18O/16O ratios measured in deep crustal xenoliths from Kilbourne Hole maar, New Mexico show that the oxygen isotopic composition of the lower crust of that region is grossly heterogeneous. Xenoliths derived from similar crustal depths vary in Δ18O by nearly 6%0, although the xenoliths are individually in internal oxygen isotopic equilibrium. Mafic granulites (including charnockite), probably of mantle origin, have Δ18O of about 6.2 to 6.3%0 and correspondingly low 87Sr/86Sr ratios (0.703 to 0.707). Garnet granulites, apparently the residue of partial fusion of pelitic sedimentary rock, have Δ18O ranging from about 9%0 to 12%0 and correspondingly high 87Sr/86Sr ratios (0.71 to 0.78). No evidence is found for extensive oxygen exchange over large distances in the anhydrous lower crust beneath Kilbourne Hole. Absence of regional isotopic equilibrium indicates that even at melting or near melting temperatures the oxygen isotopic identity of the protolith can be preserved over long periods of time. We conclude that there is little reason to postulate that massive oxygen exchange with a mantle-like reservoir is a general phenomenon of the lower crust.