Vol 64, No 8 (2019)


Chronicle of the 81st annual international meeteing of the meteoritical society, july 22–27, 2018, Moscow

Ivanova M.A., Bezaeva N.S.


The most interesting and most important scientific topics related to planetology, cosmochemistry, meteoritics, impact processes, and crater formation were discussed at the 81st Annual International Meeting of the Meteoritical Society, which was first held in Russia, July 22–27, 2018. Scientists from the whole Word discussed problems of the formation of presolar grains, interplanetary dust particles and micrometeorites, refractory inclusions, volatiles, as well as the chronology of the Solar System material; carbonaceous, ordinary and enstatite chondrites, chondrules, impact structures, achondrites, geochemistry of Martian and Lunar meteorites, differentiated cosmic bodies; the most modern methods and equipment used in the study of the the Solar System material.

Геохимия. 2019;64(8):759-761
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Features of the geochemistry of the Moon and the Earth, determined by the mechanism of formation of the Earth – Moon system (report at the 81st international meteorite conference, Moscow, july 2018)

Galimov E.M.


This article discusses some features of geochemistry of the Earth and the Moon, which manifests the specificity of the mechanism of their formation by fragmentation of protoplanetary gas-dust condensation (Galimov & Krivtsov, 2012). The principal difference between this model and other hypotheses of the Earth-Moon system formation, including the megaimpact hypothesis, is that it assumes the existence of a long stage of the dispersed state of matter, starting with the formation of protoplanetary gas-dust condensation, its compression and fragmentation and ending with the final accretion to the formed high-temperature embryos of the Earth and the Moon. The presence of the dispersed state allows a certain way to interpret the observed properties of the Earth-Moon system. Partial evaporation of solid particles due to adiabatic heating of the compressing condensation leads to the loss of volatiles including FeO. Computer simulations show that the final accretion is mainly performed on a larger fragment (the Earth’s embryo) and only slightly increases the mass of the smaller fragment (the Moon embryo).This explains the relative depletion of the Moon in iron and volatile and the increased concentration of refractory components compared to the Earth. The reversible nature of evaporation into the dispersed space, in contrast to the kinetic regime, and the removal of volatiles in the hydrodynamic flow beyond the gas-dust condensation determines the loss of volatiles without the effect of isotopes fractionation. The reversible nature of volatile evaporation also provides, in contrast to the kinetic regime, the preservation of part of the high-volatile components, such as water, in the planetary body, including the Moon. It follows from the essence of the model that at least a significant part of the Earth’s core is formed not by segregation of iron in the silicate-metal melt, but by evaporation and reduction of FeO in a dispersed medium, followed by deposition of clusters of elemental iron to the center of mass. This mechanism of formation of the core explains the observed excess of siderophilic elements in the Earth’s mantle. It also provides a plausible explanation for the observed character of iron isotopes fractionation (in terms of δ57Fe‰) on Earth and on the Moon. It solves the problem of the formation of iron core from initially oxide (FeO) form. The dispersed state of the substance during the period of accretion suggests that the loss of volatiles occurred during the time of accretion. Using the fact that isotopic systems: U–Pb, Rb–Sr, 129J–129Xe, 244Pu–136Xe, contain volatile components, it is possible to estimate the chronology of events in the evolution of the protoplanetary state. As a result, agreed estimates of the time of fragmentation of the primary protoplanetary condensation and formation of the embryos of the Earth and the Moon are obtained: from 10 to 40 million years, and the time of completion of the earth’s accretion and its birth as a planetary body: 110 – 130 million years after the emergence of the solar system. The presented interpretation is consistent with the fact that solid minerals on the Moon have already appeared at least 60 million years after the birth of the solar system (Barboni et al., 2017), and the metal core in the Earth and in the Moon could not have formed before 50 million years from the start of the solar system, as follows from the analysis of the Hf-W system (Kleine et al., 2009). It is shown that the hypothesis of megaimpact does not satisfy many constraints and does not create a basis for the explanation of the geochemistry of the Earth and the Moon.

Геохимия. 2019;64(8):762-776
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Theoretical analyses of chemical and magnesium and silica isotope fractionation during vaporization of ca-al-inclusion of chondrite

Yakovlev O.I., Shornikov S.I.


