V. B. Naumov

Extreme chemical heterogeneity of granite-derived hydrothermal fluids: An example from inclusions in a single crystal of miarolitic quartz

Magma-derived fluids are important in geologic processes (e.g., metal sequestration and ore deposition) but are intrinsically transient. Samples of magmatic fluids represented by fluid inclusions in a single zoned quartz crystal from a miarolitic cavity within a porphyritic leucogranite hosting the Industrialnoe tin deposit, northeastern Russia, were studied by using modern in situ analytical methods (laser Raman spectroscopy, proton-induced X-ray emission). The fluid inclusions are either dominated by vapor or by complex multiphase brines. The inclusions within a given trapping plane have similar phase relationships; however, there are significant variations between inclusions in different healed fractures. Phase and chemical compositions of individual brine inclusions demonstrate significant compositional heterogeneity (in terms of absolute element concentrations and ratios) of high-temperature magmatic fluids accumulated in the miarolitic cavity. This finding suggests that fluids leaving a crystallizing magma may have variable initial compositions that are subsequently modified by reactions with the rocks while the fluid is in transit to a miarolitic cavity, as well as by processes in the cavity, such as mixing, crystallization, and boiling. The inferred chemical diversity and fractionation of granite-derived fluids at near-magmatic conditions imply that fluids entering a cooler hydrothermal system are extremely complex and their metallogenic signature may differ from that of related ore deposits.

Average compositions of igneous melts from main geodynamic settings according to the investigation of melt inclusions in minerals and quenched glasses of rocks

We compiled a database containing more than 480000 determinations for 73 elements in melt inclusions in minerals and quenched glasses of volcanic rocks. These data were used to estimate the mean contents of major, volatile, and trace elements in igneous melts from main geodynamic settings. The following settings were distinguished: (I) oceanic spreading zones (mid-ocean ridges); (II) zones of mantle plume activity on oceanic plates (oceanic islands and plateaus); (III) and (IV) settings related to subduction processes, including (III) zones of island-arc magmatism generated on the oceanic crust and (IV) magmatic zones of active continental margins involving the continental crust into magma generation processes; (V) intracontinental rifts and continental hot spots; and (VI) back-arc spreading centers. The histogram of SiO2 contents in the natural igneous melts of all geodynamic settings exhibits a bimodal distribution with two maxima at SiO2 contents of 50–52 wt % and 72–74 wt %. The range 62–64 wt % SiO2 comprises the minimum number of determinations. Primitive mantle-normalized spidergrams were constructed for average contents of elements in the igneous melts of basic, intermediate, and acidic compositions from settings I–V. The diagrams reflect the characteristic features of melt compositions for each geodynamic setting. On the basis of the analysis of data on the composition of melt inclusions and glasses of rocks, average ratios of incompatible trace and volatile components (H2O/Ce, K2O/Cl, Nb/U, Ba/Rb, Ce/Pb, etc.) were estimated for the igneous melts of all of the settings. Variations of these ratios were determined, and it was shown that, in most cases, the ratios of incompatible elements are significantly different between settings. The difference is especially pronounced for the ratios of elements with different degrees of incompatibility (e.g., Nb/Yb) and for some ratios with volatile components (e.g., K2O/H2O).

Average compositions of magmas and mantle sources of mid-ocean ridges and intraplate oceanic and continental settings estimated from the data on melt inclusions and quenched glasses of basalts

