  A highly homogeneous study of 23 halo field dwarf stars has achieved a Li
abundance accuracy of 0.033 dex per star. The work shows that the intrinsic
spread of the Li abundances of these stars at a given metallicity is <0.02 dex,
and consistent with zero. That is, the Spite Li plateau for halo field dwarfs
is incredibly thin. The thinness rules out depletion by more than 0.1 dex by a
rotational-induced extra-mixing mechanism. Despite the thinness of the plateau,
an increase of Li with [Fe/H] is seen, interpreted as evidence of Galactic
chemical evolution (GCE) of Li, primarily due to Galactic cosmic ray (GCR)
spallation reactions in the era of halo formation. The rate of Li evolution is
concordant with: (1) observations of spallative 6Li in halo dwarfs; (2) GCE
models; and (3) data on Li in higher metallicity halo stars. New data have also
revealed four new ultra-Li-deficient halo dwarfs, doubling the number known.
Based on their propensity to cluster at the halo main sequence turnoff and also
to exist redward of the turnoff, we hypothesise that they are the products of
binary mergers that ultimately will become blue stragglers. We explain their
low Li abundances by normal pre-main-sequence (and possibly main-sequence)
destruction in the low mass stars prior to their merging. If this explanation
is correct, then such stars need no longer be considered an embarrassment to
the existence of negligible Li destruction in the majority of field halo
dwarfs.
