Re–Os isotopic data for 20 samples from a well-characterized 140 m section across a layered sequence, ranging from plagioclase lherzolite through lherzolite to harzburgite, of the Horoman peridotite show: (1) a range in ¹⁸⁷Os/¹⁸⁸Os ratios (from 0·1158 to 0·1283) similar to that reported for other peridotitic massifs, thereby suggesting that the processes responsible for the Re–Os isotopic variation at the meter-scale and the whole-massif scale are similar; (2) that the Os isotopic ratio is controlled by the Re content through radiogenic ingrowth over a period of ∼0·9 Gy. The ultramafic and some of the mafic rocks (Type I layers) from the Horoman massif define an ‘apparent age’ of 1·12 ± 0·24 Ga in the Re–Os isochron diagram, within error of the previously reported age of 833 ± 78 Ma based on Sm–Nd isotopes. Although the Re–Os isotopic data do not define an isochron, the consistency of the ∼900 Ma age defined by both isotopic systems suggests that this age has a geologic meaning and that mafic (Type I layers) and ultramafic rocks are genetically related. A plausible explanation for the genetic relationship between the mafic and ultramafic rocks, the meter-scale compositional variations from lherzolite to plagioclase lherzolite, the suprachondritic ¹⁸⁷Re/¹⁸⁸Os ratios in some fertile peridotites, and the oldest Re depletion model age of ∼1·86 Ga obtained for Horoman rocks is a refertilization process involving reaction of a mid-ocean ridge basalt-like magma with depleted lithospheric mantle at ∼900 Ma.