Garnetite xenoliths from ultramafic diatremes in northeastern Arizona provide insights into hydration and metasomatism in the mantle. The garnetites contain more than 95% garnet, some of which has complex compositional zonation related to growth in fractures within grains. Accessory minerals include rutile, ilmenite, chlorite, clinopyroxene, and zircon. Zircon grains in one rock were analyzed in situ to determine U–Pb ages and Hf isotopic compositions. Most U–Pb analyses plot on or near concordia in the range 60–85 Ma but a few are discordant. The range in ¹⁷⁶Hf/¹⁷⁷Hf is about 0·2818–0·2828, with grains zoned to more radiogenic Hf from interiors to rims. The garnetite protolith contained zircons at least 1·8 Ga in age, and garnet and additional zircon crystallized episodically during the interval 85–60 Ma. The garnetites are interpreted as mantle analogues of rodingites, formed in metasomatic reaction zones caused by water–rock interactions in Proterozoic mantle during late Cretaceous and Cenozoic subduction of the Farallon plate. Associated eclogite xenoliths may have been parts of these same reaction zones. The rodingite hypothesis requires serpentinization in the mantle wedge 700 km from the trench, beginning 5–10 Myr before tectonism related to low-angle subduction.