Based on the generalization of the compositions of melt inclusions and quenched glasses from basaltic rocks, the average compositions of magmas were estimated for mid-ocean ridges (MOR), intraplate continental environments (CR), and ocean islands and plateaus (OI). These compositions were used to constrain the average contents of trace and volatile elements in mantle sources. A procedure was developed for the estimation of the average contents of incompatible elements, including volatiles (H2O, Cl, F, and S), in the mantle. A comparison of the obtained average contents for the depleted mantle (DM) with the available published estimates showed that the contents of most incompatible trace elements (H2O, Cl, F, Be, B, Rb, Sr, Zr, Ba, La, Ce, Nd, Sm, Eu, Hf, Ta, Th, and U) can be reliably estimated from the ratio of K to the desired trace element in the MOR magmas and the average content of K in the DM. For Nb, Ti, P, S, Li, Y, and heavy REE, we used the ratios of their contents to an element with a similar degree of incompatibility in MOR magmas (U for Nb and Dy for the other elements). This approach was used to determine the average contents of incompatible elements in oceanic plume mantle (OPM) and the subcontinental mantle of intraplate settings or continental plume mantle (CPM). It was shown that the average composition of both suboceanic and subcontinental mantle plumes is moderately enriched compared with the DM in the most incompatible elements (K, U, Ba, and La) and volatile components (H2O, Cl, and F). The extent of volatile component enrichment in the plume mantle (500–1500 ppm H2O) is insufficient for a significant depression of the mantle solidus. Therefore, mantle plumes must be hotter than the ambient depleted mantle. The average contents of incompatible trace elements in the OPM are similar to those of the primitive mantle, which could be related either to the retention of primitive mantle material in the regions of plume generation or to DM fertilization at the expense of the deep mantle recycling of crustal materials. In the latter case, the negative anomaly of water in the trace-element distribution patterns of the OPM is explained by the participation of dehydrated crust in its formation. Variations in the compositions of magmas and their sources were considered for various geodynamic settings, and it was shown that the sources are heterogeneous with respect to trace and volatile components. The chemical heterogeneity of the magma sources and gradual transitions between them suggest that the mantle reservoirs interact with each other. Chemical variations in continental and oceanic plume magmas can be attributed to the existence of several interacting sources, including one depleted and at least two enriched reservoirs with different contents of volatiles. These variations are in agreement with the zoned structure of mantle plumes, which consist of a hot and relatively dry core, a colder outer shell with high contents of volatile components, and a zone of interaction between the plume and depleted mantle.

Magmatic inclusions in the search for natural silicate-salt melt immiscibility: methodology and examples

Immiscible phase separation during the cooling and crystallisation of magmas is an inherently fugitive phenomenon and melt inclusions may provide the only remaining evidence of this process. We detail those features of such inclusions that can both prove the existence of immiscible phase separation, and constrain the compositional signature of the process. To do so requires the combination of traditional methods (petrographic examination, microthermometry, etc.) with state of the art microbeam analytical techniques (laser Raman spectroscopy and proton-induced X-ray emission). Examples of inclusions in phenocrysts from barren and mineralised rocks are provided to illustrate the approach and validate the interpretations.

Estimation of the average contents of H2O, Cl, F, and S in the depleted mantle on the basis of the compositions of melt inclusions and quenched glasses of mid-ocean ridge basalts

Using our database of the compositions of melt inclusions and quenched glasses of basaltic magmas from mid-ocean ridges (MORB), the average concentrations and ratios of H2O, Cl, F, S, K2O, Ce, and Dy were determined in these magmas. Assuming that the concentration ratios of volatile components to K2O are constant in the MORB magmas and their sources (depleted mantle, DM), and taking an average K2O content in the DM of 72 ppm, the following average contents were estimated for the DM: 158 ppm H2O, 6.6 ppm Cl, and 8.3 ppm F. Using an S/Dy ratio of 212 for MORB melts and a Dy concentration of 0.531 ppm in the DM, the concentration of S in the DM was estimated as 113 ppm. Our value for the average content of Cl is much higher than estimates obtained by other authors. This discrepancy could be due either to the assimilation of crustal (and hydrospheric) Cl by MORB magmas or to the deep mantle recycling of Cl. The latter mechanism is supported by the statistically significant positive correlation of Cl with K2O, H2O, and F. Such a correlation is not consistent with the hypothesis of basaltic magma contamination by seawater-derived chloride brines. Similar to other surface processes, the assimilation of crustal material operates within the existing global correlations and disturbs them. Based on the average integrated degree of mantle melting and the average degree of MORB magma differentiation (0.05), the average contents of potassium and volatile components in N-MORB and E-MORB mantle sources were estimated as 39 and 126 ppm K2O, 103 and 197 ppm H2O, 4.0 and 10.7 ppm Cl, and 3.9 and 9.1 ppm F, respectively. It is not likely that normal MORB magmas can be derived from depleted mantle that experienced a previous partial melting event (for instance, during the extraction of the primordial continental crust in the Early Precambrian), which was referred to as the ultradepleted mantle. Ordinary (not ultradepleted) MORB magmas can be derived either by the melting of a zone enriched DM (for instance, progressively enriched in incompatible components with depth), which is hardly possible, or by the continuous addition (mixing) of an enriched component to the ultradepleted mantle at the expense of sediments and crustal materials involved in deep recycling.

