Yu. A. Kostitsyn

Analytical results for the material of the Chelyabinsk meteorite

This paper presents the results of the mineralogical, petrographic, elemental, and isotopic analysis of the Chelyabinsk meteorite and their geochemical interpretation. It was shown that the meteorite can be assigned to LL5-group ordinary chondrites and underwent moderate shock metamorphism (stage S4). The Chelyabinsk meteorite contains a significant fraction (approximately one-third by volume) of shock-melted material similar in composition to the main volume of the meteorite. The results of isotopic analysis suggest that the history of meteorite formation included an impact event approximately 290 Ma ago.

Geochronology of the Dovyren intrusive complex, northwestern Baikal area, Russia, in the Neoproterozoic

The paper reports newly obtained data on the geochronology of the Dovyren intrusive complex and associated metarhyolites of the Inyaptuk Formation in the Synnyr Range. The data were obtained by local LA-ICPMS analysis of zircons in samples. The U-Pb age of olivine-free gabbronorite from near the roof of the Yoko-Dovyren Massif is 730 ± 6 Ma (MSWD = 1.7, n = 33, three samples) is close to the estimated age of 731 ± 4 Ma (MSWD = 1.3, n = 56, five samples) of a 200-m-thick sill beneath the pluton. These data overlap the age of recrystallized hornfels found within the massif (“charnockitoid”, 723 ± 7 Ma, MSWD = 0.12, n = 10) and a dike of sulfidated gabbronorite below the bottom of the massif (725 ± 8 Ma, MSWD = 2.0, n = 15). The estimates are also consistent with the age of albite hornfels (721 ± 6 Ma, MSWD = 0.78, n = 12), which was produced in a low-temperature contact metamorphic facies of the host rocks. The average age of the Dovyren Complex is 728.4 ± 3.4 Ma (MSWD = 1.8, n = 99) based on data on the sill, near-roof gabbronorite, and “charnockitoid”) and is roughly 55 Ma older than the estimate of 673 ± 22 Ma (Sm-Nd; [13]). The U-Pb system of zircon in two quartz metaporphyre samples from the bottom portion of the Inyaptuk volcanic formation in the northeastern part of the Yoko-Dovyren Massif turns out to be disturbed. The scatter of the data points can be explained by the effect of two discrete events. The age of the first zircon population is then 729 ± 14 Ma (MSWD = 0.74, n = 8), and that of the second population is 667 ± 14 Ma (MSWD = 1.9, n = 13). The older value pertains to intrusive rocks of Dovyren, and the age of the “rejuvenated” zircon grains corresponds to the hydrothermal-metasomatic processes, which affected the whole volcano-plutonic sequence and involved the serpentinization of the hyperbasites. This is validated by the results of Rb-Sr isotopic studies with the partial acid leaching of two serpentinized peridotite samples from the Verblyud Sill. These studies date the overprinted processes at 659 ± 5 Ma (MSWD = 1.3, n = 3).

Zircons in gabbroids from the axial zone of the mid-atlantic ridge: U-Pb age and 176Hf/177Hf ratio (Results of investigations by the laser ablation method)

In recent decades the evolution of novel local methods of isotopic and elemental analysis such as massspectrometry of secondary ions (SHRIMP, SIMS) and laser ablation along with inductivecou� pled plasma massspectrometry (LA-ICP-MS), which allow highly sensitive analysis of separate grains of minerals, have promoted the geochronological investigation of young rocks from ocean floor within middleocean ridges (MORs). Knowledge on the age distribution of minerals along and around the rift val� leys of MORs, where the new oceanic crust presum� ably form, would favor better understanding of dynamics of the process and establishment of its gen� eral regularities and likely anomalies. The isotopic composition of hafnium, as well as that of neody� mium, are used to determine the geochemical proper� ties of minerals and rocks that in combination with the age data provide unique versatility information on their origin. Herein we present the results of isotopic study of zircons separated from gabbroids of the axial zone of the MidAtlantic Ridge (MAR). The samples were collected within the Markov Deep from the depths 3400-4300 m between