Experimental study of changes in the composition of Ca-Al-inclusions of chondrites during evaporation indicates a close relationship of chemical and isotopic fractionation of this substance. The theoretical description of the coupling is carried out using the equations of the evaporation rate of the melt component (the Hertz-Knudsen equation) and isotope Rayleigh fractionation. The form of Rayleigh equation taken in foreign literature, derived from the Hertz-Knudsen equation, faces difficulties in interpreting experimental data. The discrepancy between the experimental and model data is explained by the fact that the «ideal» isotope fractionation factor used in the Rayleigh equation does not take into account its dependence on the temperature and composition of the evaporating melt. The article presents an alternative expression of the Rayleigh equation and a new expression of the rate of evaporation of Hertz-Knudsen, taking into account the activity of the melt component. The activity of the component is determined by the acidity-basicity index of the melt Ca-Al-inclusion, which, in turn, affects the evaporative fractionation of magnesium and silicon isotopes.

Геохимия. 2019;64(8):777-793
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Thermodynamic modeling of evaporation processes of lunar and meteorite substance

Shornikov S.I.


A thermodynamic approach to modeling the processes of evaporation of lunar and meteorite matter is presented. Comparison of the results of model calculations and experimental data showed high accuracy of the developed approach in the description of thermodynamic properties of melts of lunar and meteorite substance and its behavior at evaporation. The observed regularities of melt evaporation are consistent with the thermodynamic values characterizing the residual melts.

Геохимия. 2019;64(8):794-802
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P-bearing olivines of lunar rocks: sources and localization in the lunar crust

Demidovaa S.I., Anosova M.O., Kononkova N.N., Ntaflos T., Brandstätter F.


Fragments of P-bearing olivine have been studied in lunar highland, mare and mingled meteorites and in «Apollo-14», «Luna-16, -20, -24» lunar samples. Olivine contains up to 0.5 wt.% P2O5 and has variable MG# number. It is associated with anorthite, pyroxene and accessory spinel group minerals, Ti and Zr oxides, phosphates, troilite and Fe-Ni metal. Three possible sources of P-bearing olivine were found in lunar material: 1) highland anorthositic-noritic-troctolitic rocks enriched in incompatible elements and thought to be related to high-Mg suite rocks: 2) late-stage products of mare basalts crystallization; 3) unusual olivine-orthopyroxene intergrowths either of meteoritic or lunar origin. Enrichment in incompatible elements may be resulted from both crystallization processes (source 2) and KREEP assimilation (sources 1 and 3). However following metasomatic processes can lead to some addition of phosphorus and other elements.

The rarity of P-bearing olivines points either to the low abundance or local distribution of their sources in the lunar crust. Association with mare basalts specifies the highland-mare boundary. The presence of the evolved rocks in the studied breccias suggests possible connection of some sources with recently discovered granitic domes in Procellarum Ocean. That means the P-bearing sources are mainly localized on the visible side of the Moon.

Геохимия. 2019;64(8):803-825
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Crystallisation of the metal in iie irons and possible meteorite analog

Teplyakova S.N., Lorenz C.A.


The metal of the IIE irons has evidence of fractionation in the depths of the asteroid, but the presence of a fine-grained structure is incompatible with its endogenous origin. It was assumed that the metal underwent remelting on the surface of the parent body. Data on the mineragraphy, mineral and chemical composition of IIE irons (Elga, Verkhnodniprovsk, Tobychan, Miles and Watson) indicate that the relatively fine-grained metal structure and xenomorphic schreibersite grains were probably formed by crystallization from the melt. According to the calculated data on the bulk composition of the Elga metal and on the Fe-Ni-P phase diagrams, the crystallization of the first γ-Fe grains began at ~1511°С and ended at ~1060–1100°С with the formation of polygonal crystals of cm-sized taenite and xenomorphic schreibersite along their boundaries. The identical composition of xenomorphic schreibersite, both along the borders of the taenite grains and on the rims around nonmetallic inclusions, indicates their simultaneous formation. Among four generations of schreibersite, the xenomorphic schreibersite is distinguished by a high Fe/Ni ratio. It is also noted that the higher the crystallization temperature of schreibersite, the less nickel content in this schreibersite. Similar metal structures were found in other types of meteorites: in the IAB irons and in metal of some mesosiderites, and the impact mechanism of formation is considered the most likely for them. Thus, the mechanism of formation of the IIE irons by shock remelting of fractionated metal and mixing with silicate fragments in the conditions of the parent surface may have analogues among other types of meteorites.

