Field evidence indicates that the Trinity peridotite was partially melted during its rise as a part of the upwelling convecting mantle at a spreading center. A Sm Nd mineral isochron for a plagioclase lherzolite yields an age, T = 427 ± 32 Ma and initial ε Nd = + 10.4 ∓ 0.4 which is distinctly higher than that expected for typical depleted mantle at this time. This age is interpreted as the time of crystallization of trapped melt in the plagioclase lherzolite P-T field. This time of crystallization probably represents the time when the massif was incorporated as a part of the oceanic lithosphere. The Sm Nd model age of the plagioclase lherzolite totalrock is T CHUR Nd = 3.4 AE . This suggests that the Trinity peridotite was derived from a mantle that was depleted rather early in earth history. The peridotite contains many generations of pyroxenite dikes and some microgabbro dikes. We report data for two dikes that clearly crosscut the main metamorphic fabric of the peridotite. A microgabbro dike yields a Sm Nd mineral isochron age of T = 435 ± 21 Ma and ε Nd = + 6.7 ∓ 0.3 . A pyroxenite dike yields an initial ε Nd = + 7.3 ± 0.4 . The initial ε Nd values for the pyroxenite and gabbro dikes are fairly similar to those for the depleted mantle at this time and are distinct from the lherzolite—demonstrating that they are not genetically related. Rb Sr data do not give any coherent pattern. However, some bounds can be put on initial Sr values of ε Sr ⩽ −21 for the plagioclase lherzolite and ε Sr ⩽ −8.7 for the microgabbro dike. It is plausible that the dikes represent cumulates left behind from island arc magmas that rose through the the oceanic lithosphere within the vicinity of a subduction zone. Major and trace elements and Sm Nd isotopic data indicate a multiple stage history for the Trinity peridotite; a small melt fraction was extracted from an undepleted source ∼ 3.4 AE or more ago to produce the proto-lherzolite; a large fraction of melt (∼ 12 to 23%) was extracted from the proto-lherzolite to produce the present rock; the lherzolite was then crosscut by dikes from average depleted mantle ∼ 0.44 AE ago. The data are compatible with the depleted mantle source being formed very early in earth history. Although most available data indicate that the depleted upper mantle has been relatively well stirred through time, the Trinity data suggest that very ancient Nd isotopic values are preserved and thus chemical and physical heteorgeneities are sometimes preserved in the depleted source of mid-ocean ridge basalts as well as the oceanic lithosphere which they intrude.