Computational modeling of arterial thrombus formation based on patient-specific data holds promise as a non-invasive tool for preventive diagnosis of atherosclerotic lesions. Platelet transport to the surface of a growing thrombus may be a rate limiting step in rapid thrombus formation, so accurate modeling of platelet transport may be essential for computational modeling of arterial thrombus formation. The presence of red blood cells (RBCs) in blood greatly affects platelet transport. In flowing blood, RBCs migrate away from the walls and platelets marginate toward the walls. We investigate the mechanics of platelet margination by direct simulation of cellular blood flow. We show that platelet margination can be explained by RBC-enhanced shear-induced diffusion of platelets in the RBC-filled region combined with platelet trapping in the RBC-free region. A simple continuum model is introduced based on the proposed mechanism. Using an experimental correlation for effective diffusivity in blood, the continuum model can recover experimental results from the literature over a wide range of tube diameters.