Geochemistry International最新文献

The petrography, mineralogy and geochemistry of metamorphosed Permian–Triassic to Early Triassic rocks of the Anyui gabbro-dolerite complex, composing sills in metaterrigenous rocks of the Keperveem and Malyi Anyui uplifts of western Chukotka, were studied to determine the composition of the parental melt of these rocks and to assess the mobility of elements during their metamorphism. To solve these problems, the methods of petrological and geochemical modeling of melt crystallization were applied using the COMAGMAT version 3.72 program. It was established that the rocks (hypabyssal gabbros, gabbrodiorites, and diorites) are derivatives of a single parental melt formed in a large lower crustal magma chamber. These rocks are shown to have crystallized from intraplate continental tholeiitic basaltic parental melt that had a moderately differentiated composition with Mg# 52.1 corresponding to the Cpx–Pl cotectic and exhibited signals of crustal contamination. During regional metamorphism to the greenschist facies, the contents of a number of major, minor, and trace elements in the most of the studied rocks have been changed, with the estimated relative mobility of elements increasing as follows: Eu, V < Mn < Zn, U, Co < Cu, Pb < Sr < Fe, Ba, K, Rb < Ni < Cs < Mg < Ca, Na < Li. The elements immobile during metamorphism were Si, Al, Ti, P, REE (except Eu), Y, Sc, Nb, Ta, and probably also Zr, Hf, and Th (although the contents of the latter in some rocks may reflect the presence of xenogenic accessory minerals). The COMAGMAT program was applied to model the phase crystallization sequence established based on petrographic and mineralogical data on rocks, and the parameters of the compositions of the coexisting minerals during the fractionation stages of the parental melt before magnetite started to crystallize. The application of the petrological–geochemical modeling method in combination with data on the geochemistry and mineralogy of the gabbroids thus allows one to evaluate not only the compositions of the magmas and melts and their changes during fractionation but also an input/output of elements during metamorphism and the degree of their mobility.

The distribution of dissolved molybdenum, tungsten, and vanadium was investigated in the northeastern part of the Black Sea down to a depth of 320 m. The depth of hydrogen sulfide appearance (the onset of the anaerobic zone) in the studied area was about 165 m (at a potential density of ∼16.2 kg m^–3). Water samples with dissolved (<0.45 μm) species and dissolved plus labile particulate species of the elements were collected in July 2016 and 2017. The concentration of dissolved Mo increased with depth in the oxic zone, from 36 to 39 nmol/kg, and showed no difference from the sum of dissolved and particulate forms. In the anoxic zone, molybdenum decreased when the hydrogen sulfide concentration exceeded ∼8 μM and reached 3.3 nmol/kg at 320 m. The tungsten concentration decreased from 160 pmol/kg at the surface to 113 pmol/kg at the redox interface (in the suboxic layer at depth 150 m) in the presence of particulate manganese. As Mn oxyhydroxides dissolved in the hydrogen sulfide zone, W concentrations increased to 221 pmol/kg at a depth of 180 m, along with an increase in dissolved Mn. The distribution of W at the redox interface is controlled by the sorption properties of Mn oxide. Dissolved vanadium was depleted at a depth of 5 m and increased with depth in the oxic zone to 13 nmol/kg, with a decrease in the suboxic zone (down to 7.1 nmol/kg). In the anoxic zone, a maximum of V concentration (up to 15.2 nmol/kg) was observed, coinciding with the maximum of dissolved Mn. The calculated balance of Mo and V in the Black Sea showed that about 1200 t of Mo and 1200 t of V are annually buried in the sediments. Tungsten is thought to be supplied in significant amounts to the Black Sea in the form of suspended and colloidal matter in riverine waters, and this matter passes then into seawater in the process of suboxic diagenesis in sediments.

Solid bitumens in the Mir kimberlite pipe form vein-like segregations several centimeters in size. They are distributed irregularly in the pipe body, regardless of the kimberlite breccia varieties distribution. The bitumen content in the kimberlites ranges from 0.001 to 0.12 wt %. Bitumen-rich areas are confined to the pipe contacts and faults. The question of the origin of bitumens in kimberlites is hotly debated. Therefore, this work is aimed at determining their origin in the Mir pipe. The studied bitumen-bearing kimberlite samples were collected in the Mir pipe, from depths of 100 and 130 m. The chloroform bitumoid was extracted from bitumen. The carbon isotope compositions of bitumoids were determined, and biomarker analysis was carried out by gas chromatography mass spectrometry. Biomarker hydrocarbons were detected in the bitumoid, indicating a biogenic origin of the organic matter. The following biomarkers were identified: n-alkanes, isoprenoids Pr and Ph, tri- and pentacyclic terpanes–hopanes, and steranes. The pristane to phytane ratio Pr/Ph = 0.8 indicates reducing conditions of formation corresponding to marine environment. The oddness ratio of n-alkanes, sterane and hopane maturity indices show that the thermal maturity of organic matter (OM) corresponds to the initial stage of the oil formation. The low content of long-chain alkanes and the predominance of C29 over C27 among regular steranes (C29/C27 = 2.2) allow us to assume that the initial biota for the OM of the bitumen sample under study could be phytoplankton. The δ¹³C_VPDB values of the studied bitumoid (from –29.4 to –31.6‰) correspond to the isotopic compositions of bitumoids in the Paleozoic and Mesozoic sediments of the Siberian Platform (from –25.8 to –33.8‰) and differ significantly from the isotopic composition of endogenous carbon (from –2 to –10‰). According to these features, the investigated bitumen from the Mir pipe is of marine origin.

