  The Magellanic Stream (MS) is a massive and extended tail of multi-phase gas
stripped out of the Magellanic Clouds and interacting with the Galactic halo.
In this first paper of an ongoing program to study the Stream in absorption, we
present a chemical abundance analysis based on HST/COS and VLT/UVES spectra of
four AGN (RBS 144, NGC 7714, PHL 2525, and HE 0056-3622) lying behind the MS.
Two of these sightlines yield good MS metallicity measurements: toward RBS 144
we measure a low MS metallicity of [S/H]=[S II/H I]=-1.13+/-0.16 while toward
NGC 7714 we measure [O/H]=[O I/H I]=-1.24+/-0.20. Taken together with the
published MS metallicity toward NGC 7469, these measurements indicate a uniform
abundance of ~0.1 solar along the main body of the Stream. This provides strong
support to a scenario in which most of the Stream was tidally stripped from the
SMC ~1.5--2.5 Gyr ago (a time at which the SMC had a metallicity of ~0.1
solar), as predicted by several N-body simulations. However, in Paper II of
this series (Richter et al. 2013), we report a much higher metallicity (S/H=0.5
solar) in the inner Stream toward Fairall 9, a direction sampling a filament of
the MS that Nidever et al. (2008) claim can be traced kinematically to the LMC,
not the SMC. This shows that the bifurcation of the Stream is evident in its
metal enrichment. Finally we measure a similar low metallicity [O/H]=[O I/H
I]=-1.03+/-0.18 in the v_LSR=150 km/s cloud toward HE 0056--3622, which belongs
to a population of anomalous velocity clouds near the South Galactic Pole. This
suggests these clouds are associated with the Stream or more distant structures
(possibly the Sculptor Group, which lies in this direction at the same
velocity), rather than tracing foreground Galactic material.