Native silicon and iron silicides in the Dhofar 280 lunar meteorite

The Dhofar 280 lunar highland meteorite is the first one in which native silicon was identified in association with iron silicides. This association is surrounded by silicate material enriched in Si, Na, K, and S and occurs within an impact-melt matrix. Compared to the meteorite matrix, the objects with native Si and the silicate material around them show high Al-normalized concentrations of volatile elements and/or elements with low sensitivity to oxygen but are not any significantly enriched in refractory lithophile elements. Some lithophile elements (V, U, Sm, Eu, and Yb) seem to be contained in reduced forms, and this predetermines REE proportions atypical of lunar rocks and a very low Th/U ratio. The admixture of siderophile elements (Ni, Co, Ge, and Sb) suggests that the Si-bearing objects were contaminated with meteorite material and were produced by the impact reworking of lunar rocks. The high concentrations of volatile elements suggest that the genesis of these objects could be related to the condensation of silicate vapor generated during meteorite impacts. The reduction of silicon and other elements could take place in an impact vapor cloud, with the subsequent condensation of these elements together with volatile components. On the other hand, condensates of silicate vapor could be reduced by impact reworking of impact breccias. Impact-induced vaporization and condensation seem not to play any significant role in forming the composition of the lunar crust, but the contents of the products of such processes can be locally relatively high. The greatest amounts of silicate vapor were generated during significant impact events. For example, more than 70% of the total mass of lunar material evaporated in the course of impact events should have resulted from the collision of the Moon with a cosmic body that produced the Moon’s largest South Pole-Aitken basin.

Geochemistry of the Submarine Vityaz Ridge at the Pacific Slope of the Kurile Island Arc

This paper reports the results of geological studies at the submarine Vityaz Ridge carried out during cruises 37 and 41 on the R/V “Akademik Lavrent’ev” in 2005 and 2006. The studied area is located at the near-island trench of the slope in the central part of the Kurile island arc. Morphologically, it consists of two parts: inner volcanic arc represented by the Great Kurile Range and outer arc corresponding to the submarine Vityaz Ridge. Diverse rocks that compose the basement and sedimentary cover of the ridge were recovered by dredging. Based on K-Ar dating and geochemistry, the volcanics were divided into Paleocene, Eocene, late Oligocene, and Pliocene-Pleistocene complexes. Each of the distinguished complexes reflects the tectonomagmatic stage in the ridge evolution. The geochemical and isotope data on the volcanics indicate the contribution of ancient crustal material in magma source and, correspondingly, the formation of this structure on the continental basement. Two-stage model ages, TDM2, vary in a wide range from zero values in the mafic rocks to 0.77 Ga in felsic varieties, pointing to the presence of Precambrian protolith in the source of the felsic rocks of the Vityaz Ridge. The Pliocene-Pleistocene volcanics are classed with the tholeiitic, calc-alkaline, and subalkaline series, which differ in alkali contents and REE fractionation. The values of (La/Sm)N and (La/Yb)N ratios vary from 0.74 and 0,84 in the tholeiitic varieties to 1.19 and 1.44 in the calcalkaline and 2.32 and 3.73 in the subalkaline rocks. All three varieties occur within the same volcanic edifices and were formed during differentiation of magmatic melt that were channeled along fault zones from the mantle source slightly enriched in crustal component

Relationships between the chemical and isotopic (Sr, Nd, Hf, and Pb) heterogeneity of the mantle

