E. O. Dubinina

A kinetic model of isotopic exchange in dissolution-precipitation processes

Abstract A model is derived for the isotopic fractionation between a mineral and a fluid phase in three dissolution-precipitation processes: the transformation of an unstable mineral into a stable one, and mineral-fluid mass transfer with partial or complete dissolution of the starting mineral. It is assumed that (1) the isotopic fractionation factor between the freshly formed portion of the mineral and the fluid is constant (not necessarily equilibrium) and (2) isotopic exchange before the dissolution and after precipitation is negligible. The model takes into account the possible variation in composition of the precipitating solid. Early formed solids are regarded as isolated from the system. The evolution of isotopic compositions of fluid, freshly formed mineral and bulk solid are considered. Mineral synthesis, Ostwald ripening, and repeated recrystallization are described as the examples of different types of dissolution-precipitation processes. To illustrate the model, numerical calculations of isotopic compositions and the fraction of isotopic exchange (F) for the system silica gel-quartz-water at 500°C are performed. The F value depends on the mass of the mineral transferred through the fluid and the mineral/fluid ratio for all the processes considered. The first type of dissolution-precipitation process is characterized by the F variation from zero up to values above unity. In the case of repeated dissolution-precipitation process, F shows an oscillatory character, as opposed to a diffusive model of isotopic exchange. During Ostwald ripening, F strongly depends on the crystal size distribution of the initial sample.

Sources and fluid regime of quartz-carbonate veins at the Sukhoi Log gold deposit, Baikal-Patom Highland

Complex (δ18O, δ13C, 87Sr/86Sr, 143Nd/144Nd, and REE composition) data were obtained on quartz-carbonate veins in metasedimentary rocks to elucidate the material sources and to evaluate fluid regime during low-sulfide gold-quartz ore mineralization at the Sukhoi Log deposit. In order to use an oxygen isotopic thermometry for quartz veins, we calibrated empirical dependence of fractionation factors between vein quartz and altered wall rocks. The temperature range of quartz equilibration with wall rocks was evaluated at 380–190°C. Independent temperatures obtained using this thermometer indicate that the vein ankerite can be both earlier and later than vein quartz. The isotopic systematics (δ13C and δ18O) of ankerite in the quartz-carbonate veins, carbonates in the ore-hosting shales of the Khomolkho and Imnyakh formations both within and outside mineralized zones at the deposit indicate that the ore-hosting rocks and veins in the mineralized zone contain incoming carbonate, which was most probably borrowed from the carbonate rocks of the Imnyakh Formation. REE composition of vein ankerite shows that these elements were transported by fluid as carbonate complexes. The behavior of the Eu/Eu* and (La/Yb)n ratios and Mn of the vein ankerite suggest that during carbonate crystallization the system was closed with respect to fluid. Sr-Nd isotope systematics indicates that the isotopic parameters of the vein ankerite were formed with the participation of metasedimentary host rocks of both the Imnyakh and Khomolkho formations, which are contrastingly different in Nd isotopic composition. A fluid/rock ratio during metasomatic processes in the wall rocks was calculated for two scenarios of their thermal history: with a continuously operating heat source beneath the Sukhoi Log structure and with a linear cooling of the structure. The effective integral W/R ratios calculated lie within the range of 0.046–0.226 and suggest that the veins were produced with the metamorphic fluid. Low W/R ratios are inconsistent with the mechanism of vein quartz crystallization due to fluid oversaturation with respect to SiO2 at decreasing temperature. We believe that the main mechanism responsible for the origin of these veins was variations of fluid oversaturation due to pressure variations (pressure solution mechanism). This hypothesis is consistent with the reported isotopic-geochemical characteristics of the wall rocks at the Sukhoi Log deposit.

