L. V. Sazonova

Olivine from the Pionerskaya and V. Grib kimberlite pipes, Arkhangelsk diamond province, Russia: Types, composition, and origin

We report the first systematic study of different textural varieties of olivine (olivine from peridotite xenoliths, macrocryst-type Ol-I, and phenocryst-type zoned Ol-II) from two diamondiferous kimberlite pipes of the Arkhangelsk diamond province (V. Grib and Pionerskaya) differing in geologic setting, geochemical and isotopic characteristics, and diamond content. Approximately 550 olivine analyses were obtained by the EPMA technique using the precise method of Sobolev et al. (2007) adapted at the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russian Academy of Sciences (Kargin et al., 2014). Olivines from the V. Grib moderate-Ti kimberlites, which are similar to Group I kimberlites in geochemical and Sr-Nd isotopic characteristics and rich in diamond, are dominated by high-Mg# low-Ti Ol-I formed owing to the fractional crystallization of a carbonate-rich protokimberlite melt interacting with orthopyroxene-bearing peridotite material; the fraction of high-Ti Ol-I produced by the metasomatic alteration of peridotite under the influence of silicate aqueous fluids is significantly lower; and xenocrysts weakly affected by metasomatic agents (melts and fluids) occur in minor amounts. Olivines from the low-Ti Pionerskaya kimberlites, which are similar to Group II kimberlites in geochemical and Sr-Nd isotopic characteristics and show a moderate diamond content, are dominated by high-Ti Ol-I, and xenocrysts weakly affected by metasomatic agents are also abundant. In the kimberlites of both pipes, the cores of Ol-II crystals are usually composed of low-Ti olivine similar in composition to Ol-I; both high-Ti and low-Ti olivine cores occur in the Pionerskaya pipe; whereas cores corresponding to high-Ti Ol-I were never found in the V. Grib pipe. The outer zones of olivine and small olivine grains in the groundmass show considerable variations in minor element contents within a narrow Mg# range. It is suggested that the high-Ti rims of Ol-II from the V. Grib and Pionerskaya kimberlites were produced by the late crystallization of kimberlite melt, and the low-Ti rims on the outer zones of Ol-II in the Pionerskaya kimberlites were formed by late-stage equilibration with an aqueous fluid separated from the kimberlite melt and/or possible kinetic effects. Our study revealed the diversity of olivine origin in the kimberlites and showed that there is no single mechanism of olivine formation.

Genesis of ringwoodite during metamorphism induced by impact waves: Experimental data

Ringwoodite, a high-density olivine modification, was first synthesized by loading plagioclase-biotite-quartz schist containing garnet and staurolite by impact waves. Ringwoodite was identified in the impact-thermal aggregates that replaced biotite (together with a mineral corresponding to spinel in chemical composition and with a biotite residue). The physical parameters under which ringwoodite was synthesized in this experiment (Pimp ∼ 20–30 GPa and T ∼ 1060–1500°C) include a pressure approximately 1.5 times higher than that in static analogous experiments. The ringwoodite was formed via the regrouping of and the associated removal and addition of material, as follows from the development of ringwoodite after biotite, a mineral of principally different composition. Component migration was reliably confirmed by the microprobe mapping of the chemistries of the original and newly formed minerals, which makes the origin of the ringwoodite similar to the origin of diamond (togorite) in the Kara astrobleme, where the impact loading was up to ten times higher than the static pressure.

High-pressure polymorph modifications of some minerals in impactites: Geological observations and experimental data

The paper presents a review of literature data on the polymorph modifications of SiO2 (coesite and stishovite), C (diamond and lonsdaleite), Mg2SiO4 (ring woodite), and MgSiO3 (majorite) found in naturally occurring impact structures (astroblemes) and synthesized in experiments. Much attention is devoted to the description of ringwoodite and a high-density phase of pyroxene composition, which were obtained by the authors in experiments on the shock-wave loading of rocks under pressures of 30–70 GPa. The high-density polymorph modifications listed above are demonstrated to be formed by the following three mechanism transforming the original material: (i) crystallization from an impact melt, (ii) martensite phase transition, and (iii) migration phase transition (the latter two mechanisms occur in the solid phase). Both in nature and in laboratory experiments, high-density polymorph modifications are formed under the effect of shock waves under dynamic pressures exceeding the static pressure values by factors from 1.5–2 to 10. We were the first to experimentally obtain ringwoodite and the high-pressure phase of pyroxene composition via the diaplectic transformations of biotite and garnet. Our experiments confirm calculation data on the disproportionation of the shock-wave energy between minerals composing a rock according to their volumetric contents. The experimental data also confirm our earlier conclusions that the pressures under which the shock-thermal decomposition of minerals starts are controlled by the types of the crystalline structures of these minerals.