The unique association of the epidote-group REE minerals, as well as the evolution of lanthanides and actinides, titanium and vanadium in their composition are described. Allanite-(Ce), often with a ferriallanite-(Ce) core, allanite-(Y) in an outer zone, and a rim of REE epidote forms pseudomorphs after chevkinite-(Ce) and perrierite-(Ce), as well as isolated crystals. They form intergrowths with biotite and are developed near it in quartz gabbronorite-dolerites and gabbronorite-diorites of the island-arc hypabyssal mafic Pervomaysk–Ayudag complex of the Middle Jurassic–Early Bajocian age in the Mountain Crimea. Zonal allanite-(Ce) of similar composition with REE epidote rims is widespread in the quartz diorites and plagiogranites of the same complex, where it is often developed in granophyric quartz–oligoclase intergrowths. Brown ferriallanite-(Ce) enriched in Ti (±Th) forms cores in brown allanite-(Ce) crystals enriched in Ti (±V) (up to 4.9 wt % TiO₂). The Ti-rich (up to 3.5 wt % TiO₂) allanite-(Ce) developed after ilmenite is extremely rich in vanadium (up to 4.5 wt % V₂O₃). Light colored low-titanium allanite-(Ce) has grown on the titanium-enriched allanite-(Ce). The distribution of REE and yttrium in the allanite-(Ce) and ferriallanite-(Ce) is as follows: Ce > La > Nd > Y > Pr > Sm ∼ Dy ∼ Gd > Er ∼ Tb. The outer zones of allanite-(Ce) crystals and REE epidote are relatively enriched in Nd, with Nd > La in some of them. The yttrium and REE show peculiar relations in allanite-(Y): Y ( gg ) Ce ∼ Nd ∼ Dy ∼ Er > La ∼ Gd ∼ Yb. Allanite-(Ce) in the Crimean gabbroids is noticeably richer in La, Ti, V and poorer in Y, Sm, Gd in comparison with allanite-(Ce) in Crimean plagiogranitoids. The distribution of REE and yttrium in allanite-(Ce) of Crimean plagiogranitoids is close to that of common granites, differing in an increased proportion of Gd. The coloration reasons and sources of Crimean magmatic allanite are considered. Allanite was partially replaced by monazite-(Ce) during regional metamorphism under the prehnite-pumpellyite facies conditions.

Experience in using our previous equation for the prediction of water solubility in silicate melt revealed that the calculated water contents for some experiments at pressures of 5–20 kbar are unrealistically high compared with the experimental measurements. The sample containing the results of 412 experiments used in the previous study was significantly expanded by adding experiments from the MELT database. Using the new total sample containing 1241 experiments, the set of variables describing the effect of melt composition on water solubility was revised. The newly calibrated equation predicts water solubility in silicate melts with an uncertainty of no higher than ±0.01 mole fraction or ±0.25 wt % at pressures from one atmosphere to 20 kbar and temperatures from 825 to 1550 K. The sample size used for the optimization allows the equation to be applied over a wide range of silicate melt composition from komatiitic basalt to rhyolite.

In order to determine the physico-chemical conditions for the formation of Precambrian сarbon-bearing rocks (CBR) of the Trans-Onega region, the compositions of their minerals and fluid inclusions (FI) in quartz were studied in samples collected at different mine workings of the Zazhogino ore field. The obtained FI density values in the rocks fluctuate in a sufficiently wide range. This indicates the existence of several stages of changes in the temperature regime of fluids during epigenesis. The densities and concentrations of FI solutions from the host silicified lydites are close to those of the highest-density varieties from quartz veinlets in the CBR. Some FI are partially decrepitated, which indicates superimposed processes in the post-capture period. Based on the studies of eutectic temperatures, it is suggested that mineral-forming solutions in the inclusions throughout the entire ore field contain magnesium chloride. The study of the chemical composition of the minerals of the CBR and host rocks allowed us to identify various sulfides, oxides, carbonates, phosphates, silicates, and aluminosilicates. Among them, mineral assemblages suitable for calculating the temperatures and pressures of mineral formation were selected and the corresponding calculations were carried out. Comparison of the results of the FI study and mineral thermobarometry allowed us to estimate the PT conditions that existed during the CBR epigenesis. The maximum values of the greenschist-facies temperatures and pressures during CBR formation correspond to T = 410–400°C and P = 2.4–2.1 kbar. Chlorites were formed at T = 240–200°C and P = 0.5–0.4.

The paper presents experimental results on the evaporation of the main types (A and B) of Ca–Al inclusions of chondrites in a Knudsen tungsten cell in the temperature range of 1000–2400°C. A peculiarity of the evaporation of the inclusions is that the silicon component is preserved in the residual melt to high temperatures exceeding 2200°C. At the same time, magnesium oxide completely evaporates from the melt, which leads to a decrease in the spinel content and even its disappearance during the crystallization of the residual melt at 2000°C. Magnesium evaporation leads, in turn, to an increase in the Al₂O₃ content in the melt and, accordingly, to an increase in the content of gehlenite in melilite during its crystallization, as is observed in the high-temperature rims of subtype B1 inclusions. Experiments have also shown that the high-temperature evaporation of Ca–Al inclusions occurs in an oxygen-rich atmosphere compared to carbonaceous chondrite matter. In this regard, it is assumed that the condensation of the first mineral products of the inclusions occurred in the oxygen-rich atmosphere of the star (O-rich AGB) outside the protosolar system.