Contamination of magmas, via assimilation of country rock, is widely recognized as an important process in the thermal and chemical evolution of magmas emplaced into continental crust. Its importance at mid-ocean ridges has, however, received much less consideration. Lavas from the fast-spreading East Pacific Rise show evidence for contamination through an enrichment in seawater-derived chlorine that is not observed in basalts from slow-spreading ridges. In this study magmatic amphiboles in plutonic rocks from the Oman ophiolite—an intermediate- to fast-spreading ridge analogue—have been analyzed to determine the extent of this contamination in the lower oceanic crust. These data suggest an enrichment in chlorine throughout much of the lower ocean crust. If this enrichment is representative of lower oceanic crust formed at typical intermediate- to fast-spreading ridges, assimilation may be a more important process at mid-ocean ridges than previously recognized.