  Knowing the isotopic composition of Theia, the proto-planet which collided
with the Earth in the Giant Impact that formed the Moon, could provide
interesting insights on the state of homogenization of the inner solar system
at the late stages of terrestrial planet formation. We use the known isotopic
and modeled chemical compositions of the bulk silicate mantles of Earth and
Moon and combine them with different Giant Impact models, to calculate the
possible ranges of isotopic composition of Theia in O, Si, Ti, Cr, Zr and W in
each model. We compare these ranges to the isotopic composition of carbonaceous
chondrites, Mars, and other solar system materials. In the absence of
post-impact isotopic re-equilibration, the recently proposed high angular
momentum models of the Giant Impact ("impact-fission", Cuk & Stewart, 2012; and
"merger", Canup, 2012) allow - by a narrow margin - for a Theia similar to
CI-chondrites, and Mars. The "hit-and-run" model (Reufer et al., 2012) allows
for a Theia similar to enstatite-chondrites and other Earth-like materials. If
the Earth and Moon inherited their different mantle FeO contents from the bulk
mantles of the proto-Earth and Theia, the high angular momentum models cannot
explain the observed difference. However, both the hit-and-run as well as the
classical or "canonical" Giant Impact model naturally explain this difference
as the consequence of a simple mixture of two mantles with different FeO.
Therefore, the simplest way to reconcile the isotopic similarity, and FeO
dissimilarity, of Earth and Moon is a Theia with an Earth-like isotopic
composition and a higher (~20%) mantle FeO content.
