  We investigate if there is a difference in the lithium abundances of stars
belonging to two halo populations of F and G main-sequence stars previously
found to differ in [alpha/Fe] for the metallicity range -1.4 < [Fe/H] < -0.7.
Li abundances are derived from the LiI 6707.8 A line measured in
high-resolution spectra using MARCS model atmospheres. Furthermore, masses of
the stars are determined from the logTeff - logg diagram by interpolating
between Yonsei-Yale evolutionary tracks. There is no significant systematic
difference in the lithium abundances of high- and low-alpha halo stars. For the
large majority of stars with masses 0.7 < M/M_sun < 0.9 and heavy-element mass
fractions 0.001 < Z < 0.006, the Li abundance is well fitted by a relation
A(Li) = a0 + a1 M + a2 Z + a3 M Z, where a0, a1, a2, and a3 are constants.
Extrapolating this relation to Z = 0 leads to a Li abundance close to the
primordial value predicted from standard Big Bang nucleosynthesis calculations
and the WMAP baryon density. The relation, however, does not apply to stars
with [Fe/H] < -1.5. We suggest that metal-rich halo stars were formed with a Li
abundance close to the primordial value, and that lithium in their atmospheres
has been depleted in time with an approximately linear dependence on stellar
mass and Z. The lack of a systematic difference in the Li abundances of high-
and low-alpha stars indicates that an environmental effect is not important for
the destruction of lithium.
