  We investigate the chemical evolution of odd-numbered elements such as sodium
(Na) and aluminum (Al) during the early epochs of the Galactic halo with the
use of a model that reproduces the observed box-shaped distribution of
extremely metal-poor stars in the [Na, Al/Mg] versus [Mg/H] plane. Our model is
constructed under the assumptions that those stars retain the elemental
abundance patterns produced by individual Type II supernovae (SNe), and that
the yields of the odd elements depend on the initial metallicity, z, of their
SN progenitors. As a result, recent abundance determinations that clarify how
the [Na, Al/Mg] ratios of field stars have evolved to the solar values enable
us to deduce how the yields of these odd elements depend on z. The observed
trends in these abundances, in particular the very large scatter (over 1 dex in
[Al/Mg]) requires that the Al yield scales as m_Al proportional to z^0.6 for
[Mg/H]<-1.8, while the observed [Na/Mg] trend requires that the Na yield scales
as m_Na proportional to z^-0.4 for [Mg/H]<-1.8 and m_Na proportional to z^0.4
for [Mg/H]>-1.8. It is found that the predicted frequency distribution of stars
in the [Na/Mg] versus [Mg/H] diagram is very sensitive to the assumed form of
the primordial IMF, and that its slope is steeper than the Salpeter IMF. The
necessity to match the observed abundance patterns of odd elements and the
frequency distribution of extremely metal-poor stars should provide useful
constraints on nucleosynthesis calculations of metal-free massive stars as well
as on theories of their formation.
