The isotope effect, E = (D-alpha/D-beta - 1)/(root m(beta)/m(alpha) - 1), o
f cobalt diffusion in the deeply supercooled liquid state of the bulk metal
lic glass formers Zr46.7Ti8.3Cu7.5Ni10Be27.5 and Zr65Cu17.5Ni10Al7.5 has be
en measured employing the radio-tracers Co-57 and Co-60 in conjunction with
serial sectioning by means of ion-beam depth profiling. In both alloys a s
mall mass dependence with E approximate to 0.10 is observed. This dependenc
e has no temperature dependence in a range of 120 K in the supercooled liqu
id state encompassing four orders of magnitude in the diffusivity. We sugge
st that these results show that long-range diffusion in a deeply supercoole
d liquid is not mediated by viscous flow but proceeds by collective hopping
processes involving about 10 atoms. (C) 1999 Elsevier Science B.V. All rig
hts reserved.