Strontium isotope and trace element compositions of olivine-hosted melt inclusions from the Song Da ultramafic volcanic suite, northern Vietnam: Implications for chemical heterogeneity in mantle plumes

Abstract

Ultramafic magmas are the ideal tracеrs of mantle composition, as they are derived from high degrees of partial melting and their compositions approach that of mantle peridotite. The Permian Song Da ultramafic volcanic rocks in northern Vietnam represent a rare example of well-preserved Paleozoic picrites of the Emeishan Large Igneous Province (ELIP), with the most Mg-rich olivine phenocrysts of up to Fo_93.5. As such, they are invaluable for constraining temperature, pressure, and composition of the ELIP mantle source. Here, we report the results of a study of olivine-hosted melt inclusions from ultramafic lavas of both low-Ti and high-Ti types in the Song-Da zone of the ELIP, providing new information on the concentrations of mobile trace elements, volatile components, and Sr isotope compositions of parental melts. Our data suggest a significant difference between the mantle source compositions of low-Ti and high-Ti primary melts of the Song Da zone. The latter melt likely originally contained 2.7 ± 1.2 wt% CO₂ and 0.6 ± 0.1 wt% H₂O, but lost most of its volatile inventory during degassing. Its enrichment in trace elements is attributed to a low degree of partial melting (less than 9 %) of a PREMA-type peridotitic source, as evidenced by the bulk rocks Nd and melt inclusions Sr isotopic data, contaminated by metasomatic fluids or recycled components bringing Pb, Sr, and Na. The low-Ti komatiite-type Song Da melts contain an excess of H₂O (H₂O/Ce up to 5500), similarly to some Archean komatiites, show a decoupling of Nd and Sr isotopes, and reveal mixing of at least two unusual components: (1) one with low initial ⁸⁷Sr/⁸⁶Sr (down to 0.7040), low Nb/U and Ce/Pb ratios and relatively enriched in La, Th, U, Pb, and Sr, and (2) the other with high initial ⁸⁷Sr/⁸⁶Sr (up to 0.7080), high Nb/U and Ce/Pb ratios, and strongly depleted in highly incompatible lithophile elements such as Ba, Rb, Nb, K, U, Th, La, and Sr. We suggest that component (1) reflects contamination with lower continental crust or other subduction-related constituents in the mantle source, while component (2) represents partial melt derived from dehydrated seawater-altered recycled harzburgite that was part of a subducted oceanic slab 300 to 500 Ma in age. The latter component can be recognized only in the melts that are exceptionally depleted in highly incompatible elements. The data from this study and Kazzy et al. (2024) suggest that the ELIP was produced by a mantle plume with a maximum potential temperature (T_p) of ∼1600 °C at ∼260 Ma. Because of its high temperature, this plume passed through the hydrated mantle transition zone (MTZ) in a partially molten state, which allowed it to entrain in its hottest part H₂O and components of the subducted 300–500 Ma Paleo-Tethyan oceanic plate stagnated in the MTZ for ∼200 Ma. An alternative source of dehydrated serpentinites of recycled Paleo-Tethyan slab in the Emeishan mantle plume could be a core-mantle boundary if this slab had been subducted shortly (less than 240 Ma) before the initiation of this plume. This option requires a fast Tethyan crust recycling rate of >2.3 cm/year. Low-Ti komatiite-type melts of the Song Da area came from the hottest part of the Emeishan mantle plume and contain traces of these components, whereas the high-Ti Song Da melts came from the coldest part of the Emeishan mantle plume (Tp ∼1450 °C) and reflect a PREMA-type mantle source metasomatized by addition of a component enriched in Na, Sr, and Pb.