  We present results of a detailed chemical analysis performed on 23
main-sequence turnoff stars having -3.4<=[Fe/H]<=-2.2, a sample selected to be
highly homogeneous in T_eff and log(g). We investigate the efficiency of mixing
in the early Galaxy by means of the [Mg/Fe] ratio, and find that all values lie
within a total range of 0.2 dex, with a standard deviation about the mean of
0.06 dex, consistent with measurement errors. This implies there is "little or
no intrinsic scatter" in the early ISM, as suggested also by the most recent
results from high-quality VLT observations. These results are in contrast with
inhomogeneous Galactic chemical evolution (iGCE) models adopting present
supernova (SN) II yields, which predict a peak-to-peak scatter in [Mg/Fe] as
high as 1 dex at very low metallicity, with a corresponding standard deviation
of about 0.4 dex. We propose that cooling and mixing timescales should be
investigated in iGCE models to account for the apparent disagreement with
present observations. The contrast between the constancy and small dispersion
of [Mg/Fe] reported here and the quite different behaviour of [Ba/Fe]
indicates, according to this interpretation, that Mg and Ba are predominantly
synthesised in different progenitor mass ranges.
