2. Given that $A B C D-A_{1} B_{1} C_{1} D_{1}$ is a unit cube, $O_{1}$ is the center of the base $A_{1} B_{1} C_{1} D_{1}$. $M$ is the midpoint of edge $B B_{1}$. Then the volume of the tetrahedron $O_{1} A D M$ is $\qquad$ .