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.
Bibliographical noteFunding Information:
We thank Nilanjan Chatterjee for conducting microprobe analyses of some olivine grain mounts and melt inclusions, Brian Montleone for help with SIMS analyses of the melt inclusions, Galina Pashkova and Svetlana Panteeva for major and trace element analyses of the whole rock samples. Reviews by J. Brian Balta, Nicolas T. Arndt and three anonymous reviewers are greatly appreciated. Analytical works for the whole-rocks and melt inclusions were funded respectively by the Russian Science Foundation project no. 16-17-10068 to AVI and EID and National Science Foundation grant OPP-1025513 to SBM. VSK acknowledges Australian Research Council (discovery grants DP1092823 and DP130100257 ). AVI acknowledges a Fulbright Scholarship during his stay in the USA. FTIR analyses were funded by the Russian Foundation for Basic Research project no. 16-35-00317 and the projects of the Ministry of Education and Science of the Russian Federation no. 14.B25.31.0032 and no. 14.Y26.31.0018 to MVK, KDL and DAZ.
- Melt inclusion
- Siberian Traps