  We analyze the mean rest-frame ultraviolet (UV) spectrum of Type Ia
Supernovae (SNe Ia) and its dispersion using high signal-to-noise Keck-I/LRIS-B
spectroscopy for a sample of 36 events at intermediate redshift (z=0.5)
discovered by the Canada-France-Hawaii Telescope Supernova Legacy Survey
(SNLS). We introduce a new method for removing host galaxy contamination in our
spectra, exploiting the comprehensive photometric coverage of the SNLS SNe and
their host galaxies, thereby providing the first quantitative view of the UV
spectral properties of a large sample of distant SNe Ia. Although the mean SN
Ia spectrum has not evolved significantly over the past 40% of cosmic history,
precise evolutionary constraints are limited by the absence of a comparable
sample of high quality local spectra. Within the high-redshift sample, we
discover significant UV spectral variations and exclude dust extinction as the
primary cause by examining trends with the optical SN color. Although
progenitor metallicity may drive some of these trends, the variations we see
are much larger than predicted in recent models and do not follow expected
patterns. An interesting new result is a variation seen in the wavelength of
selected UV features with phase. We also demonstrate systematic differences in
the SN Ia spectral features with SN light curve width in both the UV and the
optical. We show that these intrinsic variations could represent a statistical
limitation in the future use of high-redshift SNe Ia for precision cosmology.
We conclude that further detailed studies are needed, both locally and at
moderate redshift where the rest-frame UV can be studied precisely, in order
that future missions can confidently be planned to fully exploit SNe Ia as
cosmological probes [ABRIDGED].