Shock metamorphism of some rock-forming minerals: Experimental results and natural observations

The products of shock metamorphism in the Jänisjärvi astrobleme in Karelia, Russia, are compared with the results of experiments in which spherical converging shock waves affected a spherical rock sample. The sample was loaded by a broad spectrum of shock pressures, which increased from ∼20 GPa at the periphery of the rock sphere to > 200 GPa at its center. Experiments with rocks metamorphosed under the effect of spherical converging shock waves imitate collisions of cosmic bodies with the Earth’s surface, when transformations in rocks and minerals are induced by a single impact event. The shock-thermal decomposition of mafic minerals occurs in the same succession in nature and the experiments, with some differences between natural and experimentally produced shock-thermal aggregates likely accounted for by the smaller sizes of the experimental impact rock sample and, correspondingly, its more rapid quenching. Our shock experiments were the first to synthesize ringwoodite that was rich in Al2O3 and should be referred to as aluminous ringwoodite. The mineral was produced not via the martensite transition of olivine but by means of biotite replacement coupled with the migration of elements. The transformations of minerals by shock waves (amorphization and shock-thermal decomposition) were determined to be controlled mainly by the crystal structures of these minerals. The experimental products provide evidence of the migration of chemical elements within the crystal structure. The structural setting of ions in a mineral determines the onset of element migrations and the intensity of this process.

SHOCK METAMORPHISM OF SOME ROCK-FORMING MINERALS

The shock metamorphism of schist consisting of garnet, biotite, quartz, and plagioclase is studied under shock wave loading of a sample in steel recovery ampoules of plane geometry. A maximum shock pressure was reached during several circulations of waves in the sample (stepwise shock compression) and varied within the range 19–52 GPa. The recovered samples were examined by the methods of scanning electron microscopy and microprobe and X-ray phase analysis. The results were compared with natural impactites and with shock-induced alterations in minerals loaded by a spherical convergent wave. It is established that, given a plane geometry of loading (stepwise shock compression), solid-state transformations at the lattice level (migration of chemical elements and formation of shock thermal aggregates) are not observed in all of the studied minerals, in contrast to natural impact processes and spherical geometry experiments. Under the conditions of our experiments, minerals melt at higher pressures than in the case of natural impact processes and spherical geometry experiments. However, for each mineral studied, the mechanical strain patterns at close shock pressures are, on the whole, the same for all of the aforementioned three variants of shock wave loading.

Crystallochemical Structure of Rock‐Forming Minerals and Peculiarities, Sequence and Completeness of Physicochemical Transformations in Weak and Strong Shock Waves

Investigation results on shock metamorphism of rock taken from the environs of the natural meteoritic crater Janisjarvy, Karelia, Russia, before and after its spherical explosive compression in the laboratory‐scale experiments, are presented. Elements migration in stress‐wave of increasing along the radius intensity in the solid state, as well as peculiarities of melting of polyminerals rock on isentropes under rarefaction and at the Hugoniots have been observed. A correlation has been established between main characteristics of shock melt glasses, obtained in rock samples of the Janisjarvy crater at meteoritic impact and during explosive laboratory‐scale experiments.

Kimberlite age in the Arkhangelsk Province, Russia: Isotopic geochronologic Rb–Sr and 40Ar/39Ar and mineralogical data on phlogopite

