Olivine O isotope and trace element constraints on source variation of picrites in the Emeishan flood basalt province, SW China

Abstract

Like many continental flood basalt (CFB) provinces in the world, the source mantle compositions (peridotites or pyroxenites) of the Emeishan CFB province in SW China are still debated. We have used the combination of olivine O isotopes determined in situ using SIMS and trace element abundances measured in situ using LA-ICP-MS to address this fundamental problem. The newly discovered picrites in the Emeishan CFB province at Yumen are used in this study. In addition, we have also analyzed primitive olivine phenocrysts with known O isotope ratios from four other picrite occurrences (Maoniuping, Tanglanghe, Wuguijing and Wulongba) in the Emeishan CFB province for trace element abundances. Based on whole rock compositions the picrite samples from Wulongba belong to the low-Ti group (Ti/Y < 500) and those from the other locations belong to the high-Ti group (Ti/Y > 500). Mineral chemical data reveal that the two different groups of picrites can be distinguished by using Ti/Al of olivine phenocrysts and Cr-spinel inclusions enclosed in olivine. Specifically, the high-Ti picrite group is characterized by 10Ti/Al (molar) >0.9 and >0.5 for olivine phenocrysts and Cr-spinel inclusions, respectively. In the mantle source discrimination diagrams based on olivine 100Mn/Fe versus 100Ni/Mg or 100Ca/Fe, both groups of picrites plot between the suggested fields of peridotite and pyroxenite mantle sources. In the same type of diagrams based on olivine Mn/Zn and 10000Zn/Fe, both groups of picrites plot exclusively in the fields for a peridotite mantle source. The mean δ18O value of olivine phenocrysts from the Yumen high-Ti picrites is 5.19 ± 0.25‰ (1σ, n = 35), which is significantly lower than that for the Wulongba low-Ti picrites (5.6 ± 0.15‰) but is similar to values (5.1 ± 0.15 to 5.3 ± 0.15‰) for the high-Ti picrites from elsewhere (Maoniuping, Talanghe and Wuguijing) in the Emeishan CFB province. The mean δ18O values of olivine phenocrysts from the Emeishan high-Ti picrites at different locations are all within the range of typical mantle olivine values (5.1 ± 0.3‰). Based on these results, we conclude that the high-Ti picrites in the Emeishan CFB province were primarily derived from a modally enriched peridotite mantle source. Our new olivine Zn-Mn-Fe data are consistent with the view that the Wulongba low-Ti picrites were derived from a lithospheric peridotite mantle source.