Vol 64, No 2 (2019)
- Year: 2019
- Articles: 8
- URL: https://journals.eco-vector.com/0016-7525/issue/view/706
- DOI: https://doi.org/10.31857/S0016-752520192
ARTICLES
Long-lasting influence of the Discovery plume on tholeiitic magmatism in the South Atlantic: data on basalts recovered by hole 513a, dsdp leg 71
Abstract
The paper presents the very first data on concentrations of major and trace elements; Sr, Nd, and Pb isotopic ratios of rocks; and the composition of olivine phenocrysts of 38-Ma basalts recovered by Hole 513a (DSDP Leg 71) in the South Atlantic. The bulk-rock samples and the chilled glasses are mildly magnesian (7–8 wt % MgO) and bear elevated FeO and low Na2O concentrations, as is typical of MORB of the
TOR-1 type. Olivine phenocrysts (Fo84.5–88) in these rocks contain concentrations of trace elements
(Ni, Mn, Cr, and Zn) that are typical of classic MORB, which are produced by partial melting mantle peridotite. The rocks are strongly depleted in incompatible elements [(La/Sm)n ~ 0.6] but have elevated
Ba/Nb, K/Nb, and Pb/Ce ratios and Cu, Ag, and Au concentrations that are 1.5–4 times higher than in typical depleted MORB (N-MORB) and in most rift basalts in the South Atlantic. Isotope compositions of the basalts (average ratios 206Pb/204Pb ~ 18.0; 207Pb/204Pb ~ 15.6, 208Pb/204Pb ~ 38.0, 143Nd/144 Nd ~ 0.5130, and 87Sr/86Sr ~ 0.7040) are close to those in modern tholeiites from the southern MAR segment (SMAR) north of the Agulhas Fracture Zone. The data indicate that the magmas were derived from a strongly depleted mantle source that contained a minor (~3%) admixture of an enriched component, which is discernible in the magmas of the Discovery hotspot. The composition of the source, which is more depleted than DM, and the high degrees of melting of this source explain why the basalts from DSDP Hole 513a are enriched in chalcophile elements. It is believed that spreading magmatism at 45°–48° S in SMAR as far back as 40 Ma was already affected by the Discovery hotspot. This hotspot might be related to the Tristan plume system, and its origin and long-lasting influence on spreading magmatism in the South Atlantic are regarded as evidence of the extensive effect of the Tristan plume.
107-127
Genesis of diamonds and paragenetic inclusions under lower mantle conditions: the liquidus structure of the parental system at 26 GPa
Abstract
The patterns of the joint genesis of diamonds, and their paragenetic inclusions under lower-mantle conditions, are controlled by the liquidus melting conditions of the multicomponent diamond-forming system. The boundary compositions of this system are evident from the generalized data produced by the analytical mineralogy of paragenetic inclusions in lower-mantle diamonds. We studied the structure of the liquidus of a theoretical diamond-forming system in a physicochemical experiment with P–T parameters typical of the depths of 670–800 km. The compositions of the parental melts/solutions for diamonds and paragenetic inclusions correspond to the multicomponent system MgO–FeO–СаО–SiO2–MgCO3–FeCO3–CaCO3–Na2CO3–C. Its primary melting ёwas controlled by the peritectic phase relations at the solidus, which was revealed by experimental studies of polythermal sections of the system and their phase diagrams. Of key importance is the effect of the “stishovite paradox,” in which the peritectic reaction between the ultrabasic bridgmanite phase and melt coincides with the formation of basic oxide associations of periclase–wustite solid solutions and stishovite. The peritectic reaction of bridgmanite is a fundamental feature of the diamond-forming system, and it determines the major defining characteristics in its liquidus structure. Elements of the peritectic liquidus provide the physicochemical basis for the evolution of growth melts that yield diamonds and their paragenetic minerals. On the basis of the experimental results, we constructed a fractional diagram of the syngenesis of diamonds and inclusions. It clearly illustrates the solution–melt mechanism by which diamond genesis occurs, as well as the sequence of growth trapping of primary inclusions by diamonds under the lower-mantle conditions. The physicochemical factors of the genesis of diamonds and their primary inclusions agree: they are generalized in a compositional diagram of lower-mantle diamond-forming media, and they provide a natural basis for the genetic classification of inclusions of rock-forming and accessory minerals in diamonds.