The paper reports detailed data on phlogopite from kimberlite of three facies types in the Arkhangelsk Diamondiferous Province (ADP): (i) massive magmatic kimberlite (Ermakovskaya-7 Pipe), (ii) transitional type between massive volcaniclastic and magmatic kimberlite (Grib Pipe), and (iii) volcanic kimberlite (Karpinskii-1 and Karpinskii-2 pipes). Kimberlite from the Ermakovskaya-7 Pipe contains only groundmass phlogopite. Kimberlite from the Grib Pipe contains a number of phlogopite populations: megacrysts, macrocrysts, matrix phlogopite, and this mineral in xenoliths. Phlogopite macrocrysts and matrix phlogopite define a single compositional trend reflecting the evolution of the kimberlite melt. The composition points of phlogopite from the xenoliths lie on a single crystallization trend, i.e., the mineral also crystallized from kimberlite melt, which likely actively metasomatized the host rocks from which the xenoliths were captured. Phlogopite from volcaniclastic kimberlite from the Karpinskii-1 and Karpinskii-2 pipes does not show either any clearly distinct petrographic setting or compositional differentiation. The kimberlite was dated by the Rb–Sr technique on phlogopite and additionally by the 40Ar/39Ar method. Because it is highly probable that phlogopite from all pipes crystallized from kimberlite melt, the crystallization age of the kimberlite can be defined as 376 ± 3 Ma for the Grib Pipe, 380 ± 2 Ma for the Karpinskii-1 pipe, 375 ± 2 Ma for the Karpinskii-2 Pipe, and 377 ± 0.4 Ma for the Ermakovskaya-7 Pipe. The age of the pipes coincides within the error and suggests that the melts of the pipes were emplaced almost simultaneously. Our geochronologic data on kimberlite emplacement in ADP lie within the range of 380 ± 2 to 375 ± Ma and coincide with most age values for Devonian alkaline–ultramafic complexes in the Kola Province: 379 ± 5 Ma; Arzamastsev and Wu, 2014). These data indicate that the kimberlite was formed during the early evolution of the Kola Province, when alkaline–ultramafic complexes (including those with carbonatite) were emplaced.

Mesoproterozoic olivine gabbronorites of the Bashkirian anticlinorium, the South Urals: Parental melts and specifics of magma evolution

This paper is devoted to detailed study of picritic rocks (olivine melanogabbronorites) and comagmatic gabbrodolerites from sills and dikes in the central part of the Bashkirian meganticlinorium. These rocks are ascribed to the Kama-Belsk magmatic province (KBP) that was formed in the eastern East European Platform (EEP) in the Mesoproterozoic time. The study of minerals (EMPA, SIMS), rocks, and their oxygen isotope compositions showed the contribution of crustal contamination, fractional crystallization and cumulus processes in their formation. The geochemical indicators of crustal contamination (Nb/Nb*, (Nb/La)n, δ18O, and others) show strong variations, which indicates uneven crustal contribution in the parental melts during rock formation (10–25%). The study of weakly contaminated (δ18O = 5.3‰) olivine melanogabbronorites (MgO = 22.55 wt %) from the small Ishlya-1 subvolcanic body, which contain subordinate amount of cumulus (24%), high-magnesian olivine (Fo91.3), and high-Cr spinel (cr# 0.67), as well as HREE depleted clinopyroxenes, allowed us to retrieve the composition of parental melt. The latter contained about 20 wt % MgO and was formed by 19–26% melting of mantle source (potential mantle temperature Tm of 1530–1545°C).Geochemical characteristics of KBP reflect the formation of primary melts by melting of mantle column at different depths, mixing of the melts, and significant contamination by crustal material. The dominant role in the formation of the rocks of the Ishlya area and Mashak Complex was played by derivatives of spinel peridotites, while the rocks of the Bakal-Satka area were derived from garnet peridotites.The melts of olivine melanogabbronorites crystallized within a pressure range from 7 to 2 kbar and temperature from 1470°C to 1000–950°C at fO2 close to QFM. The comparison of model crystallization trend of parental melt with compositions of basic and ultrabasic rocks of the province showed that the observed compositional diversity was provided by the accumulation of clinopyroxene-plagiolcase (dolerites and gabbrodolerites) and olivive-plagioclase (picrites and picrobasalts) cumulates. Obtained estimates of conditions of magma generations with Tm significantly exceeding (at 150–165°C) that of asthenosphere indicate that the Mesoproterozoic magmatism in the eastern part of EEP was related to the ascent of deep-seated plume. Preliminary results of comparative analysis of petrological-geochemcial characteristics of KBP and Mesoproterozoic basic rocks of Northern Greenland (Upton et al., 2005; Kalsbeek and Frei, 2006) revealed significant similarity of these within-plate manifestations, which in both cases bear signs of plume nature and distinct crustal contamination.