Геохимия. 2019;64(8):826-836
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Liquid immiscibility in regions of localized shock melting in meteorite Elga

Khisina N.R., Wirth R., Abdrakhimov A.M.


The regions of localized shock melting (melt pockets) in one of silicate inclusions in IIE iron meteorite Elga were investigated with EMPA, SEM, TEM and Raman spectroscopy. It has been established that the mechanism of formation of melt pockets in Elga is of a mixed nature, associated not only with the melting in situ of the silicate matrix, but also with the intrusion of portions of the melted schreibersite-oxide rim inside the silicate inclusion. Melt pockets have an emulsion texture, which is a sign of phase separation by liquid immiscibility in high-temperature shock melts. The emulsion texture, formed by droplet-shaped exsolutions of siderite in the schreibersite matrix of one of the melt pockets, has all the features of phase separation by liquid immiscibility at superliquidus temperatures and thus convincingly indicates the extraterrestrial origin of siderite in the Elga meteorite.

Геохимия. 2019;64(8):837-847
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Raman spectroscopy of high-pressure phases in shocked L6 chondrite NWA 5011

Litasov K.D., Badyukov D.D.


In the paper we present results of studies of thick shock melt veins in NWA 5011 L6 chondrite. The veins contain a wide variety of high-pressure phases that correspond to contrast values of pressure-temperature parameters on equilibrium phase diagrams. Olivine was transformed to ringwoodite and wadsleyte, orthopyroxene to majorite, akimotoite, and bridgmanite glass, maskelenite is converted to jadeite (+SiO2) and lingunite, apatite to tuite, and chromite to the phase with the calcium ferrite (mCF-FeCr2O4) structure. ) The peak PT shock parameters for NWA 5011 seem highest among the ones for other shocked chondrites according to wide occurrence of lingunite and bridgmanite glass and are considerable higher than 25 GPa and 2500 K. Akimotoite crystals in a quenched matrix of shock melt veins were found for the first time. Probably, they initially crystallized as bridgmanite, since akimotoite is not a liquidus phase in related systems. Plagioclase-chromite aggregates have been established, which characterize the late stages of the shock process and are formed during successive crystallization from isolated pockets of the impact melt.

Геохимия. 2019;64(8):848-858
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Shock-wave experiment with the Chelyabinsk LL5 meteorite: experimental parameters and the texture of the shock-affected material)

Petrova E.V., Grokhovsky V.I., Kohout T., Muftakhetdinova R.F., Yakovlev G.G.


The shock experiment with Chelyabinsk LL5 light lithology material was performed as a spherical geometry shock. The material experienced shock and thermal metamorphism from the initial S3–4 up to complete melt stage. Temperature and pressure realized were estimated above 2000°С and 90 GPa. Textural shock effects were studied by the means of optical and electron microscopy. By the only experimental impact, all the range of the shock pressures and temperatures was realized. Four zones were revealed from the petrographic analysis: 1 – melt zone, 2 – melted silicates zone, 3 – black ring zone, 4 – weakly shocked initial material. Several features of the material texture were noted: displacement of the metal and troilite phases from the central melt zone; mixed lithology zone formation (light-colored chondrules within the silicate melt); dark-colored lithology ring formation; generation of radial-oriented shock veins. Thus, at the experimental fragment, four texture zones were formed. These zones correspond to the different lithology types of the Chelyabinsk LL5 meteorite, which could be found in different fragments of the meteoritic shower from UrFU collection. The results obtained prove that the shock wave loading experiment could be used for space shock modeling. Therefore, the processes of the small bodies of the Solar system could be experimentally modeled at the laboratory conditions.

Геохимия. 2019;64(8):859-868
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Meteorites minerals

Ivanov A.V., Yaroshevskiy А.A., Ivanova M.A.


“The Meteorite Minerals Catalog” is the first edition in Russia prepared in the 21st century. It includes all the minerals found in meteorites, approved by the Committee on New Minerals and MMA Minerals Names, approved before January 1, 2017, and mineral phases. The Russian and English names, chemical composition, as well as meteorites or meteorite groups, which are characterized by the considered minerals are given for all minerals and mineral phases. Mainly the first description of all minerals and phases and references to publications are also given in the Catalog. Samples of minerals whose origin is associated with specific processes are also presented: these are pre-solar meteorite minerals, refractory and ultra-refractory solar condensates, impact minerals of meteorites and products of the terrestrial weathering of meteorites.

Геохимия. 2019;64(8):869-932
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