V. A. Dorofeeva

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.

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.

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.

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).

Volatiles in basaltic magmas of ocean islands and their mantle sources: I. Melt compositions deduced from melt inclusions and glasses in the rocks

Statistical analysis of a data bank of the compositions of glasses and melt inclusions in minerals from ocean-island basalts. The initial database contains more than 45 000 published analyses of ocean-island igneous rocks from around the world. Much attention was given to the contents of volatiles (H2O, Cl, F, and S) and their ratios to one another and to nonvolatile components of close incompatibility (Ti, P, K, and Ce). The average compositions of melt inclusions are similar to those of glasses of the rocks, including volatiles, with consideration for a somewhat higher degree (by approximately 20%) of the differentiation of glasses. The average compositions of ocean-island melts differ from those of mid-ocean basalts in having wider variations and elevated contents of some of the most incompatible elements (Sr, Nb, Ta, Ba, U, Th, and others), as well as H2O, F, and Cl. Based on the correlation of volatiles to one another and to incompatible elements, three groups of ocean-island basalts are distinguished: (I) low-K, P, Ti magma compositions approximating mid-ocean ridge magmas, (II) high-K, Ce, P, and Ti magmas that resemble continental rift magmas but differ from them in low H2O content, and (III) high-K, H2O, Ce, P, and Ti magmas close to continental rift magma. All three types of the melts were found only in the Hawaiian Archipelago, whereas other ocean islands are dominated by any one of these types. The distinguished melt types presumably reflect the differences (heterogeneity) in the compositions of the sources.

Peralkaline silicic melts of island arcs, active continental margins, and intraplate continental settings: Evidence from the investigation of melt inclusions in minerals and quenched glasses of rocks

Based on the analysis of data on the composition of melt inclusions in minerals and quenched glasses of igneous rocks, we considered the problems of the formation of peralkaline silicic magmas (i.e., whose agpaitic index, the molar ratio AI = (Na2O + K2O)/Al2O3, is higher than one). The mean compositions of peralkaline silicic melts are reported for island arcs and active continental margins and compared with the compositions of melts from other settings, primarily, intraplate continental areas. Peralkaline silicic rocks are rather common in the latter. Such rocks are rare in island arcs and active continental margins, but agpaitic melts were observed in inclusions in phenocrysts of plagioclase, quartz, pyroxene, and other minerals. Plagioclase fractionation from an alkali-rich melt with AI < 1 is considered as a possible mechanism for the formation of peralkaline silicic melts (Bowen’s plagioclase effect). However, the analysis of available experimental data on plagioclase-melt equilibria showed that natural peralkaline melts are almost never in equilibrium with plagioclase. For the same reason, the melting of the majority of crustal rocks, which usually contain plagioclase, does not produce peralkaline melts. The existence of peralkaline silicic melt inclusions in plagioclase phenocrysts suggests that plagioclase can crystallize from peralkaline melts, and the plagioclase effect may play a certain role. Another mechanism for the formation of peralkaline silicic magmas is the melting of alkali-rich basic and intermediate rocks, including the spilitized varieties of subalkali basalts.

Comparison of major, volatile, and trace element contents in the melts of mid-ocean ridges on the basis of data on inclusions in minerals and quenched glasses of rocks

Using our database on major, trace, and volatile element contents in melt inclusions in minerals and quenched glasses of volcanic rocks reported in the literature, we compared the mean contents of 71 chemical elements in melts from the mid-ocean ridges (MORB) of the Atlantic, Pacific, and Indian oceans and determined the mean MORB composition for all the oceans of the Earth (global MORB composition). Mean ratios of incompatible trace and volatile components (H2O/Ce, K2O/Cl, Nb/U, Ba/Rb, Ce/Pb, Nb/U, etc.) were calculated for magmatic melts from all the oceans. Variations of these parameters were estimated, and significant differences between the melts of the Atlantic and Pacific oceans were established.

Physicochemical formation parameters of hydrothermal mineral deposits: Evidence from fluid inclusions. II. Gold, silver, lead, and zink deposits

Information from a database, which was compiled and continuously updated by the authors of this paper and now includes information from 19500 publication on fluid and melt inclusions in minerals, is used to summarize results on the physicochemical formation parameters of hydrothermal Au, Ag, Pb, and Zn deposits. The database provides information on fluid inclusions in minerals from 970 Pb-Zn, 220 Au-Ag-Pb-Zn, and 825 Au-Ag deposits in various settings worldwide. Histograms for the homogenization temperatures of fluid inclusion are presented for the most typical minerals of the deposits. In sphalerite, most homogenization temperatures (1327 measurements) of fluid inclusions lie within the range of 50–200°C with a maximum at 100–200°C for this mineral from Pb-Zn deposits and within the range of 100–350°C (802 measurements) with a maximum at 200–300°C for this mineral from Au deposits. Data are presented on fluid pressures at Au (1495 measurements) and Pb-Zn (180 measurements) deposits. The pressure during the preore, ore-forming, and postore stages at these deposits ranged from 4–10 to 6000 bar. The reason for the high pressures during preore stages at the deposits is the relations of the fluids to acid magmatic and metamorphic processes. More than 70% of the fluid pressures values measured at Pb-Zn deposits lie within the range of 1–1500 bar. Au-Ag deposits are characterized by higher fluid pressures of 500–2000 bar (61% of the measurements). The overall ranges of the salinity and temperature of the mineral-forming fluid at Au-Ag (6778 measurements) and Pb-Zn (3395 measurements) deposits are 0.1–80 wt % equiv. NaCl and 20–800°C. Most measurements (∼64%) for Au-Ag deposits yield fluid salinity <10 wt % equiv. NaCl and temperatures of 200–400°C (63%). Fluids at Pb-Zn deposits are typically more saline (10–25 wt % equiv. NaCl, 51% measurements) and lower temperature (100–300°C, 74% measurements). Several measurements of the fluid density fall within the range of 0.8–1.2 g/cm3. The average composition of volatile components of the fluids was evaluated by various techniques. The average composition of volatile components of fluid inclusions in minerals is calculated for hydrothermal W, Au, Ag, Sn, and Pb-Zn deposits, metamorphic rocks, and all geological objects. The Au, Ag, Pb, and Zn concentrations in magmatic melts and mineral-forming fluids is evaluated based on analyses of individual inclusions.