  The study of silicon (Si) isotopes in ocean island basalts (OIB) has the
potential to discern between different models for the origins of geochemical
heterogeneities in the mantle. Relatively large (several per mil per atomic
mass unit) Si isotope fractionation occurs in low-temperature environments
during biochemical and geochemical precipitation of dissolved Si, where the
precipitate is preferentially enriched in the lighter isotopes relative to the
dissolved Si. In contrast, only a limited range (tenths of a per mil) of Si
isotope fractionation has been observed from high-temperature igneous
processes. Therefore, Si isotopes may be useful as tracers for the presence of
crustal material within OIB mantle source regions that experienced relatively
low-temperature surface processes in a manner similar to other stable isotope
systems, such as oxygen. Characterizing the isotopic composition of the mantle
is also of central importance to the use of the Si isotope system as a basis
for comparisons with other planetary bodies (e.g., Moon, Mars, asteroids).
  Here we present the first comprehensive suite of high-precision Si isotope
data obtained by MC-ICP-MS for a diverse suite of OIB. Samples originate from
ocean islands in the Pacific, Atlantic, and Indian Ocean basins and include
representative end-members for the EM-1, EM-2, and HIMU mantle components. On
average, Si isotope compositions of OIB are in general agreement with previous
estimates for the Si isotope composition of bulk silicate Earth. Nonetheless,
some small systematic variations are present; specifically, most HIMU-type
(Mangaia; Cape Verde; La Palma, Canary Islands) and Iceland OIB are enriched in
the lighter isotopes of Si, consistent with recycled altered oceanic crust and
lithospheric mantle in their mantle sources.