128-144
Regularities of distribution of rare-earth elements in the Kivakka olivinite-gabbronorite layered intrusion (Northern Karelia)
Abstract
This study discusses the presentation of rare-earth elements (REEs) in the rocks of the Kivakka Olivinite-Gabbronorite Layered Intrusion in North Karelia. It aimed to provide a detailed petrographic description of the mineral parageneses that are present in the studied section of the massif. We found that the same minerals can manifest in both intercumulative and cumulative positions, depending on the degree of melt fractionation. At the same time, their quantity, which determines both the petrographic characteristics and the name of the rock, is not a criterion for their presence during the cumulus phases. We analyzed the concentrations of REEs in rocks and rock-forming minerals in the Kivakka massif, considering the REE concentration vertically and in the critical zones of contrast interbedding. In the study area, REEs are present as incoherent elements and accumulate in the residual melt, together with U, Th, Zr, and other incoherent elements, which make them a useful indicator of the degree of melt fractionation. In some cases, they can reflect different structural and genetic characteristics, such as the degree of cumulus density in a specific type of cumulative paragenesis. The presence of hydrothermal changes is best reflected by a change in Ce concentration. The preservation of the stability of the configuration and the slopes of the lines on the spider diagrams for REE indicate that the process of crystallization differentiation took place in a closed system.
145-167
Origin of Skarns at Migmatization on Ol’khon Island, Lake Baikal, Russia
Abstract
Migmatites on the western shore of Ol’khon Island host unusual rocks: zoned lenses of hedenbergite–garnet–epidote–anorthite metasomatites coupled with the migmatites. No intrusive granites were found nearby. The skarn-forming process operated at the interface of the granite gneiss and skarn protolith (perhaps, carbonate rocks). The composition of the metasomatites is analogous to that of calcic skarns with high Al2O3, FeO, and CaO concentrations. The compositions and relations of the minerals provide evidence of the successive development of the hedenbergite–anorthite outer zone, dominantly anorthite–garnet main zone, and quartz-enriched inner zone, with all of the zones parallel to contact with the granite gneiss. The granite gneiss itself is also likely of metasomatic nature, as follows from its supraeutectic concentration of potassic feldspar in the leucosome and low crystallization temperatures. A minimum of the Gibbs free energy (calculated with the SELECTOR-C program package) was reached at 8 kbar and temperatures of 600– 625°C. These parameters are lower than the melting temperature of the granite eutectic, and the absence of melt is confirmed by the absence of melt inclusions in minerals of the granite gneisses. This indicate that the driving force of the process was migmatizing silicic–potassic solutions. The P–T parameters of the skarns are close to the foregoing values. The very high Sr and Ca and low Mg concentrations suggest that the protolith of the skarns was calcite marble. The enrichment of the skarns in the granitophile elements suggests that the skarns were produced simultaneously with and in genetic relation to the migmatization processes. The metasomatites were formed before the partial melts were derived, early in the course of the granite-forming processes and provide important information for better understanding the metasomatic process responsible for the exchange of chemical elements between the rocks.
