Vol 64, No 9 (2019)


Iron and its compounds in the earth’s core: new data and ideas

Pushcharovsky D.Y.


Iron-the most abundant chemical element in the Earth’s core, which it is more than 85% by weight. The remaining ~15% weight of the nucleus is accounted for Ni, and some lighter elements: Si, C, S, O, and H. New data of the transformations of iron and its compounds under the influence of high temperature and pressure, which modeling conditions in the Earth’s core, are analyzed. Structural types of mineralogically possible polymorphic modifications of iron and its compounds in deep geospheres in the Earth’s core are considered. New data about changes in the electronic structure of iron atoms at high pressure are presented. Scientific ideas were expanded and new ideas were put forward about the forms of concentration of chemical elements at ultra-high temperature and pressure. It is concluded that modern views on the specific features and properties of the Earth’s mantle and core are based not only on the results of geological and geophysical methods, but are clarified using micro-mineralogical and crystallographic approaches.

Геохимия. 2019;64(9):936-947
pages 936-947 views

The inclusions of SiO2 in sublithospheric diamonds

Zedgenizov D.A., Ragozin A.L., Kagi H., Yurimoto H., Shatsky V.S.


The specific features of the mineralogy of SiO2 inclusions in sublithospheric diamonds are described in this study. Such diamonds are characterized by a complex growth history with stages of growth and dissolution and postgrowth processes of deformation and crushing. The nitrogen content in all studied crystals does not exceed 71 ppm and nitrogen is detected only as B-defects. The carbon isotope composition of diamonds varies widely from -26.5 to -6.7 ‰ of δ13С. SiO2 inclusions associate with omphacitic clinopyroxenes, majoritic garnets, CaSiO3, jeffbenite and ferropericlase. All SiO2 inclusions are coesite, which is often accompanied by micro-blocks of kyanite. These phases are suggested to represent the product of the retrograde transformation of the primary Al-stishovite. Significant internal stresses in the inclusions and deformations around them can be evidence of thise phase transformation. The heavier oxygen isotope composition of SiO2 inclusions in sublithospheric diamonds (up to 12.9 δ18O) indicates the crustal origin of their protoliths. The observed anti-correlation of δ18O of SiO2 inclusions and δ13C of their host diamonds reflects the processes of interaction of slab-derived melts with reduced mantle rocks at depths above 270 km.

Геохимия. 2019;64(9):948-957
pages 948-957 views

The compositional peculiarities of microinclusions in diamonds from the Lomonosov deposit (Arkhangelsk province)

Kriulina G.Y., Iskrina A.V., Zedgenizov D.A., Bobrov A.V., Garanin V.K.


The data on the composition of microinclusions in diamonds from the Lomonosov deposit are reported. The studied diamonds include coated (n = 5) and cubic (n = 5) crystals. The determined range of the degree of nitrogen aggregation (4−39 %B1) in diamonds does not support their links with kimberlite magmatism, but their short occurrence in the mantle at higher temperatures is probable as well. The composition of melt/fluid microinclusions in these samples varies from essentially carbonatitic to significantly silicate. It is shown that the content of MgO, CaO, Na2O, Cl and P2O5 decreases with increasing content of silicates and water. Different mechanisms of generation and evolution of diamond-forming media are considered to explain the observed variations.

Геохимия. 2019;64(9):958-966
pages 958-966 views

Synthesis and Raman spectra of K-Ca double carbonates: K2Ca(CO3)2 bütschliite, fairchildite, and K2Ca2(CO3)3 at 1 ATM

Arefiev A.V., Podborodnikov I.V., Shatskiy A.F., Litasov K.D.


Here we present results on synthesis of double K−Ca carbonates at atmospheric pressure in closed graphite capsules. The mixtures of K2CO3 and CaCO3 corresponding to stoichiometry of K2Ca(CO3)2 and K2Ca2(CO3)3 were used as starting materials. The low-temperature modification of K2Ca(CO3)2 was synthesized by a solid-state reaction at 500°C during 96 h. The high-temperature modification of K2Ca(CO3)2 as well as the K2Ca2(CO3)3 compound were synthesized both by a solid-state reaction at 600°C during 72 h and during cooling of the melt from 830 to 650°C for 30 min. The obtained carbonates were studied by Raman spectroscopy. The Raman spectrum of bütschliite is characterized by the presence of an intense band at 1093 cm1 and several bands at 1402, 883, 826, 640, 694, 225, 167 and 68 сm1. The Raman spectrum of fairchildite has characteristic intense bands at 1077 and 1063 cm1, and several bands at 1760, 1739, 719, 704, 167, 100 сm1. In the Raman spectrum of K2Ca2(СO3)3 intense bands at 1078 and 1076 cm1 and several bands at 1765, 1763, 1487, 1470, 1455, 1435, 1402, 711, 705, 234, 221, 167, 125 and 101 сm1 were found. The collected Raman spectra can be used to identify carbonate phases entrapped as microinclusions in phenocrysts and xenoliths from kimberlites and other alkaline rocks.

Геохимия. 2019;64(9):967-973
pages 967-973 views

Phase relations in the model pyrolite at 2.5, 3.0, 7.0 GPа and 1400–1800°c: evidence for the formation of high-chromium garnets

Matrosova E.А., Bendeliani А.А., Bobrov A.V., Kargal’tsev A.A., Ignat’ev Y.A.