The reasons for the isotopic heterogeneity of the mantle are analyzed in this paper on the basis of published isotopic data. It was shown that the observed variations in the Sr, Nd, Hf, and Pb isotopic compositions of oceanic basalts cannot be explained by mixing of a finite number of homogeneous reservoirs (components). The considerable variations in the contents of Rb, Sr, Sm, Nd, Lu, Hf, U, Th, and Pb and ratios of these and other trace elements in tholeiitic basalts indicate that the chemical heterogeneity of mantle-derived rocks is inherited in part from their sources. Oceanic tholeiitic basalts show a tight correlation between the variances of Nd, Hf, Sr, and Pb isotopic ratios and the variances of respective radiogenic additions that could be accumulated in these rocks over a time period of 〈t〉 = 1.8 Gyr. This paradox clearly indicates that variations in all the mentioned isotopic systems in the mantle cannot be understood without the analysis of the geochemical heterogeneity of rocks.The close to lognormal distributions of lithophile trace elements in oceanic tholeiitic basalts and the character of correlations between them suggest that magmatic differentiation was the major mechanism of the formation of chemical heterogeneity in the mantle. The role of metasomatism in the global transport of trace elements and formation of the geochemically heterogeneous mantle is probably rather limited. Intrusive processes within the mantle could result in the development of chemical and, after a period of time, isotopic anomalies in the mantle. Simple calculations show that long-lived geochemical anomalies related to alkaline magmatism could be responsible for EM-I type isotopic anomalies, and geochemical anomalies produced in the mantle by enriched tholeiitic melts could be sources of EM-II type isotopic anomalies. The analysis of the distribution of the isotopic compositions of mantle-derived igneous rocks in various “isochron” coordinates suggested that the formation of geochemical anomalies in the mantle is a long-term process lasting for hundreds of millions of years. Nonetheless, trends approaching 4.5 Ga were never observed in such diagrams, i.e., the mantle is in general rejuvenated in all isotopic systems. Both on global and local scales, there are no mantle domains that have remained geochemically closed and isolated since the Earth’s formation. The entire mantle is involved in material exchange processes.The development of isotopic systems in the mantle was explored by means of statistical modeling accounting for the tendency of a continuous increase in the chemical heterogeneity of the mantle source and the tendency of obliteration of the isotopic heterogeneity owing to the convective mixing in the mantle. The modeling demonstrated that the character of the isotopic heterogeneity of the mantle is statistically consistent with the character of its chemical heterogeneity. The mantle isotopic anomalies HIMU, EM-I, and EM-II were generated by two simultaneous processes: the magmatic differentiation of mantle material and its not very efficient mixing.

U-Pb zircon dating of the lunar meteorite Dhofar 1442

Dhofar 1442 is one of the few lunar KREEP-rich meteorites, which contains KREEP norites and KREEP gabbronorite as well as low-Ti basalts and highly evolved granophyres. Zircon is a typical accessory mineral of KREEP rocks. U-Th-Pb dates of 12 zircon grains (four of them were in two lithic clasts, and the others were fragments in the meteorite matrix) indicate that the zircons belong to at least two groups of different age: “ancient” (∼4.31 Ga) and “young” (∼3.95 Ga), which correspond to two major pulses of KREEP magmatism in the source region of the Dhofar 1442 meteorite. The zircon of the “young” group was most probably related to the crater ejecta of the Mare Imbrium Basin. The rock fragments dated at approximately 3.95 Ga have the composition of KREEP gabbronorite. The parental rocks of the zircon of the “ancient” group in the Dhofar 1442 meteorite are uncertain and could be highly evolved granophyres. This hypothesis is supported by the high Th (100–300 ppm) and U (150–400 ppm) contents. These zircon fragments of the “ancient” group, higher than in the “young” group (<50 ppm Th and <70 ppm U) and are typical of zircon from lunar granitic rocks. The composition of the products of KREEP magmatism in the source region of the Dhofar 1442 meteorite could vary from predominantly granitic to KREEP gabbronoritic at 4.3–3.9 Ga.

The Ruiga intrusion: A typical example of a shallow-facies paleoproterozoic peridotite-gabbro-komatiite-basaltic association of the Vetreny Belt, Southeastern Fennoscandia

