Home – Home – Jornals – Russian Geology and Geophysics 2023 number 8

This Special issue of Russian Geology and Geophysics is a collection of papers on current problems of experimental mineralogy, petrology, and geochemistry discussed at the XVIII Russian Conference on Experimental Mineralogy (5-10 September 2022, Vinogradov Institute of Geochemistry, Irkutsk). The scope of considered issues ranges from laboratory modeling of mineral formation processes in different tectonic settings to technical mineralogy. The reported experiments are run at pressures and temperatures corresponding to crustal and mantle conditions.

Experimental studies aimed at determining the conditions for the formation of diamond and graphite as a result of the redox interaction of reduced mantle rocks and oxidized rocks of the slab in a wide temperature range, including the conditions of both "cold" and "hot" subduction, were carried out on a "split-sphere" multianvil high-pressure apparatus (BARS) in the (Fe,Ni)-(Mg,Ca)CO₃ system, at 6.3 GPa and 800-1550 °C for 35-105 h, using the «sandwich» assembly. We have established that the interaction of Fe,Ni metal and carbonate is due to the creation and propagation of a redox front, at rates from 1.3 (800 °C) to 118 µm/h (1550 °C). At T < 1200 °С, this interaction leads to the formation of alternating reaction zones (from the reduced center to the oxidized periphery): metal → metal + wüstite/magnesiowüstite → magnesiowüstite + graphite ± Mg,Fe,Ca carbonates → magnesite + aragonite. In this case, in the reduced part of the samples, the formation of a Ni,Fe metal phase strongly enriched in Ni (up to 65-70 wt.% vs. the initial 10 wt.%) was recorded. At higher temperatures, the formation of Fe,Ni metal-carbon (≥1200 °C) and carbonate (≥1330 °C) melts was observed. We have found that the presence of nickel precludes the formation of carbides in the reduced part of the sample and ensures stable diamond crystallization at 1400-1550 °C both in metal-carbon and carbonate melts. Our experiments demonstrate that diamonds from the metal-carbon melt are characterized by inclusions of taenite and magnesiowüstite. The morphology of these diamonds is determined by the {111} layer-by-layer grown faces, and their indicator characteristics are nitrogen-vacancy and nickel-related (884 nm) centers at 1400 °C or nickel-nitrogen centers (S3, 598 nm, 727 nm, 746 nm, etc.) at 1550 °C. For diamonds formed in the carbonate melt, the morphology is determined by the {100} and {111} (vicinal-growth) faces; carbonates are identified as inclusions; and nitrogen-vacancy centers H3, NV⁰, and NV^- are fixed in the photoluminescence spectra. Experiments show that the indicator of the metal-carbonate interaction temperature is the degree of structural perfection of graphite, which increases in the range of 800-1550 °C.

Subduction of marine carbonates is accompanied by numerous transformations and interactions, including reactions with reduced mantle rocks. At depths of 250-300 km, carbonates enter mantle zones where metallic iron can be stable. The interaction of carbonates with metals is one of the mechanisms of the release of elemental carbon and the formation of diamond. These processes are also accompanied by carbon isotope fractionation and can result in a significant isotopic heterogeneity of mantle carbon. In this work we study the partitioning of carbon isotopes between carbon and carbon-bearing phases obtained in experiments on the interaction of FeNi alloy with (Mg,Ca)CO₃, which simulates mantle-crust redox reactions in the temperature range 800-1550 °C and at a pressure of 6.3 GPa. It has been established that at 800-1000 °C, the carbon of carbonate is reduced at the metal/carbonate interface and dissolves in the FeNi alloy. This process leads to a 17-20 ‰ depletion of the metal in the heavy carbon isotope. At temperatures above 1330 °C, the fractionation of carbon isotopes between carbonate and metal-carbon melts is reduced to 8.5 ‰, approaching the thermodynamic calcite-cohenite isotope equilibrium. At temperatures above 1400 °C, diamond crystallizes from metal-carbon and carbonate melts, which leads to isotopic depletion of the metal-carbon melt. As a result, the measured carbon isotope fractionation between the carbonate and metal-carbon melts increases and moves away from the thermodynamic CaCO₃-Fe₃C equilibrium line. The carbonate-metal redox interaction is supposed to be one of the probable mechanisms of the formation of isotopically light carbon in the mantle at the expense of the marine carbonate sediments subducted into the mantle. This mechanism also provides the formation of anomalous isotopically heavy carbonates found in kimberlites of the Siberian Platform.