Inclusions of silicate and sulfate melts in chrome diposide from the Inagli deposit, Yakutia, Russia

Melt inclusions were studied in chrome diopside from the Inagli deposit of gemstones in the Inagli massif of alkaline ultrabasic rocks of potassic affinity in the northwestern Aldan shield, Yakutia, Russia. The chrome diopside is highly transparent and has an intense green color. Its Cr2O3 content varies from 0.13 to 0.75 wt %. Primary and primary-secondary polyphase inclusions in chrome diopside are dominated by crystal phases (80–90 vol %) and contain aqueous solution and a gas phase. Using electron microprobe analysis and Raman spectroscopy, the following crystalline phases were identified. Silicate minerals are represented by potassium feldspar, pectolite [NaCa2Si3O8(OH)], and phlogopite. The most abundant minerals in the majority of inclusions are sulfates: glaserite (aphthitalite) [K3Na(SO4)2], glauberite [Na2Ca(SO4)2], aluminum sulfate, anhydrite (CaSO4), gypsum (CaSO4 × 2H2O), barite (BaSO4), bloedite [Na2Mg(SO4)2 × 4H2O], thenardite (NaSO4), polyhalite [K2Ca2Mg(SO4)4 × 2H2O], arcanite (K2SO4), and celestite (SrSO4). In addition, apatite was detected in some inclusions. Chlorides are probably present among small crystalline phases, because some analyses of aggregates of silicate and sulfate minerals showed up to 0.19–10.3 wt % Cl. Hydrogen was identified in the gas phase of polyphase inclusions by Raman spectroscopy. The composition of melt from which the chrome diopside crystallized was calculated on the basis of the investigation of silicate melt inclusions. This melt contains 53.5 wt % SiO2, considerable amounts of CaO (16.3 wt %), K2O (7.9 wt %), Na2O (3.5 wt %), and SO3 (1.4 wt %) and moderate amounts of Al2O3 (7.5 wt %), MgO (5.8 wt %), FeO (1.1 wt %), and H2O (0.75 wt %). The content of Cr2O3 in the melt was 0.13 wt %. Many inclusions were homogenized at 770–850°C, when all of the crystals and the gas phase were dissolved. The material of inclusions heated up to the homogenization temperature became heterogeneous even during very fast quenching (two seconds) producing numerous small crystals. This fact implies that most of the inclusions contained a salt (rather than silicate) melt of sulfate-dominated composition. Such inclusions were formed from salt globules (with a density of about 2.5 g/cm3) occurring as an emulsion in the denser (2.6 g/cm3) silicate melt from which the chrome diopside crystallized.

Average composition of basic magmas and mantle sources of island arcs and active continental margins estimated from the data on melt inclusions and quenched glasses of rocks