Nature of ore-forming fluids of the Dal’negorsk deposit: Isotopic and geochemical parameters of the altered host rocks

Oxygen isotopic composition was studied in the altered host rocks of the Dal’negorsk borosilicate deposit in order to establish a boron source and the origin of ore-forming fluids responsible for deposition of economic borosilicate ore. The relationships between oxygen isotopic composition and geochemistry of the altered igneous rocks occupying various structural and temporal positions in the ore zone were studied, including premineral high-potassium minor intrusions located in the zones of datolite mineralization; alkali basalt, gabbro, and breccia from the sedimentary framework of the deposit; and postmineral basaltic andesite, basalt, and dolerite dikes. It was suggested that interaction of aqueous fluid with host rocks brought about not only variation in oxygen isotopic composition but also shifts in geochemistry of these rocks, especially as concerns the chemical elements contained in ore-forming fluid. The disturbance of oxygen isotopic system is typical of all studied rocks: δ18O values sharply decrease indicating interaction with aqueous fluid at elevated temperatures. The lowest δ18O (from −2.9 to +0.1‰) is characteristic of the premineral high-potassium and ultrapotassium minor intrusions from skarn-datolite zone. Igneous rocks from the sedimentary framework of the deposit have δ18O of +2 to −0.9‰ The δ18O of postmineral basaltic andesite, basalt, and dolerite dikes varies from 0 to +7‰ with increasing distance from the ore zone. The oxygen isotopic composition of aqueous fluid evidences its exogenic origin. The geochemical and isotopic characteristics of ore-forming fluid show that it could have been deep-seated subsurface water similar to the contemporary water of the Alpine fold zone, which contain up to 700–1000 mg/l B and is distinguished by high K, Li, Rb, Cs contents and high K/Na ratio. Similar geochemistry is characteristic of the fluid inclusions in quartz from ore zones. It cannot be ruled out that continental evaporites were a source of boron as well. The relationships between δ18O, K-Ar age, and geochemical parameters of premineral and postmineral altered intrusive bodies allow us to suggest that the subsurface B-bearing water discharged through narrow channels controlled by premineral basaltic bodies. The discharge was probably initiated by emplacement of basalt and dolerite dikes.

Stable isotope evidence for the Bottom Convective Layer homogeneity in the Black Sea

The Black Sea is the largest euxinic basin on the Earth. The anoxic zone consists of the upper part water mass stratified by density, and the lower water mass homogenized relative to density (depth >1750 m), named the Bottom Convective Layer. To assess homogeneity and possible exchange of matter across the upper and lower boundaries of the Bottom Convective Layer, new data on stable isotope composition of S, O and H were obtained. Samples were collected in August 2008 and March 2009 from two stations located in the eastern central part of the Black Sea.Distribution of δ18O and δD values of water for the entire water column did not vary seasonally. Appreciable differences were marked for δD value variation in the picnocline area (water depth 200-400 m) and in the BCL 5 m above the bottom that might be caused by penetration of intrusions with elevated portion of shelf modified Mediterranean Water. Observed linear relationship between δ18O (or δD) and salinity indicates that mixing water and salt occurs at the same time, and the deep water of the Black Sea has two end members: the high-salinity Mediterranean seawater and freshwater input.In the Bottom Convective Layer, the average δ34S (H2S) was -40.6 ± 0.5‰ and did not vary seasonally. At the bottom (depth > 2000 m), 34S depletion down to –41.0‰ was observed. Our δ34S (SO4) data are by 2-3‰ higher than those measured previously for the Bottom Convective Layer. Sulfate from the aerobic zone with δ34S (SO4) = +21‰ corresponds to ocean water sulfate and that has not been subjected to sulfate reduction. Average δ34S (SO4) values for depths > 1250 m were found to be +23.0 ± 0.2‰ (1σ). Sulfur isotope composition of sulfate does not change in the Bottom Convective Layer and on its upper and lower boundaries, and does not depend on the season of observation.

Chemical composition and metal capacity of magmatic gases of Gorelyi volcano, Kamchatka

The high-temperature (865°C according to data of an IR-camera) gases and the crater lake water of Gorelyi volcano were sampled in September 2011. The gas and condensate were collected from the active crater in bocca, which was formed in June 2010. The chemical composition and the δ18O and δD values of the condensate and lake water were analyzed. In comparison with the isotopic data on volcanic condensates of the adjacent Mutnovskii volcano, the data points of samples from Gorelyi volcano fall to the mixing line of meteoric and magmatic water. The isotopic characteristics of the water component of the fluid mostly correspond to the high-temperature equilibrium with silicate rocks or melts; the portion of local meteoric water is ~25%. The high metal content of gases of the volcano is determined. The total removal of Cu, Zn, Pb, Mo, Cd, In, Sn, Tl, and Bi by gases of Gorelyi volcano (~30 t/year) comparable to that of the Kudryavyi volcano (20–80 t/year) indicates the high ore-bearing capacity of the volcanic gas fluids.

