Дренажные растворы зоны выщелачивания гипергенного профиля метабазальтов. Зависимость состава дренажных растворов от уровня метаморфизма субстратаКлимова Е.В. 1 , Матреничев В.А. 2 1 Институт геологии и геохронологии докембрия РАН, Санкт-Петербург,

The paper presents newly obtained original data on the morphology, internal structure (as seen in cathodoluminescence images, CL), and composition of more than 400 zircon grains separated from gabbroids and plagiogranites (OPG) sampled at the axial zone of the Mid-Atlantic Ridge (MAR). The zircons were analyzed for REE by LA-ICP-MS and for Hf, U, Th, Y, and P by EPMA. Magmatic zircon in the gabbroids crystallized from differentiating magmatic melt in a number of episodes, as follows from systematic rimward increase in the Hf concentration, and also often from the simultaneous increase in the (U + Th) and (Y + P) concentrations. These tendencies are also discernible (although much less clearly) in zircons from the OPG. Zircon in the OPG is depleted in REE compared to the least modified zircons in the gabbro, which suggests that the OPG were derived via partial melting of gabbro in the presence of seawater-derived concentrated aqueous salt fluid. Another reason for the REE depletion might be simultaneous crystallization of zircon and apatite. The CL-dark sectors, which are found in practically all of the magmatic zircon grains, have Y/P (a.p.f.u.) ≫ 1 which most likely resulted from OH accommodation in the zircon structure, a fact suggesting that the OPG parental melt contained water. High-temperature hydrothermal processes induced partial to complete recrystallization of zircon (via dissolution-reprecepitation), a process that was associated with ductile and brittle deformations of the zircon-hosting rocks. The morphology of the hydrothermal zircons varies depending on pH and silica activity in the fluid from weakly corroded subhedral crystals with typical vermicular microtopography of the crystal faces to completely modified grains of colloform structure. Geochemically, the earlier hydrothermal transformations of the zircons resulted in their enrichment in La and other LREE, except only Ce, whose concentration, conversely, decreases compared to that of the unmodified magmatic zircons. The hydrothermal zircon displays a reduced Ce anomaly and its most altered domains typically host minute inclusions of xenotime, U and Th oxides and silicates, and occasionally also baddeleyite, which suggests that the hydrothermal fluid was reduced and highly alkaline. These features were acquired by the seawater-derived fluid when it circulated within the axial MAR zone area due to phase separation in the H2O–NaCl system and particularly as a result of fluid interaction with the abyssal peridotites of oceanic core complexes. Our data demonstrate that zircon is a sensitive indicator of tectonic and physicochemical processes in the oceanic crust.

Precambrian weathering profiles (or regoliths) were formed on the Earth's surface in contact with meteoritic water. This signifies that their mineral and bulk composition should reflect the environmental conditions at the time of weathering. The aim of this study is to reconstruct the main mineralogical and chemical trends involved in the formation of Precambrian weathering regoliths by means of geochemical modeling verified through laboratory experimentation. We have demonstrated that, in a wide range of modeling conditions, weathering profiles of basalts consist mostly of clay minerals (smectites and illite). Carbonate minerals may also be deposited within the profile at the initial stage of weathering, which are later dissolved during a further process. The time, necessary for complete dissolution of primary minerals and crystallization of authigenic minerals is between 100 and 140 Kyr. Modeling results indicate that Neoarchean and Paleoproterozoic (2.8-2.0 Ga) weathering profiles were formed under an atmosphere with pCO(2) not much higher than 25 PAL and climatic conditions close to those found in modern times. Weathering under CO2-rich (pCO(2) = 1 and 10 bar) and CH4-rich atmospheres as well as high temperature (50 degrees C and 75 degrees C) conditions do not produce compounds corresponding to the analyzed samples. (C) 2013 Elsevier B.V. All rights reserved.

New data on the stratigraphy and isotopic age of supracrustal rocks from the lower part of the section constituting the northeastern limb of the Lekhta structure (northern Karelia) and their relationships with the basement are considered. Geological-petrographic, lithological-geochemical, and isotopic data are used to define three formations united into the Okhta Group. Immediate relationships between volcanics of the greenstone belt and granitoids of the basement represented by the oldest (for the Baltic Shield: 2.8 Ga) continental weathering crust after granites, are discussed. Isotopic age of volcanics and granite gneisses of the basement indicates that the Lopian supracrustal complex of the Lekhta structure was deposited in a period lasting 16 myr with duration of periods corresponding to formation of the Okhta and Pebozero groups being as long as 8 and 11 myr, respectively. In the regional stratigraphic scale, the entire Archean part of the supracrustal section in the Lekhta structure should be attributed to the Middle Lopian.

