  Motivated by the recent report of a high Li6 ``plateau'' extending to low
metallicities in Galactic halo stars, we study the energetics of an early
production of Li6 through the interaction of energetic particles with the
interstellar medium. We then explore the potential of various candidate sources
of pre-galactic energetic particles and show that, in general, they fail to
satisfy the observational and theoretical requirements (especially if the Li6
plateau is at a considerably higher level than observed). Succesful candidates
appear to be: supernova explosions with abnormally low metal yield; the massive
black hole in the Galactic center (provided that it was formed early on and
that it was then radiating much more efficiently than today); and, perhaps, an
early accretion phase of supermassive black holes in galaxies. Assuming that
Li6 is indeed pre-galactic, we study the galactic evolution of the light
isotopes Li7, Li6, and Be in a self-consistent way. We find that the existence
of a Li6 plateau is hard to justify, unless a fine-tuned and
metallicity-dependent depletion mechanism of Li6 in stellar envelopes is
invoked. The depletion of Li6 should be different, both in magnitude and in its
metallicity dependence, than the depletion required to explain current
observations of Li (mostly Li7) in halo stars. If the recently reported Li6
``plateau'' is confirmed, our analysis suggests important implications for our
understanding of the production, evolution, and stellar depletion of the Li
isotopes.
