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Relatively low Rb/Sr and Sm/Nd ratios are typical in peridotites from at least three examples (Nunivak, Kiama, St. Paul’s Rocks) postulated to be metasomatized mantle. With time, such metasomatized mantle will develop isotopic characteristics unlike most Na2O-rich alkaline basalts (but similar to some K2O-rich alkaline basalts). Consequently, it is unlikely that old metasomatized mantle is the source of Na2O-rich alkaline basalts. The relative abundance of metasomatized spinel peridotites indicates that such geochemically anomalous mantle may be common at relatively shallow depths. Mantle metasomatism largely may occur at the Moho, where the density contrast between crustal rocks and mantle peridotite leads to the trapping and crystallization of alkaline basalts. Residual melts and fluids derived from these alkaline magmas are the agents of metasomatism.

It is also emphasized that most metasomatized mantle peridotites, even those with high La/Ce ratios, crystallized in an open system; i.e., some of the metasomatic fluid escaped during or following crystallization. Thus, it is difficult to infer the specific fluid compositions from bulk rock compositions without considering elemental fractionation between solids and fluids.

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