Volatile concentrations in olivine-hosted melt inclusions from meimechite and melanephelinite lavas of the Siberian Traps Large Igneous Province: evidence for flux-related high-Ti, high-Mg magmatism

The origin of high-Mg melts remains one of the most highly debated questions in igneous petrology. There are two contrasting points of view, namely, (1) melt initiation in a rising high-temperature plume, and (2) mantle melting due to fluxing by water. To address this question we determined H₂O, CO₂, F, Cl and S concentrations in olivine-hosted melt inclusions of high-Mg volcanic rocks of the Siberian Traps Large Igneous Province that bracket the main pulse of volcanism at about 252–250 Ma, and can be classified as melanephelinites and meimechites. Both rock types belong to the high-Ti rock series. Correcting measured H₂O, CO₂, F, Cl and S concentrations in homogenized primary meimechite melt inclusions to primary meimechite melt composition using experimental melt compositions resulted in corrected melt-inclusion, volatile compositions of ˜3.88 wt% H₂O, ˜1477 ppm CO₂, ˜4214 ppm F, ˜2.08 wt% Cl and ˜2490 ppm S. These values are viewed as minimum estimates for the original volatile concentrations in the melt because of the high probability for degassing during melt crystallization and/or during experiment homogenization. Olivine-hosted homogenized melt inclusions from melanephelinites yielded lower corrected concentrations of ˜1.06 wt% H₂O, ˜998 ppm CO₂, ˜3242 ppm F, ˜607 ppm Cl and ˜2131 ppm S. We also measured water concentrations in clinopyroxenes of melanephelinites by FTIR, obtaining values as high as 133 ppm H₂O, which corresponds to 0.91 wt% in the melt, in general agreement with data obtained by SIMS on the olivine-hosted melt inclusions. Olivine grains from melanephelinites are characterized by evolved compositions (Fo 0.80–0.86). Extrapolation to a primitive melanephelinite melt by simple fractional crystallization suggests that it could also contain high H₂O concentrations (up to ˜ 3 wt%). Analyzed meimechite and melanephelinite whole-rock samples are characterized by trace-element patterns that are typical of mantle-derived melts and by Sr-Nd isotope ratios that exclude crustal contamination or derivation from ancient lithospheric mantle. Thus, high volatile concentrations can be attributed to sublithospheric mantle source regions. This supports the notion that high-Mg melts form by volatile fluxing of the asthenospheric mantle rather than by decompression melting under relatively dry conditions of a rising abnormally high-temperature mantle plume.