The composition of the fluid in carbonate- and chlorine-bearing pelite was experimentally studied at 3.0 GPa and 750 and 900 ºC, using the diamond trap method. The results of inductively coupled plasma atomic-emission spectrometry (ICP AES) data and mass balance calculations showed that a supercritical fluid formed in the studied system at 3.0 GPa and 750 °C. The fluid is Si- and Al-rich and contains 30–50 wt.% H₂O + CO₂ and up to 1 wt.% Cl. The contents of other major elements decrease in the order: K > Na > Сa ≈ Fe > Mg > Mn >> Ti ≈ P. Compared with supercritical fluids appeared in the systems pelite–H₂O and eclogite–H₂O, the fluid with high CO₂ and Cl contents is richer in Fe, Ca, Mg, and Mn but poorer in Si. Silicate melt generated in this system at 900 ºС has a composition typical of pelitic melt. Our experiments reveal a set of fingerprints of element fractionation between a supercritical fluid and solids forming an eclogite-like association, namely, high mobility of P, Sr, and B and low mobility of Li and S. Thus, a supercritical fluid compositionally similar to the pelitic melts generated in subduction zones can transfer significant amounts of both volatiles (H₂O, CO₂, Cl, and P) and major components to the regions of arc magma generation. It is important that supercritical fluids should have trace element signatures of diluted low-temperature fluids.

Alkaline chlorides are important constituents of carbonatitic inclusions in magmatic minerals from kimberlites and lamproites, mantle xenoliths from kimberlites, and diamonds from kimberlites and placers around the world. This indicates the participation of alkali chlorides, along with carbonates, in the processes of melting of mantle rocks, which makes it important to study chloride-carbonate systems at mantle pressures. In this work, we studied the phase relations in the NaCl-CaCO₃-MgCO₃ system at 3 GPa in the range of 800-1300 °С using a multianvil press. It has been found that the NaCl-CaCO₃ and NaCl-MgCO₃ binaries have the eutectic type of T-X diagram. The halite-calcite eutectic is situated at 1050 °C and Na2# = 36, while the halite-magnesite eutectic is located at 1190 °C and Na2# = 77, where Na2# = 2NaCl/(2NaCl + CaCO₃ + MgCO₃) · 100 mol.%. In the NaCl-CaCO₃-MgCO₃ ternary, subsolidus assemblages are represented by halite and calcium-magnesium carbonates. Just below solidus, two assemblages are stable: halite + magnesite + dolomite and halite + dolomite-calcite solid solution. The minimum on the liquidus/solidus surface corresponds to the halite-Ca_0.84Mg_0.16CO₃ dolomite eutectic, located at about 1000 °С with Na2#/Ca# = 34/84, where Ca# = Ca/(Ca + Mg) · 100 mol.%. At Ca# ≤ 73, the melting is controlled by the halite + dolomite = magnesite + liquid ternary peritectic, located at 1050 °C with Na2#/Ca# = 31/73. According to the data obtained, it can be assumed that at 3 GPa the solidi of NaCl-bearing carbonated peridotite and eclogite are controlled by the peritectic reaction halite + dolomite = magnesite + liquid, located at about 1050 °C. The melting is accompanied by the formation of a chloride-carbonate melt containing (wt.%): NaCl (35), CaCO₃ (56), and MgCO₃ (9).

Feldspar group minerals (feldspars) form up to 60 vol.% of the Earth’s crust. The knowledge of their stability under extreme conditions (high-pressure and high-temperature) allow to better understand the processes, that occur in the subduction and collision processes. This review focuses on the behavior of feldspars with paracelsian topology (seven mineral species: three borosilicates, two aluminosilicates and two beryllophosphates) at elevated temperatures and pressures. Partly, new data on high-temperature behavior of paracelsian BaAl₂Si₂O₈ (based on in situ high-temperature powder X-ray diffraction) provided. The high-temperature studies of 5 feldspar minerals with paracelsian topology (danburite, maleevite, pekovite, paracelsian, slawsonite) revealed that all of them are stable at least up to 800 °C. Among all of them only paracelsian undergoes polymorphic transition (at 930 °C), whereas all other minerals decompose or amorphisize. The structural deformations of these minerals demonstrate the different anisotropy degree upon heating, whereas the average volume expansion is similar for all of them (α_V = 23 ×10^-6 ºC^-1). High-pressure behavior was studied for six of seven minerals with paracelsian topology (danburite, meleevite, pekovite, paracelsian, slawsonite, hurlbutite). The studied minerals undergo transformations with the stepwise increasing of coordination number of frame-forming cations from 4 to 5 and 6 upon compression The discovering of unusual structural units under extreme conditions (e.g., fivefold-coordinated polyhedral) can influence on the concentration and transport processes of trace elements that should be taken into account when interpreting geochemical and geophysical data. The crystal structure stability range of studied minerals highly depends on the chemical composition of frame-forming cations: aluminosilicates are the least stable and undergo the phase transitions below 6 GPa; borosilicates preserve their initial crystal structure up to ~20 GPa; beryllium phosphates do not undergo phase transformations up to 75 GPa. It has been shown that transformations pathway of isostuctural compounds highly depends on the chemical composition of both extraframework and frame-forming cations that involves the difficulties with predictions of their behavior under extreme conditions.

