A Nd and Sr isotopic study of the Trinity peridotite Implications for mantle evolution

Field evidence is reported which indicates that the Trinity peridotite in Northern California was partially melted during its rise as part of the upwelling convecting mantle at a spreading center. A Sm-Nd mineral isochron for a plagioclase Iherzolite yielded an age of about 427 Ma which is significantly higher than that expected for depleted mantle during this period. The age is interpreted as the time of crystallization of trapped melt in the plagioclase Iherzolite P-T field, and probably represents the time when the massif was incorporated as a part of the oceanic lithosphere. The Sm-Nd model age of the plagioclase Iherzolite total rock is 3.4 AE. This suggests that the peridotite was derived from a mantle that was depleted 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 isotropic values are preserved and chemical and physical heterogeneities are sometimes preserved in the depleted source of midocean ridge basalts as well as the oceanic lithosphere which they intrude.