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.

Fractionation factor of 238U and 235U isotopes in the process of hydrothermal pitchblende formation: A numerical estimate

Owing to the rapid increase in available data on the natural variations of the 238U/235U ratio, new isotopic geochemical mark of redox processes are beginning to emerge. In this connection, numerical estimates of the 238U and 235U fractionation factor (α(UIV−UVI)) accompanying the reduction UVI → UIV are needed. Such an estimate has been obtained for hydrothermal pitchblende formation based on results of high-precision (±0.06‰) measurements of the 238U/235U ratio in local microsamples of coarse spherulitic pitchblende from carbonate-pitchblende veins at the Oktyabr’sky deposit (Strel’tsovsky uranium ore field, eastern Transbaikal region). For this purpose, we used the formation temperature of hydrothermal pitchblende and a maximum estimate of the fractionation factor for 238U and 235U isotopes in the solution-solid phase system under normal (25°C) conditions (Murphy et al., 2014). The most probable isotopic fractionation factor accompanying pitchblende crystallization from hydrothermal solution at T = 320−250°C falls into the interval α(UIV−UVI) = 1.00020−1.00023.

Oxygen isotopes effects due to dehydration of the blueschist: The experimental data under P-T conditions of subduction zone

The oxygen isotope composition of minerals from metamorphic rocks carries information on the nature of the protolith, the temperature of the metamorphism peak, and the isotope composition and content of the fluid phase participating in the metamorphic process. Among the main sources of fluid in highgrade meta� morphism are hydrous minerals, for example, those delivered to the subduction zone with sedimentary and metamorphic rocks of the oceanic slab. The process of dehydration should result in the oxygen isotope shift in the dehydrated rock (restite), the surrounding crustal rocks, and the overlying rocks of the mantle wedge. Currently it is generally accepted that the oxygen isotope effect under metamorphic dehydration is very low (<1 ‰) (1). Moreover, small isotope shifts become poorly reflected against the background of the variable lithology of rocks in metamorphic complexes. More and more rocks with anomalously light or widely ranging oxygen isotope composition are found in granulitic and eclogitic metamorphic complexes. Signif� icant oxygen isotope lightening ( δ 18 O up to -11‰) was observed in eclogite and gneiss of the Dabie-Sulu belt in China; moderate oxygen isotope lightening (up to -4‰), in eclogite of the Kokchetav massif; and extreme oxygen isotope lightening (δ 18 O from -16 to

On the nature of ore-bearing fluids on the Dal’negorskoe borosilicate deposit (Primor’e)

Study of the oxygen isotopic composition in highpotassic rocks of preore magmatic bodies from thezones of pay datolite mineralization in skarns of theDal’negorskoe borosilicate deposit allowing us torefine the nature of oreforming fluids was undertakenfor the first time. The concepts on the relationshipsbetween datolite mineralization with magmas of alkaline basaltic chambers and parental juvenile fluids prevailed in special publications [1–4]. As this took place,high and ultrapotassic magmatic bodies usuallycalled trachyte or latite were considered as late orebearing phases of the alkaline basaltic chamber. However, mineralogical and geochemical investigations ofthese rocks [5] demonstrated that these were preoredykes of Paleogene basalt intruded in the zone of afluid canal, so oreforming solutions modified basaltsto a state of highpotassic rocks. Alteration was carriedout by aqueous fluids enriched in Ba, K, B, and Rband depleted in lowsoluble elements, namely Zr, Nb,Ta, La, and Ce. The estimations of oxygen isotopeshifts and their correlations in pre and postore igneous rocks of the deposit obtained in this study confirmthese assumptions.The Dal’negorskoe deposit (44°34

Isotopic parameters of meteoric waters in fractured porous rocks of the Tulukuev ore deposit

Isotopic analysis of hydrogen and oxygen was performed using the DeltaPlus mass spectrometer (ThermoQuest, Finnigan). Gaseous hydrogen from water samples was obtained by the decomposition of water on hot powder of metallic chrome at 800 ° C. Determination of the oxygen isotopic composition was based on the method of isotopic balance of water with ee 2 [5]. The δ D and δ 18 e values were determined accurate to 0.3 and 0.2 ‰, respectively. We used the following MAGATE reference samples: OH-1, OH-2, OH-3, and OH-4. All results presented in Figs. 1 and 2 are shown relative to the international standard V-SMOW.