Based on study of partial melting in the model pyrolite, it is shown that garnets synthesized at 7.0 GPa in a temperature range of 1400–1800°C are characterized by an excessive Si content (in relation to 3 f.u.), stable admixture of Cr2O3, and, thus, represent a solid solution of the pyrope–majorite–knorringite composition. Increase in the Cr/Al value in the starting composition results in increase of this ratio in garnet. With increasing temperature, the concentration of Cr2O3 decreases in restite and increases in melt. Cr/Al increases in all garnets from the zone of restite and from the quenched melt aggregate. Estimates of the bulk compositions of restite formed by partial melting of the model pyrolite at 2.5 and 3.0 GPa show that the concentration of Cr in it is higher than that in the starting composition. All minerals from the zone of restite are characterized by the high Cr concentrations, and upon partial melting in the spinel-depth facies, Cr is redistributed to restite. Our results show that the formation of high-chromium garnets relates to the protolith with the high Cr/Al value formed as a residue from partial melting in the spinel-depth facies and further transported to the garnet facies.

Геохимия. 2019;64(9):974-985
pages 974-985 views

Evolution of diamond-forming systems of the mantle transition zone: ringwoodite peritectic reaction (Mg,Fe)2SiO4 (experiment AT 20 GPa)

Spivak А.V., Litvin Y.А., Zakharchenko Е.S., Simonova D.А., Dubrovinsky L.S.


The peritectic reaction of ringwoodite (Mg,Fe)2SiO4 and silicate-carbonate melt with formation of magnesiowustite (Fe,Mg)O, stishovite SiO2 and Mg, Na, Ca, K-carbonates is revealed by experimental study at 20 GPa of melting relations of the multicomponent MgO−FeO−SiO2−Na2CO3−CaCO3−K2CO3 system of the Earth’s mantle transition zone. A reaction of CaCO3 and SiO2 with the formation of Ca-perovskite CaSiO3 is also detected. It is shown that the peritectic reaction of ringwoodite and melt with the formation of stishovite physic-chemically controls the fractional ultrabasic-basic evolution of both magmatic and diamond-forming systems of the deep horizons of the transition zone up to its boundary with the Earth’s lower mantle.

Геохимия. 2019;64(9):986-994
pages 986-994 views

Formation of iron hydride and iron carbide from hydrocarbon systems at ultra high thermobaric conditions

Serovaiskii A.Y., Kolesnikov A.Y., Kutcherov V.G.


The chemical interaction of hydrocarbon systems and iron-bearing minerals was investigated under extreme thermobaric conditions, corresponding to the Earth upper mantle. As a result of the reaction, the formation of iron carbide and iron hydride was detected. The experiments were carried out in diamond anvils cells with laser heating. Natural petroleum from the Korchaginskoe deposit and a synthetic mixture of paraffin hydrocarbons were used as hydrocarbon systems, and pyroxene-like glass and ferropericlase (57Fe enriched) as iron bearing minerals. The experiments were carried out in the pressure range of 26–95 kbar and temperature range of 1000–1500°C (±100°C). As a result of the experiments, the formation of iron hydride was detected at pressure of 26–69 kbar (corresponds to a depth of 100–200 km), and a mixture of iron carbide and iron hydride at pressure of 75–95 kbar (corresponds to a depth of 210–290 km). The formation of hydrides and iron carbides as a results of the interaction of hydrocarbon systems with iron-bearing minerals may indicate the possible existence of these compounds in the upper mantle.

Геохимия. 2019;64(9):995-1002
pages 995-1002 views

MgCO3 + SiO2 reaction at pressures to 32 GPa studied using in situ X-ray diffraction and synchrotron radiation

Litasov K.D., Shatskiy A.F.


The results of the experimental study of the decarbonation and melting reactions in the MgCO3–SiO2 system at pressures up to 32 GPa using multi-anvil technique, in situ X-ray diffraction and synchrotron radiation have been reported. At 3–7 GPa and 1400–1700 K, the reaction proceeds with the release of carbon dioxide and the formation of enstatite. At 9–13 GPa and 1850–1930 K, clinoenstatite, carbonate-silicate melt, and CO2 were found among the reaction products. At 16 GPa and 1825 K, the reaction is accompanied by the formation of wadsleyite and at higher temperature by the formation of a carbonated melt, with a Mg/Si ratio close to wadsleyite, stishovite and CO2 fluid. At this pressure, which coincides with the wadsleyite-stishovite assemblage stability field in the MgSiO3 phase diagram, a decrease in the reaction temperature by about 100 K is observed. At higher pressures, the reaction proceeds with the formation of the MgSiO3 (akimotoite or bridgmanite) + melt assemblage. The reaction temperature at 25–35 GPa does not change and is about 2000 K. With a further increase in temperature to 2100 K, bridgmanite melts incongruently, reacting with a carbonate-silicate melt to form stishovite. The composition of the eutectic mixture shifts towards MgCO3 with increasing pressure. The studied reaction marks the upper temperature limit of the stability of magnesite and the free phase of SiO2 in the Earth’s mantle and generally coincides with the mantle adiabat at depths of 300–900 km.

Геохимия. 2019;64(9):1003-1012
pages 1003-1012 views

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