We present spectroscopic ages , metallicities , and [ \alpha /Fe ] ratios for 70 globular clusters in M31 that were derived from Lick line-index measurements .
A new interpolation technique of age-metallicity and \alpha /Fe-diagnostic grids is used to account for changes in index strength as a response to abundance-ratio variations , in particular for all of the Balmer-line Lick indices .
In addition to a population of old ( > 10 Gyr ) globular clusters with a wide range of metallicities , from about -2.0 dex to solar values , we find evidence for a population of intermediate-age globular clusters with ages between \sim 5 and 8 Gyr and a mean metallicity [ Z/H ] \approx - 0.6 .
We also confirm the presence of young M31 globular clusters that were recently identified by Beasley et al .
( 2004 ) , which have ages \la 1 Gyr and relatively high metallicities around -0.4 dex .
The M31 globular cluster system has a clearly super-solar mean [ \alpha /Fe ] = 0.14 \pm 0.04 dex .
Intermediate-age and young objects show roughly solar abundance ratios .
We find evidence for an age- [ \alpha /Fe ] relation in the sense that younger clusters have smaller mean [ \alpha /Fe ] ratios .
From a comparison of indices , mostly sensitive to carbon and/or nitrogen abundance , with SSP model predictions for nitrogen-enhanced stellar populations , we find a dichotomy in nitrogen enhancement between young and old M31 globular clusters .
The indices of objects older than 5 Gyr are consistent with a factor of three or higher in nitrogen enhancement compared to their younger counterparts .
Using kinematical data from Morrison et al .
( 2004 ) we find that the globular cluster sub-population with halo kinematics is old ( \ga 9 Gyr ) , has a bimodal metallicity distribution , and super-solar [ \alpha /Fe ] .
Disk globular clusters have a wider range of ages , are on average more metal-rich , and have a slightly smaller mean [ \alpha /Fe ] ratio .
A cross-correlation of structural parameters for M31 globular clusters with spectroscopically derived ages , metallicities , and [ \alpha /Fe ] ratios shows a correlation between half-light/tidal radius and metallicity , which is most likely due to the correlation of half-light/tidal radius and galactocentric distance .
We compare our results for M31 globular clusters with those obtained with the same technique for globular clusters in the Milky Way , Large Magellanic Cloud , M81 , and other spiral galaxies in the Sculptor group .
Finally , we compare the globular cluster systems of the two Local Group spirals , M31 and Milky Way , with their integrated bulge light .