Re–Os data obtained by LAM–ICPMS techniques from sulfides within the Bardane peridotite body in the Western Gneiss Region of the Norwegian Caledonides indicate a Proterozoic, or possibly Archean, origin for the peridotite, probably as depleted residue formed through melt extraction in the mantle. Sm–Nd mineral ages from primary garnet-bearing assemblages in a megacrystic orthopyroxenite within the peridotite indicate the mantle was invaded by melts that crystallized in the garnet-stability field during the mid-Proterozoic. However, microdiamonds within this megacrystic orthopyroxenite formed well after these processes, during the Paleozoic Caledonian Orogeny. Clinopyroxenes show enriched trace element patterns and radiogenic Sr and non-radiogenic Nd isotopic ratios, indicating fertilization of the Bardane peridotite with incompatible elements, presumably including carbon. Metasomatism may have occurred while the peridotite was part of the mantle wedge above an Iapetus subduction zone or it may have occurred after the lenses were introduced into the crust. Textural and field relationships and Sm–Nd mineral results from secondary garnet-bearing assemblages indicate the microdiamonds formed during Caledonian metamorphism, probably during the Scandian Orogeny, when the peridotite body was introduced from the mantle into a subducting crustal gneiss complex and carried to mantle depths within the diamond-stability field. Texturally and mineralogically similar megacrystic orthopyroxenites on the nearby island of Otrøy lack microdiamonds, suggesting they underwent all the above processes except the metasomatic introduction of carbon.