Chemical composition of melts of the Early Eocene volcanic center at Cape Khairyuzova,western Kamchatka: Evidence from inclusions in minerals

Original authors’ data on the mineralogy and composition of melt inclusions in two samples show that the Early Eocene magmatic rocks at Cape Khairyuzova were formed by mixing melts of mafic, intermediate, and acid composition, which were derived from different sources. The mafic melt was rich in MgO, and its temperature was 1100–1150°C. The temperature of the acid melt varied from 1070 to 1130°C. The melts are also different in concentrations of trace elements and in their ratios. All three melt types are enriched in LILE and LREE and depleted in HFSE and were likely derived in suprasubductional environments. The mafic and intermediate magmas were formed by melting a mantle wedge and subsequent fractionation of the melts. The acid melts could be formed by melting crustal rocks when they were overheated in the newly formed orogen of significant thickness. When ascending, the mantle melts could mix in variable proportions with acid melts in crustal chambers.     

Evidence for the volcanoclastic genesis of the Carnian diamondiferous bed in the Ust’-Olenek area (from petrographic and geochemical data)

A detailed petrographic study of a 0.4–1.2 m thick Carnian diamondiferous bed of the Angardam-Tasa Ridge (Ust’-Olenek area, northern Siberia) was performed. It showed that the bed earlier referred to as a terrigenous deposit might be interpreted as an intensely altered volcanoclastic lapilli tuff of basic and, probably, ultrabasic composition. The tuff occurs within a 100 m thick Ladinian–Carnian volcanosedimentary sequence. It consists mainly of resurgent material: altered lithoclasts (nodules of basic-ultrabasic? lithology), devitrified volcanic glass, fragments of autolithic breccia, etc. The thin volcanoclastic sheet is supposed to have formed within a sedimentation basin as a result of phreatomagmatic eruptions of kimberlite volcanoes. The substantiation of the volcanogenic origin of the Carnian diamondiferous bed might be a new approach to elucidate the primary source of widespread placer diamonds in Arctic Siberia.     

Basaltic melts in olivine from alkaline pumice of Primor’e: Evidence from the study of melt inclusions

Doklady Earth Sciences -     

Genesis of massive sulfide deposits in the Verkhneural’sk ore district, the South Urals, Russia: Evidence for magmatic contribution of metals and fluids

Melt inclusions and aqueous fluid inclusions in quartz phenocrysts from host felsic volcanics, as well as fluid inclusions in minerals of ores and wall rocks were studied at the Cu-Zn massive sulfide deposits in the Verkhneural’sk ore district, the South Urals. The high-temperature (850–1210°C) magmatic melts of volcanic rocks are normal in alkalinity and correspond to rhyolites of the tholeiitic series. The groups of predominant K-Na-type (K₂O/Na₂O = 0.3–1.0), less abundant Na-type (K₂O/Na₂O = 0.15–0.3), and K-type (K₂O/Na₂O = 1.9–9.3) rhyolites are distinguished. The average concentrations (wt %) of volatile components in the melts are as follows: 2.9 H₂O (up to 6.5), 0.13 Cl (up to 0.28), and 0.09 F (up to 0.42). When quartz was crystallizing, the melt was heterogeneous, contained magnetite crystals and sulfide globules (pyrrhotite, pentlandite, chalcopyrite, bornite). High-density aqueous fluid inclusions, which were identified for the first time in quartz phenocrysts from felsic volcanics of the South Urals, provide evidence for real participation of magmatic water in hydrothermal ore formation. The fluids were homogenized at 124–245°C in the liquid phase; the salinity of the aqueous solution is 1.2–6.2 wt % NaCl equiv. The calculated fluid pressure is very high: 7.0–8.7 kbar at 850°C and 5.1–6.8 kbar at 700°C. The LA-ICP-MS analysis of melt and aqueous fluid inclusions in quartz phenocrysts shows a high saturation of primary magmatic fluid and melt with metals. This indicates ore potential of island-arc volcanic complexes spatially associated with massive sulfide deposits. The systematic study of fluid inclusions in minerals of ores and wall rocks at five massive sulfide deposits of the Verkhneural’sk district furnished evidence that ore-forming fluids had temperature of 375–115°C, pressure up to 1.0–0.5 kbar, chloride composition, and salinity of 0.8–11.2 (occasionally up to 22.8) wt % NaCl equiv. The H and O isotopic compositions of sericite from host metasomatic rocks suggest a substantial contribution of seawater to the composition of mineral-forming fluids. The role of magmatic water increases in the central zones of the feeding conduit and with depth. The dual nature of fluids with the prevalence of their magmatic source is supported by S, C, O, and Sr isotopic compositions. The TC parameters of the formation of massive sulfide deposits are consistent with the data on fluid inclusions from contemporary sulfide mounds on the oceanic bottom.     

Fluid inclusion studies of the Felbertal scheelite deposit

Fluid inclusion measurements on quartz, scheelite, beryl, fluorite and calcite in the metamorphosed Felbertal scheelite deposit display two main types of fluid inclusions:

1. H₂O-CO₂ fluid inclusions are characterized by variable amounts of CO₂ up to 18 wt.%. They show two or three phases at room temperature. The bulk homogenization temperatures for the inclusions range between +269 °C and +357 °C. The calculated salinities are between 2.2 and 7.8 wt.% NaCl equivalent. For the late CO₂-bearing fluid inclusions a methane component is evident from microthermometrical data (T_mclath >10.0 °C combined with T_mCO₂
2. Aqueous, two-phase fluid inclusions with salinities in the range between 0 and 11 wt.% NaCl equivalent. Their homogenization temperatures are scattered between 100 °C and 360 °C.

Both types of fluid inclusions are of Alpine origin. They do not record the conditions of the original tungsten ore formation in pre-Alpine (Upper Proterozoic) time. However, it was possible to deduce a path for the fluid evolution and the combined ore redeposition during the retrograde Alpine metamorphism and tectonism from microthermometrical and petrographical studies.     

The helical feature calculation for the velocity field of a developing tropical cyclone

The paper presents a part of the results obtained within the framework of the research program on numerical analysis of helical features of the developing tropical cyclone, by using the regional ETA model of the atmosphere. A set of integral characteristics and of gridded diagnostics is selected and substantiated, allowing revealing and tracing evolution of large-scale helical-vortex instability associated with specific properties of helical convective turbulence under condition of tropical cyclogenesis. The helicity vertical flux (helicity index) is calculated following the technique by Kurganskii for two tropical cyclones: Man-Yi (2007) and Wilma (2005). It is shown that the helicity index changes during the tropical cyclone evolution reflect adequately the tendencies of pressure and velocity changes and thus the index can be used as an indicative characteristic of intensity of the developing tropical cyclone.     

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).     

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.