Study of olivine-hosted melt and spinel inclusions from the Song Da ultramafic volcanic suite, northern Vietnam: Compositions, crystallization temperatures, and origin of the low-Ti komatiite-like and high-Ti primary melts

The Permian (~260 Ma) Song Da volcanic suite in Vietnam is one of very few known occurrences of Phanerozoic ultramafic volcanic rocks that are similar in composition to komatiites. Despite continuous efforts to determine the primary melt composition of Song Da ultramafic lavas, the concentrations of the volatile and mobile elements are still poorly constrained due to widespread alteration and low-grade metamorphism of bulk rocks. This study reports high-precision in-situ major- and trace element abundances in host olivine and inclusions of melt and Cr-spinel from the Song Da ultramafic lavas. Two different types were identified: low-Ti lavas, previously described as komatiites, and newly discovered Ti- and Na-rich picrites. The application of olivine-melt Sc/Y, olivine-spinel Al, and olivine-melt Fe/Mg geothermometers indicates crystallization temperatures of up to 1450 °C for the Song Da low-Ti suite, which are within the range of komatiite crystallization temperatures, and up to 1330 °C for the high-Ti picrites. These conditions correspond to mantle potential temperatures of 1590 °C and 1450 °C, respectively. The estimation of oxygen fugacity, based on V partitioning between olivine and melt and Fe2+/Fe3+ between spinel and melt, indicates that low-Ti melts crystallized in a closed system under reduced conditions starting from one to half an order of magnitude below the QFM buffer. The high-Ti melt crystallized at higher oxygen fugacity (∆QFM +0.5) in a buffered open system. The primary melt of the Song Da komatiites contained 0.7 wt% H2O, which was likely entrained from the hydrated Mantle Transition Zone (MTZ) by a partially molten plume. Our results indicate that the Song Da low-Ti ultramafic volcanics were likely derived from an ultramafic komatiite-like parental melt with an MgO content between 21 and 23 wt%. It was produced by a high degree (>26%) of partial melting of a depleted mantle source. The high-Ti picrite melt has 17–18 wt% MgO and was produced by a lower degree of partial melting (<9%) in a colder part of the same plume.