We report new experimental data on the interaction of igneous melts with hydrogen at temperatures of 1100-1250 °C and hydrogen pressures of 1-100 MPa in strongly reducing conditions: f_O2 = 10^-12-10^-14. The experiments were conducted using an original high-gas-pressure unit equipped with a unique device that provides long-term experiments at high temperatures and pressures of hydrogen. The experiments used natural samples of igneous rocks: the magnesian basalt of the Northern Breakthrough of the Tolbachik Volcano (Kamchatka) and the andesite of the Avacha Volcano (Kamchatka). On the basis of the experiments, the following features of the process of interaction of hydrogen with igneous melts have been established: (1) Despite the high reduction potential of the H₂-igneous melt system, the reactions of hydrogen oxidation and complete reduction of oxides of metals of variable valence in the melt do not go to the end. The cessation of redox reactions in basaltic and andesitic melts is due to the formation of H₂O in the melt, which buffers the reduction potential of hydrogen; (2) The initially homogeneous igneous melt becomes heterogeneous: The formed H₂O dissolves in the melt and in the fluid phase (at first pure hydrogen), and melts of variable, more acidic composition and small metallic isolations of the liquation structure are formed; (3) The complex process of metal-silicate liquation in magmatic melts when they interact with hydrogen can be carried out at real magma temperatures in nature (≤1200 °C), significantly lower than the corresponding melting points of iron and its alloys with nickel and cobalt; (4) The structure and dimensions of the experimentally established metal isolations are consistent with natural data on the finds of small quantities of native metals, primarily iron and its alloys with nickel and cobalt, in igneous rocks of different compositions and genesis.

Distribution of a wide range of elements in the systems with magnetite, hematite and sphalerite is studied by the method of thermogradient hydrothermal synthesis combined with internal fluid sampling at 450 °C temperature and 100 MPa pressure. The distribution and cocrystallization coefficients are determined; the literature and original data on these coefficients are summarized. The possibility of obtaining the reproducible data on elements distribution in the mineral - solution system in the occurrence of many typomorphic elements is substantiated. This considerably increases the experiment efficiency. A significant advantage of using cocrystallization coefficients rather than "conventional" distribution coefficients expressed by the ratio of the element concentrations in crystal and solution (fluid) is shown. The features of behavior and occurrence of elements in hydrothermal systems are provided with physico-chemical evidence, through application of cocrystallization coefficients. The examples of the behavior of typomorphic trace elements in sphalerite are considered, which support the theoretical analysis. The major (Fe, Mn, Zn and possibly Cu) and secondary (Ti, V, Al, and Co) components of ore-forming solutions are estimated according to the compositions of magnetite and hematite from hydrothermal ore deposits of various types. The similarity in compositions of magnetite and hematite does not prove their coformation from a single fluid, quite the reverse, and this fact indicates different compositions of fluids from which the minerals were deposited.

The modeling experiments were conducted to study transport of ore-forming components in the lithosphere, taking into account the possibility of ore matter remobilization under endogenous conditions. The experiments, which included temperature gradient-based ones, were conducted at T = 500-680 °C and P = 1.5-5.0 kbar on high gas pressure devices (HGPD) in highly concentrated water-salt solutions of alkaline specifics. The experiments consisted of two stages. During the first stage, we tested the possibility of recrystallization of the ore matter of "black smokers" in the presence of basalt at 500 ℃ and 5 kbar and water-salt fluids at a concentration of up to 5 wt.%. At the second stage, mechanisms of ore-forming components transport ( P-T parameters: 450-650 ℃ and up to 5 kbar) were studied under conditions of a temperature gradient (0.3-0.4 °C/mm). The duration of the experiments was 14 days. The test products were: oceanic basalts, granite model mixtures (Fsp + Qz), as well as various sulfide minerals, oxides and noble metals (Au, Pt). It has been shown that at T 680-650 °C, intensive recrystallization and deposition of sulfide minerals (sphalerite, galena, chalcopyrite, pyrite, cooperite, etc.) along with feldspars, micas and quartz, takes place. Intensive transport of both the main petrogenic (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K) and ore-forming elements (Ni, Cu, Zn, As, Pb, Cd, Pt, Au, Hg, Bi), and a joint transport of silicate and ore matter is established. Some ore elements are either included into compositions of solid solutions or present as impurities in ore-forming minerals: Fe, Ni, Cu → pyrite, pyrrhotite; Pb, Au, As, Bi, Zn → galena; Zn, Cd, Fe, Mn, Cu → sphalerite; As → galena, orpiment, realgar, gold; Hg → gold. The obtained data attest to the possibility of modeling ore mineralization mechanisms. The experimental results apply to explain the genesis of the Zun-Kholba gold-quartz-sulfide deposit and describe the processes of epigenetic transformations of primary ores in polymetallic deposits, on the example of the Ozernoe Pb-Zn deposit. The discussed mechanisms can be extended to explain the genesis of other ore deposits occurring in the zones of tectonic-magmatic activation.

We consider high-purity quartzites of the Gargan quartz-bearing zone of East Sayan. The main productive varieties of quartzites have been identified. The structures, textures, chemical composition, and degree of enrichment of the quartzites and the mineral and fluid inclusions in them have been studied. Quartz concentrates of high and ultrahigh purity have been obtained from superquartzite and compact quartzite.