  We analyze a sample of 3,944 low-resolution (R ~ 2000) optical spectra from
the Sloan Digital Sky Survey (SDSS), focusing on stars with effective
temperatures 5800 < Teff < 6300 K, and distances from the Milky Way plane in
excess of 5 kpc, and determine their abundances of Fe, Ca, and Mg. We followed
the same methodology as in the previous paper in this series, deriving
atmospheric parameters by chi2 minimization, but this time we obtained the
abundances of individual elements by fitting their associated spectral lines.
Distances were calculated from absolute magnitudes obtained by a statistical
comparison of our stellar parameters with stellar-evolution models. The
observations reveal a decrease in the abundances of iron, calcium, and
magnesium at large distances from the Galactic center. The median abundances
for the halo stars analyzed are fairly constant up to a Galactocentric distance
r ~ 20 kpc, rapidly decrease between r ~ 20 and r ~ 40 kpc, and flatten out to
significantly lower values at larger distances, consistent with previous
studies. In addition, we examine the [Ca/Fe] and [Mg/Fe] as a function of Fe/H
and Galactocentric distance. Our results show that the most distant parts of
the halo show a steeper variation of the [Ca/Fe] and [Mg/Fe] with iron. We
found that at the range -1.6 < [Fe/H] < -0.4 [Ca/Fe] decreases with distance,
in agreement with earlier results based on local stars. However, the opposite
trend is apparent for [Mg/Fe]. Our conclusion that the outer regions of the
halo are more metal-poor than the inner regions, based on in situ observations
of distant stars, agrees with recent results based on inferences from the
kinematics of more local stars, and with predictions of recent galaxy formation
simulations for galaxies similar to the Milky Way.
