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We present the first measurements of [Fe/H] and [$\alpha$/Fe] abundances,obtained using spectral synthesis modeling, for red giant branch stars in M31'sgiant stellar stream. The spectroscopic observations, obtained at a projecteddistance of 17 kpc from M31's center, yielded 61 stars with [Fe/H]measurements, including 21 stars with [$\alpha$/Fe] measurements, from 112targets identified as M31 stars. The [Fe/H] measurements confirm theexpectation from photometric metallicity estimates that stars in this region ofM31's halo are relatively metal-rich compared to stars in the MW's inner halo:more than half the stars in the field, including those not associated withkinematically identified substructure, have [Fe/H] abundances $> -1.0$. Thestars in this field are $\alpha$-enhanced at lower metallicities, while[$\alpha$/Fe] decreases with increasing [Fe/H] above metallicities of [Fe/H]$\gtrsim -0.9$. Three kinematical components have been previously identified inthis field: the giant stellar stream, a second kinematically cold feature ofunknown origin, and M31's kinematically hot halo. We compare probabilistic[Fe/H] and [$\alpha$/Fe] distribution functions for each of the components. Thegiant stellar stream and the second kinematically cold feature have verysimilar abundance distributions, while the halo component is more metal-poor.Although the current sample sizes are small, a comparison of the abundances ofstars in the giant stellar stream field with abundances of M31 halo and dSphstars from the literature indicate that the progenitor of the stream was likelymore massive, and experienced a higher efficiency of star formation, than M31'sexisting dSphs or the dEs NGC147 and NGC185.

Elemental Abundances in M31: First Alpha and Iron Abundance Measurements in M31's Giant Stellar Stream