The Ruiga differentiated mafic-ultramafic intrusion in the northwestern part of the Vetreny Belt paleorift was described for the first time based on geological, petrological, geochronological, and geochemical data. The massif (20 km2 in exposed area) is a typical example of shallow-facies peridotite-gabbro-komatiite-basalt associations and consists of three zones up to 810 m in total thickness (from bottom to top): melanogab-bronorite, peridotite, and gabbro. In spite of pervasive greenschist metamorphism, the rocks contain locally preserved primary minerals: olivine (Fo75–86), bronzite, augite of variable composition, labradorite, and Cr-spinels. A mineral Sm-Nd isochron on olivine melanogabbronorite from the Ruiga Massif defines an age of 2.39 ± 0.05 Ga, while komatiitic basalts of the Vetreny Belt Formation were dated at 2.40–2.41 Ga (Puchtel et al., 1997). The rocks of the Ruiga intrusion and lava flows of Mt. Golets have similar major, rare-earth, and trace element composition, which suggests their derivation from a single deep-seated source. Their parent magma was presumably a high-Mg komatiitic basalt. In transitional crustal chambers, its composition was modified by olivine-controlled fractionation and crustal contamination, with the most contaminated first portions of the ejected melt. In terms of geology and geochemistry, the considered magmatic rocks of the Vetreny Belt are comparable with the Raglan Ni-PGE komatiite gabbro-peridotite complex in Canada (Naldrett, 2003).

Modern problems of geochemical and U-Pb geochronological studies of zircon in oceanic rocks

We present results of zircon LA-ICP-MS U–Pb, Lu–Hf, and trace-element study in combination with whole-rock Sm-Nd and Rb-Sr isotope data on the magmatic rocks of the Markov Deep and Ashadze hydrothermal field (Mid-Atlantic Ridge). Zircon from three gabbronorite samples in the Markov Deep defined an U–Pb ages between 0.90 ± 0.02 and 2.00 ± 0.05 Ma, with the youngest age found in the deepest sample. Zircons from four samples of gabbros and trondhjemites of the Ashadze Field have identical ages: from 1.04 ± 0.07 to 1.12 ± 0.09 Ma. Plagioclase troctolite from the Markov Deep (sample I-1069/19) contains exotic zircon grains with ages widely ranging from 90 Ma to 3.2 Ga, which is inconsistent with age of the rocks in the Mid-Atlantic Ridge. Several hypotheses are discussed to explain the origin of such exotic grains, in particular, their formation at mantle depths, or reaching these depths with subducted crust, and others. Experimental study of zirconium solubility shows that the mafic and ultramafic melts could be oversaturated with respect to zirconium only at unrealistically high contents, which usually do not occur in the corresponding rocks. Entrapped xenogenic zircon must be dissolved in the mafic and ultramafic melts and its finds in these rocks presumably indicate its disequilibrium precipitation. Zircon could be formed in the intrusive mafic rocks at the final stages of fractional crystallization, which explains the presence of own zircon in gabbroids. Zircon is very stable in crustal magmatic processes, especially at lowered activity of alkalis, but almost instantly (on geological scale) loses radiogenic lead by diffusion way under upper mantle conditions (1300–1500°C). While applying REE distribution for interpreting zircon origin, as many as possible elements should be analyzed to discriminate between intrinsic zircon element distribution and anomalies caused by defects in its structure.

Geochemistry and age of the complex of alkaline metasomatic rocks and carbonatites of the Gremyakha-Vyrmes massif, Kola Peninsula

This paper presents new geochemical data on the complex of alkaline metasomatic rocks and carbonatites, which hosts the rare-metal mineralization of the Gremyakha-Vyrmes massif. The contents of major and trace, including rare-earth elements were determined in the albitites, aegirinites, and carbonatites. Two types of the rare-metal ores are distinguished: niobium albitite and zirconium aegirinite ores. It was shown that the albitites and aegirinites have similar trace element distribution patterns, being most geochemically close to the foidolites. The carbonatites, albitites, and aegirinites were dated by Rb-Sr and Sm-Nd methods at 1887 ± 58 Ma, which corresponds to the formation age of the Gremyakha-Vyrmes massif. The ultrabasic rocks, foidolites, alkaline metasomatic rocks, and carbonatites were formed successively within a relatively narrow range. The geological observations and geochemical data led us to conclude that the emplacement of the fluid-saturated carbonatite solutions-melts at the final stages of the massif formation against a background of fault tectonics caused a pervasive metasomatism of the ultrabasic and alkaline rock complexes and, as a result, the formation of the alkaline albitites and aegirinites. The carbonatites could be sources of rare-metals, while foidolites served as a geochemical barrier, and their metasomatic alteration led to the formation of Zr-Nb mineralization in the albitites and aegirinites.