Stepan P. Krasheninnikov

Meimechite (i.e., rare high MgO and TiO2 ultramafic rocks) concluded the Permo-Triassic Trap magmatism ca. 250 Ma-ago, known as a Siberian Large Igneous Province (SLIP) in the Meimecha-Kotui region, northern Siberia (e.g. [1]). In addition to their elevated MgO contents, meimechite’s melts display almost no crustal contamination, making them ideally suited to investigate the mantle source of the SLIP. Formerly, two opposing models were evoked for the origination of the meimechite: i) the hottest phanerozoic mantle plume [1] or ii) water fluxing of the asthenospheric mantle in a long-lived subduction zone [2]. Based on an extended analytical workflow we will shed new light on the source of these unusual rocks.Here we present new results for more than 300 olivine-hosted homogenized melt inclusions from Siberian meimechite including major, minor and trace elements, water and Sr-isotopes contents (EPMA, LA-ICP-MS and Raman spectrometry) along with the chemical composition of their host olivine (EPMA, LA-ICP-MS). When encountered, spinel inclusions were analysed by EPMA for major element abundances.We show that the Siberian meimechite crystallised from a highly magnesian (MgO > 22 wt%) parental melt deficient in H2O compared to Ce and K concentrations, which was degassed of most of its CO2 and likely part of its H2O while rising to shallower depths. Three independent geothermometers (Mg-Fe and Sc-Y olivine melt and Al olivine-spinel) confirm the high crystallisation temperature of the Siberian meimechite, ca. 1400oC. Furthermore, the calculated potential temperatures (over 1500oC) imply a mantle plume origin of the Siberian meimechite and, consequently, of the SLIP.Initial 87Sr/86Sr values of melt inclusions reveal heterogeneous populations ranging from 0.7022±0.0002 to 0.7039±0.0004 suggesting mixing between at least two depleted mantle components. The less depleted group has an average Bulk Silicate Earth (BSE) model age of 876±88 Ma, whereas the more depleted group is significantly older with an average model age of 1716±76 Ma. All source components display significantly fractionated proxies of continental crust extraction (Nb/U, Th/U and Ce/Pb [3]), indicating major events of continental crustal formation and deep recycling of residual lithosphere before the Proterozoic Eon.References:[1] – Sobolev, A.V., et al., Russ. Geol. Geophys., 2009 and references therein. [2] – Ivanov, A.V., et al., Chem. Geol., 2018. [3]- Hofmann, A.W. et al. EPSL, 1986.

The diffusive exchange of major elements in Na-series tephrite–phonolite diffusion couples with compositions relevant to the Canary Islands magmatism was determined at 300 MPa and variable H₂O concentrations (0.3 wt % to 3.3 wt %), temperatures (1150 to 1300 °C), and fO₂ levels (NNO−1.5 to NNO+1.7). Composition-dependent effective binary diffusion coefficients were determined from concentration–distance profiles. Results show a wide range of diffusivities for different cations, consistently following the sequence Na ≫ Al ≫ K ≥ Mg = Fe = Ca > Si > Ti, with a mild diffusivity contrast (0.2–0.8 log units) between tephritic and phonolitic melts. Na is the fastest component, with diffusivities falling ∼1.0 log units above those of Si for any given condition. An anomalously fast Al diffusion is observed, with D_Al falling ∼0.4 log units above Si and ∼0.6 log units below Na, suggesting a prevalence of Al–alkali coupling across our range of run conditions. The relationships between log D and H₂O content in melt for all cations in an intermediate composition are strongly nonlinear and can be fitted using an exponential function with a convergence in diffusion coefficients for different temperatures with increasing H₂O content. Thus, Arrhenius analyses result in a decrease in activation energies from 222–293 kJ mol⁻¹ at 1.7 wt % H₂O to 48–112 kJ mol⁻¹ at 3.0 wt % H₂O. These results provide new data on chemical interdiffusion in highly alkaline Na-rich melts and suggest that H₂O content plays a key role in increasing the chemical efficiency of magma mixing at low temperatures. The obtained dataset is used to test chemical controls of magma mixing in the El Abrigo ignimbrite, Tenerife, where banded pumices involving basanitic–tephritic to phonolitic magmas are common in several compositionally bimodal ignimbrite units.

Besides H2O, CO2 and sulphur, halogens (F, Cl, Br, I) are important volatile components in magmas. The extremely high chemical activity of halogens in melts and liquids leads to a significant influence on (a) magmatic properties, (b) the degassing of magma, (c) the extraction, transport and deposition of metals, (d) the chemistry of volcanic emissions and (e) the composition of the atmosphere. Indeed, their geochemical behaviour can be used as a key indicator of the genetic conditions and evolution of magma.In July 2021 a joint interdisciplinary campaign of petrologists, chemists and atmospheric physicists took place at Mt Etna volcano, Italy. Due to the favourable volcanic activity at Mt Etna (frequent paroxystic activity characterized by lava fountaining) we were able to collect a unique dataset of simultaneously sampled fresh tephra fallout, in-situ gas samples (multiGAS, alkaline traps, 1,3,5-Trimethoxybenzene impregnated denuders) and spectral data with remote sensing techniques (DOAS, FTS, IFPICS) of the volcanic plume. The halogen and sulphur content was analysed in the volcanic plume as well as in the melt inclusion and glasses of the deposits. Results of the various applied techniques are presented. They allow us a direct comparison of degassing signatures (e.g., Cl/F, Br/Cl, and S/Cl) from the pre-eruptive melt to the volcanic plume.