The absence of Ce anomaly and significant positive correlation between Ce and Fe in the weathering profiles and sedimentary carbonate rocks of the Baltic Shield with an age of 2.8–2.1 Ga indicate that Fe 3+ was the main form of Fe migration in the Archean-Paleoproterozoic exogenous environments in the Baltic Shield. The Ce distribution was mainly controlled by hydrogen index rather than redox conditions.

Weathering crusts are the only reliable evidences of continental conditions existence, and often are the only source of information about exogenous processes and subsequently about conditions under which the development of biosphere occurred. Complex of diverse fossil microorganisms was discovered in result of electronic-microscope investigations. Chemical composition of discovered fossils is identical to that of the host rocks and is represented by Si, Al, Fe, Ca and Mg. Probably, microorganisms fixed in rocks played role of catalyst. Decomposition of minerals, comprising rocks, and their transformation into clayey (argillaceous) minerals, occurred most likely under influence of microorganisms. And may be unique weathering crusts of Early Precambrian were formed due to interaction between specific composition of microorganism assemblage and conditions of hypergene transformations. So it is possible to speak about colonization of land by microbes already at that time and about existence of single raw from weathering crusts (Primitive soils) to real soils.

Continental chemical weathering, as a reason for changing the Sr isotope ratio in seawater was examined only qualitively. The composition of drainage waters and river waters is fully caused by the interaction of climate and rocks, which form a drainage basin, thus the changing of the composition of the upper continental crust and atmosphere through time should affect the composition of the drainage waters and continental flow in general. The profiles of continental weathering are widely found in all stratigraphic levels in Karelia. Some of these profiles were examined and by means of mathematical modeling the amount of main elements and isotopic values of Sr were reconstructed in drainage waters of these profiles. Mathematical models are based on the Rb-Sr isotope data for the most ancient weathering profile at the Baltic shield – 2.8 Ga [1], and geochemical data for a number of Proterozoic weathering profiles. The Sr/Sr ratio at the granite source was calculated relative to their age, according to the measured contemporary value 0.70391, with the Rb/Sr contemporary ratio 0.0736. The results of the models are shown in picture 1. The majority of published data concerning the isotopic value of Sr, including data for the Tulomozerskaya formation of the Baltic Shield [2], are shown at picture 1 and most of then are inside calculated levels of Sr/Sr. Thus, the calculated values of the isotope composition of Sr can indicate possible limits for the continental flow in Early Precambrian time.

The paper presents a detailed description of the geological structure, mineralogy, and chemical composition of the Archean weathering profile of granitoids in the Baltic Shield. Based on model calculations, the primary composition of granites in the substrate and weathering profile, as well as elemental composition of solutions draining the supergene profile, are determined. Model calculations for the Proterozoic weathering profile of analogous granitoids testify to the similarity of supergene alterations in the Early Precambrian. Mineral transformations during the weathering of granitoids are mainly expressed in the formation of minerals of new micas of the illite group against the background of large-scale decomposition of plagioclase. The composition of solutions in the hypergenesis zone of Precambrian weathering profiles sharply differs from the modern composition by lower contents of Ca2+ and K+ and the higher content of Fe2+. This is only possible under reducing conditions at high acidity of the diagenetic environment (pH < 6.5). The results obtained are essential for understanding the nature of unique Precambrian rocks (sedimentary iron ore rocks). The subordinate amount of carbonate sequences in Precambrian sedimentary sequences can be related to the specific composition of continental runoff primarily governed low-Ca supergene solutions.

This paper presents the results of investigations of the geochemical and isotopic compositions of komatiites from various Late Archean greenstone belts and structures of the Baltic shield. The analysis of major trace, and rare earth elements demonstrated a considerable heterogeneity of their composition and lateral zonine, in the distribution of their characteristics. It was found that variations in the contents of FeO, Al2O3, MgO,, and REE and Al2O3/TiO2, Ti/Zr, Y/Zr, (Ce/Sm)(N), and (Gd/Yb)(N) ratios in the complexes studied are related to various factors and have diverse origins. In order to decipher the latter, isotope geochemical data and methods of petrological and geochemical modeling of rock-forming processes were applied. The analysis of regularities in the spatial and temporal distribution of greenstone belts, the lithological characteristics of volcanosedimentary complexes, and the geochemical and isotopic heterogeneities of komatiites and their mantle sources demonstrated that they can be most adequately interpreted by the model of development of two mantle plumes of different ages.