  There has been recent progress in the study of the angular momentum evolution
of low mass stars (Krishnamurthi et al 1997a). Theoretical models can now be
constructed which reproduce the angular momentum evolution of low mass open
cluster stars and the distribution of initial conditions can be inferred from
young clusters. In this poster we report on the application of these models to
the problem of rotational mixing in halo stars. The distribution of initial
conditions inferred from young clusters produces a well-defined halo lithium
"plateau" with modest scatter and a small population of outliers. Different
choices for the solar calibration produce a range of absolute depletion
factors. We show that both the dispersion and the ratio of $^6$Li depletion to
$^7$Li depletion increase as the absolute $^7$Li depletion increases. The
measured $^6$Li in HD 84937 and the dispersion in the plateau set independent
upper bounds on the $^7$Li depletion. Consistency with open clusters and the
Sun, along with claims of an intrinsic dispersion in the plateau, set a lower
bound. We derive a range of 0.2-0.4 dex $^7$Li depletion in halo field stars.
Implications for cosmology are discussed.