Based on the generalization of data on melt inclusions and quenched glasses, the average compositions of subduction (island arc and active continental margin settings) basic magmas were estimated. The main geochemical features of the average composition of these magmas are significant depletion in Nb and Ta, less significant depletion in Ti, Zr, and Sm, and enrichment in Cl, H2O, F, and P in the primitive mantlenormalized patterns. The average normalized contents of moderately incompatible HREE in these magmas are close to those in the basic magmas of other geodynamic settings. Subduction basic magmas exhibit negative correlation of Li, Y, Dy, Er, Yb, Lu, and Ti contents with MgO content. Most of incompatible elements (Nb, Ta, U, Th, LREE) do not correlate with MgO, but correlate with each other and K2O. Variations in element contents are related to crystallization differentiation, magma mixing, and possibly, participation of several sources. The water content in the island arc basic magmas varies from almost zero value to more than 6 wt %. Most compositions are characterized by weak negative correlation between H2O and MgO contents, but some compositions define a negative correlation close to that in magmas of mid-ocean ridges (MOR). Considered magmas demonstrate distinct positive correlation between MgO content and homogenization temperature, practically coinciding with that of MOR magmas. Modeling of phase equilibria revealed widening of crystallization field of olivine in the magmas of subduction zones compared to MOR magmas. This can be related to the high water content in subduction magmas. Simultaneous liquidus crystallization of olivine and clinopyroxene in subduction magmas occurs at pressure approximately 5 kbar higher than that of MOR magmas. Based on the average ratios of trace element to K2O content, we determined the average compositions for subduction magma sources. Relative to depleted mantle, they are enriched in all incompatible elements, with positive anomalies of U, Rb, Ba, B, Pb, Cl, H2O, F, and S, and negative anomalies of Th, K, Be, Nb, Ta, Li, Nd, Pb, and Ti. A general elevated content of incompatible elements indicates a reworking of the rocks of mantle wedge by fluids and melts that were released from the upper layers of subducted plate.

Physicochemical parameters of the formation of hydrothermal deposits: A fluid inclusion study. I. Tin and tungsten deposits

The author’s database, which presently includes data from more than 18500 publications on fluid and melt inclusions in minerals and is continuing to be appended, was used to generalize results on physicochemical parameters of the formation of hydrothermal deposits and occurrences of tin and tungsten. The database includes data on 320 tin and tin-tungsten deposits and occurrences and 253 tungsten and tungstentin deposits around the world. For most typical minerals of these deposits (quartz, cassiterite, tungsten, scheelite, topaz, beryl, tourmaline, fluorite, and calcite), histograms of homogenization temperatures of fluid inclusions were plotted. Most of 463 determinations made for cassiterite are in the range of 300–500°C with maximum at 300–400°C, while those for wolframite and scheelite (453 determinations) fall in the range of 200–400°C with maximum at 200–300°C. Representative material on pressures of hydrothermal fluids included 330 determinations for tin and 430 determinations for tungsten objects. It was found that premineral, ore, and postmineral stages spanned a wide pressure range from 70–110 bar to 6000–6400 bar. High pressures of the premineral stages at these deposits are caused by their genetic relation with felsic magmatism. Around 50% of pressure determinations lie in the range of 500–1500 bar. The wide variations in total salinity and temperatures (from 0.1 to 80 wt % NaCl equiv and 20–800°C) were obtained for mineral-forming fluids at the tin (1800 determinations) and tungsten (2070 determinations) objects. Most of all determinations define a salinity less than 10 wt % NaCl equiv. (∼60%) and temperature range of 200–400°C (∼70%). The average composition of volatile components of fluids determined by different methods is reported. Data on gas composition of the fluids determined by Raman spectroscopy are examined. Based on 180 determinations, the fluids from tin objects have the following composition (in mol %): 41.2 CO2, 39.5 CH4, 19.15 N2, and 0.15 H2S. The volatile components of tungsten deposits (190 determinations) are represented by 56.1 CO2, 30.7 CH4, 13.2 N2, and 0.01 H2S. Thus, the inclusions of tungsten deposits are characterized by higher CO2 content and lower (but sufficiently high) contents of CH4 and N2. The concentrations of tin and tungsten in magmatic melts and mineral-forming fluids were estimated from analysis of individual inclusions. The geometric mean Sn contents are 87 ppm (+ 610 ppm/−76 ppm) in the melts (569 determinations) and 132 ppm (+ 630 ppm/−109 ppm) in the fluids (253 determinations). The geometric mean W values are 6.8 ppm (+ 81/−6.2 ppm) in the magmatic melts (430 determinations) and 30 ppm (+ 144 ppm/−25 ppm) in the mineral-forming fluids (391 determinations).

