. Thus, we can estimate the depth provenance of mantle xenoliths from fluid density. Combining fluid-inclusion geobarometry with some other characteristics of both fluid inclusion and host mantle xenoliths enables us to assess the three-dimensional structure of the lithosphere from various viewpoints. Fluid inclusions in mantle xenoliths serve as an index for investigating both the elastic and plastic strength of mantle minerals. Changes in ambient pressure and temperature of mantle xenoliths during entrainment by host magma engender elastic changes in the volumes of constituent minerals, thereby inducing small changes in the fluid density of fluid inclusions. Thus, we can investigate elastic properties of mantle minerals from the gradation of fluid density among minerals in a mantle xenolith. In addition, the dislocation density around fluid inclusions in minerals reflects the degree of plastic deformation of the host crystal in response to the overpressure of fluid during entrainment. Thus, observations of fluid density and dislocations are an effective new probe for elucidating rheological properties of the lithospheric mantle. Consequently, fluid inclusions in the mantle xenoliths provide a unique way of exploring the lithospheric mantle.