168-181
Authigenic minerals of titanium and zirconium of the Verkhnekamskoye salt deposit
Abstract
Newly formed titanium oxides and zircons have been identified in the salt and suprasalt of the Verkhnekamskoye Salt Deposit. These features have been associated with the hydrolysis of clay material and the release of colloidal titanium and zirconium hydroxide during diagenesis, catagenesis, and hypergenesis. This is made possible by sulfate reduction and the acidic environment, which are caused by the radiation–oxidation of Fe2+ from sylvite and carnallite. Anatase is formed both in the suprasalt and salt measures and, only in the lower part of the salt measures, in the rutile. Such a distribution can be associated both with an increase in the degree of catagenetic transformation of salt rocks with depth and with the composition of coexisting sulfate or chloride brines. A typical form of anatase crystals is pinacoid, but this is replaced by a dipyramidal structure in the carnallite zone, which is the location of the most acidic brine. Three types of twins were recorded for rutile, the formation of which is attributable to the growth of a dehydrated colloid of titanium hydroxide on a finely dispersed aggregate. The impurities V, Cr, and Zr (Al, Si, and Fe) established in the titanium oxides reflect the geochemical specialization of the aluminosilicate terrigenous material sources.
182-194
Geochemistry of suspended matter in the Amazon river waters
Abstract
The duplicate samples of the Amazon River suspended matter (SM) collected during Cruise 9th of R/V Professor Shtokman were studied using modern analytical techniques. Waters of the Amazon basin are subdivided into three main types differing in SM content and chemical composition: white (turbid yellow waters of the Amazon River and Madeira tributary), clear (transparent waters of the large Xingu, Tapajós, Trombetas, and Tocantins tributaries) and black (waters with strong coffer color, high content of dissolved organics, and elevated acidity, Riu Negro). The specific features of SM from different parts of the river basin have been established. The contents and relationships of major and trace elements in SM vary within wide ranges, but in general the bulk composition of SM is close to the average composition of river SM and clay rocks, except for some samples from clear waters. The SM of the white and clear waters (blackwater samples unfortunately have not been preserved) are variably enriched in Hg. Literature data indicate that artisanal gold mining with the use of Hg (amalgamation method) has been carried out for many years in the Amazon Basin, i.e., SM enrichment in Hg is related to the anthropogenic factor. At the same time, significant (up to two orders of magnitude) enrichment of SM of the clear water Xingu and Tokantins rivers, the Amazon tributaries, in Sn, Zn, Pb, and to lesser extent, in Cu, Cd, Ag, and some other metals give grounds to suggest ore mineralization in these regions.
195-205
Short communications
A study of the diagenesis of organic matter of peat using Rock Eval pyrolysis
Abstract
We used Rock Eval pyrolysis to examine a peat collection in the Gursky field (located in the Lower Priamurye) during the early stages of diagenesis. This study focused on the top 350 cm of peat, and it also studied the producers of the organic matter. A pyrogram of peat consists of the superposition of three peaks that represent hydrocarbons that desorb at low temperatures (<300°C), intermediate temperatures
(300°C–400°C), and high temperatures (>400°C). These peaks represent the lipid, labile, and kerogen components, respectively. We found that diagenetic transformation of organic material was followed by a gradual reduction in lipid and labile components and a simultaneous increase in the kerogen component. This effect is most intense in the top portion of the studied section
206-211
Chronicle
A report on the 2018 All-Russian Annual Seminar on Experimental Mineralogy, Petrology, and Geochemistry
Abstract
On April 18–19, 2018, the regular All-Russian Annual Seminar on Experimental Mineralogy, Petrology, and Geochemistry was held in Moscow. It was organized by the Vernadsky Institute of Geochemistry and Analytical Chemistry and the Korzhinsky Institute of Experimental Mineralogy of the Russian Academy of Sciences. The seminar reviewed the latest experimental results in several main areas: phase equilibrium at high pressure and high temperatures; the processes of formation and differentiation of magmas; interactions between fluid-melt and crystals; hydrothermal equilibria and the formation of ore; synthesis of minerals; thermodynamic properties of minerals, melts, and fluids; problems with our current models of planets, meteorites, and the cosmos; physics and chemical properties of geomaterials; experimental geoecology; and methods and techniques of experimentation. Participation in the seminar included 340 specialists from 50 Russian scientific institutes and 11 foreign organizations, and more than 180 reports were presented.
212-224