Composition and chemical structure of oceanic mantle plumes

The average compositions (including H2O, Cl, F, and S contents) and chemical structure of oceanic mantle plumes were estimated on the basis of the ratios of incompatible volatile components, potassium, and some other elements in the basaltic magmas of ocean islands (melt inclusions and quenched glasses). The following average concentrations were estimated for the plume mantle: 510 ppm K2O, 520 ppm H2O, 21 ppm Cl, 55 ppm F, and 83 ppm S; these values are significantly higher than those of the depleted mantle (except for S). The abundances of H2O, Cl, and S are lower than in the primitive mantle. The normalized H2O content in the plume mantle is similar to the concentrations of similarly incompatible La and Ce but lower than the concentrations of K2O, Cl, and Sr. This is at odds with the idea of wet mantle plumes. Three types of basaltic magmas corresponding to three types of plume sources (M1, M2, and M3) were distinguished. The concentrations of incompatible elements in these reservoirs were estimated using two models, assuming either an isochemical mantle or a moderately enriched composition of plume material. The latter model gave the following average concentrations of H2O, Cl, F, and S: 130, 33, 11, and 110 ppm for M1, 110, 12, 65, and 45 ppm for M2; 530, 29, 49, and 110 ppm for M3. The plume mantle is not homogeneous, and its heterogeneity is related to the existence of three main compositions, one of which (M1) is similar to the mantle of mid-ocean ridges, and two others (M2 and M3) are moderately enriched in K2O, TiO2, P2O5, F, and incompatible trace elements. The compositions of M2 and M3 are strongly different in H2O, Cl, and S contents. The M2 mantle reservoir is significantly poorer in these components and richer in incompatible trace elements than M3. The plume mantle was formed mainly by the mixing of three sources: ultradepleted mantle, moderately enriched relatively dry mantle, and moderately enriched H2O-rich mantle. In addition to the three main components of the plume mantle, there are probably minor components enriched in chlorine and depleted in fluorine. It is supposed that all these components are entrained into the plume mantle through the mantle recycling of components of the oceanic and continental crust. The established relationships are in agreement with the zonal model of a mantle plume, which includes a hot central part poor in H2O, Cl, and S; an outer part enriched in volatile and nonvolatile incompatible elements; and enclosing mantle material interacting with the plume.

Principal Physicochemical Parameters of Natural Mineral-Forming Fluids

The authors’ database (which includes data from more than 17500 publications on fluid and melt inclusions in minerals) was used to generalize information on the principal physicochemical parameters of natural mineral-forming fluids (temperature, pressure, density, salinity of aqueous solutions, and the gas composition of the fluids). For 21 minerals, data are reported on the frequency of occurrence of the homogenization temperatures of fluid inclusions in various temperature ranges, which make it possible to reveal temperature ranges most favorable for the crystallization of these minerals. Data on 5260 determinations were used to evaluate the frequency of occurrence of certain temperature and pressure ranges of natural fluids within the temperature intervals of 20–1200°C and 1–12000 bar. Within these intervals, frequencies of occurrence were evaluated for water-dominated and water-poor or water-free fluid inclusions in minerals. The former are predominant at temperatures below 600°C and pressures below 4000 bar, whereas the latter dominate at temperatures of 600–1200°C and pressures of 4000-12000 bar. Illustrative examples are presented for visually discernible magmatic water that exists as an individual high-density phase in melt inclusions in minerals from various rocks sampled worldwide (in the Caucasus, Italy, Slovakia, United States, Uzbekistan, New Zealand, Chile, and others). Attention is drawn to the fact that extensive data testify to fairly high (>1000–1500 bar) pressures during hydrothermal mineral-forming processes. These pressures are much higher not only than the hydrostatic but also the lithostatic pressures of the overlying rocks. Data on more than 18000 determinations are used to evaluate the frequency of occurrence of certain temperature and salinity ranges of mineral-forming fluids within the intervals of 20–1000°C and 0–80 wt % equiv. NaCl and certain temperature and density ranges of these fluids at 20–1000°C and 0.01–1.90 g/cm3. Information is presented on the gas analysis methods most commonly applied to natural fluids in studying fluid inclusions in minerals in 1965–2007. The average composition of the gaseous phase of natural inclusions is calculated based on more than 3000 Raman spectroscopic analyses (the most frequently used method for analyzing individual inclusions).