Context :

Aims : We analyze a sample of 3,944 low-resolution ( R \sim 2000 ) optical spectra from the Sloan Digital Sky Survey ( SDSS ) , focusing on stars with effective temperatures 5800 \leq T _ { eff } \leq 6300 K , and distances from the Milky Way plane in excess of 5 kpc , and determine their abundances of Fe , Ca , and Mg .

Methods : We followed the same methodology as in the previous paper in this series , deriving atmospheric parameters by \chi ^ { 2 } 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 .

Results : 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 \sim 20 kpc , rapidly decrease between r \sim 20 and r \sim 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 .

Conclusions :