The study of melt inclusions in minerals is the only direct approach to estimate the contents of volatile components in silicate melts, which are essential to constrain the processes of magma generation and evolution. In particular, the content of sulfur in silicate melts is closely connected with the P-T-fO 2 conditions of mantle melting and the formation of sulfide deposits. Inclusions in high-Mg olivine can provide access to the composition of the most primitive melts, but these are the most susceptible to post-entrapment modification such as ‘Fe-loss’. The loss of Fe consequently decreases sulfur solubility and leads to silicate-sulfide immiscibility and formation of daughter sulfide globules, significantly affecting the initial proportions of S contained within the silicate part (glass) of melt inclusions.In this study, we investigate the influence that sulfide phase separation has on the accuracy of S content in melt estimations by performing heating experiments on natural samples at various temperatures and durations. Melt inclusions from MORB-like olivine-phyric rocks (Kamchatsky Mys, Far East Russia) heated for 30 minutes at 1200 °C and 1300 °C were compared with similar experiments on olivine-hosted melt inclusions over a 5 minute duration (at 1200 °C and 1350 °C). Average Fo content in host olivine is the same for all experimental sets, allowing for direct comparison of sulfur contents in the following types of melt inclusions: (i) both sulfide globules and shrinkage bubbles, (ii) only sulfide globules, (iii) only shrinkage bubbles, and (iv) completely homogenized. We found that the content of sulfur in shrinkage bubbles and its precipitates is negligible in reheated water-poor melt inclusions, whereas daughter sulfide globules are significant repositories of initial S content. Therefore, for restoring sulfur contents in parental melts the amounts stored in daughter sulfide and silicate glass should be combined.

Mesozoic (125–135 Ma) cratonic low-Ti lamproites from the northern part of the Aldan Shield do not conform to typical classification schemes of ultrapotassic anorogenic rocks. Here we investigate their origins by analyzing olivine and olivine-hosted inclusions from the Ryabinoviy pipe, a well preserved lamproite intrusion within the Aldan Shield. Four types of olivine are identified: (1) zoned phenocrysts, (2) high-Mg, high-Ni homogeneous macrocrysts, (3) high-Ca and low-Ni olivine and (4) mantle xenocrysts. Olivine compositions are comparable to those from the Mediterranean Belt lamproites (Olivine-1 and -2), kamafugites (Olivine-3) and leucitites. Homogenized melt inclusions (MIs) within olivine-1 phenocrysts have lamproitic compositions and are similar to the host rocks, whereas kamafugite-like compositions are obtained for melt inclusions within olivine-3. Estimates of redox conditions indicate that “lamproitic” olivine crystallized from anomalously oxidized magma (∆NNO +3 to +4 log units.). Crystallization of “kamafugitic” olivine occurred under even more oxidized conditions, supported by low V/Sc ratios. We consider high-Ca olivine (3) to be a fingerprint of kamafugite-like magmatism, which also occurred during the Mesozoic and slightly preceded lamproitic magmatism. Our preliminary genetic model suggests that low-temperature, extension-triggered melting of mica- and carbonate-rich veined subcontitental lithospheric mantle (SCLM) generated the kamafugite-like melts. This process exhausted carbonate and affected the silicate assemblage of the veins. Subsequent and more extensive melting of the modified SCLM produced volumetrically larger lamproitic magmas. This newly recognized kamafugitic “fingerprint” further highlights similarities between the Aldan Shield potassic province and the Mediterranean Belt, and provides evidence of an overlap between “orogenic” and “anorogenic” varieties of low-Ti potassic magmatism. Moreover, our study also demonstrates that recycled subduction components are not an essential factor in the petrogenesis of low-Ti lamproites, kamafugites and leucitites.

The shock-explosive processing of rocks by micrometeorite and meteorite bombardment on the surface of the Moon and other airless bodies of the solar system is a dominant geological and geochemical process that produces the loose layer of regolith. The paper presents results of modeling the process of a micrometeorite impact using a millisecond laser. The targets in the experiment were a sample of basalt of composition similar to those of basalts at Moon’s mare areas and a sample of basalt glass obtained by melting this basalt. At the laser “impact”, products ejected from the crater (glass spherules, droplets of various shapes, and condensates) and melting products in the crater were studied by electron microscopy, electron microprobe analysis with energy-dispersive spectrometers, X-ray diffraction, and X-ray fluorescence analysis. It has been shown that the glasses (melt drops) obtained in the experiment with the basalt target are characterized by a heterogeneous distribution of chemical composition. They were mixed in variable proportions as a result of melting the original minerals of the basalt. The spherules that have undergone significant evaporation differentiation in the experiment with a basalt target (crystalline basalt) make up about 25%. When the target was basalt glass, the proportion of the spherules (that have undergone profound evaporative differentiation) was ~90%. The glass was depleted in high- and medium-volatility components (Na₂O, K₂O, FeO, etc.). The most strongly differentiated glasses corresponded to high-alumina HASP glasses found on the Moon (Al₂O₃ > 34 wt %, SiO₂ < 32 wt %).

This paper reports the results of a study of naturally and experimentally quenched melt inclusions in magnesian olivine ( Fo77–89) from a basalt sample from the Karymsky volcanic center,...