Geochemistry and oxygen isotopic composition of olivine in kimberlites from the Arkhangelsk province: Contribution of mantle metasomatism

The paper presents data on the composition of olivine macrocrysts from two Devonian kimberlite pipes in the Arkhangelsk diamond province: the Grib pipe (whose kimberlite belongs to type I) and Pionerskaya pipe (whose kimberlite is of type II, i.e., orangeite). The dominant olivine macrocrysts in kimberlites from the two pipes significantly differ in geochemical and isotopic parameters. Olivine macrocrysts in kimberlite from the Grib pipe are dominated by magnesian (Mg# = 0.92–0.93), Ti-poor (Ti < 70 ppm) olivine possessing low Ti/Na (0.05–0.23), Zr/Nb (0.28–0.80), and Zn/Cu (3–20) ratios and low Li concentrations (1.2–2.0 ppm), and the oxygen isotopic composition of this olivine δ18O = 5.64‰ is higher than that of olivine in mantle peridotites (δ18O = 5.18 ± 0.28‰). Olivine macrocrysts in kimberlite from the Pionerskaya pipe are dominated by varieties with broadly varying Mg# = 0.90–0.93, high Ti concentrations (100–300 ppm), high ratios Ti/Na (0.90–2.39), Zr/Nb (0.31–1.96), and Zn/Cu (12–56), elevated Li concentrations (1.9–3.4 ppm), and oxygen isotopic composition δ18O = 5.34‰ corresponding to that of olivine in mantle peridotites. The geochemical and isotopic traits of low-Ti olivine macrocrysts from the Grib pipe are interpreted as evidence that the olivine interacted with carbonate-rich melts/fluids. This conclusion is consistent with the geochemical parameters of model melt in equilibrium with the low-Ti olivine that are similar to those of deep carbonatite melts. Our calculations indicate that the variations in the δ18O of the olivine relative the “mantle range” (toward both higher and lower values) can be fairly significant: from 4 to 7‰ depending on the composition of the carbonate fluid. These variations were formed at interaction with carbonate fluid, whose δ18O values do not extend outside the range typical of mantle carbonates. The geochemical parameters of high-Ti olivine macrocrysts from the Grib pipe suggest that their origin was controlled by the silicate (water–silicate) component. This olivine is characterized by a zoned Ti distribution, with the configuration of this distribution between the cores of the crystals and their outer zones showing that the zoning of the cores and outer zones is independent and was produced during two episodes of reaction interaction between the olivine and melt/fluid. The younger episode (when the outer zone was formed) likely involved interaction with kimberlite melt. The transformation of the composition of the cores during the older episode may have been of metasomatic nature, as follows from the fact that the composition varies from grain to grain. The metasomatic episode most likely occurred shortly before the kimberlite melt was emplaced and was related to the partial melting of pyroxenite source material.

PHASE TRANSFORMATIONS OF ENSTATITE IN SPHERICAL SHOCK WAVES

The analysis of available theoretical evaluations and experimental data reveals discrepancies and makes it possible to formulate the goals for the comprehensive study of the behavior of enstatite MgSiO3 in shock isentropic waves of various scale and intensity. The paper presents the layout and results of an explosion experiment on the compression of an enstatite sphere with spherical shock waves and the subsequent recovery of the experimental material and its examination in discrete zones (along the sphere radius) that were produced by shock waves in the material. The latter were examined with the application of scanning electron microscopy, Raman spectroscopy, and X-ray diffraction analysis. The comparison of the systematic variations in the texture, chemistry, and phase composition of enstatite along the sphere radius with calculated pressure P(R, t) and temperature T(R, t) values led us to the following conclusions: enstatite starts melting on an isentrope upon pressure relief after shock wave compression at σxx ≈ 80 GPa and melts on the front of the spherically converging shock wave at σxx ≈ 160 GPa and T ≈ 6300 K. Our laboratory experiments with shock waves were the world’s first in which enstatite was loaded with spherical converging shock isentropic waves and which provided evidence that shock wave-loaded MgSiO3 shows certain morphological and mineralogical features never before detected in this mineral loaded with plane shock wave of smaller amplitude and duration. Goals are formulated for the further studying of shock wave-loaded materials, and the necessity is discussed for conducting an explosion experiment with a five to seven times greater spherical system in order to increase the duration of the shock wave loading impulse.