The Collaborative International Dictionary of English v.0.48:

geochemistry \ge`o*chem"is*try\
   (j[=e]`[-o]*k[e^]m"[i^]s*tr[y^]), n. [Gr. ge`a, gh^, the
   earth + chemistry.]
   The study of the chemical composition of, and of actual or
   possible chemical changes in, the crust of the earth. --
   Ge`o*chem"ic*al, a. -- Ge`o*chem"ist, n.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

geochemistry
    n 1: the chemistry of the earth's crust