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listelement.badge.dso-typeЭлемент, GEOCHEMISTRY AND AGE OF METAMORPHIC ROCKS OF THE KHAVYVEN HIGHLAND, EASTERN KAMCHATKA(2007) Tararin I.A.; Badredinov Z.G.; Dril' S.I.The metamorphic rocks of the Khavyven Highland in eastern Kamchatka were determined to comprise two complexes of metavolcanic rocks that have different ages and are associated with subordinate amounts of metasediments. The complex composing the lower part of the visible vertical section of the highland is dominated by leucocratic amphibole-mica (±garnet) and epidote-mica (±garnet) crystalline schists, whose protoliths were andesites and dacites and their high-K varieties of the island-arc calc-alkaline series. The other complex, composing the upper part of the vertical section, consists of spilitized basaltoids transformed into epidote-amphibole and phengite-epidote-amphibole green schists, which form (together with quartzites, serpentinized peridotites, serpentinites, and gabbroids) a sea-margin ophiolitic association. The high LILE concentrations, high K/La, Ba/Th, Th/Ta, and La/Nb ratios, deep Ta-Nb minima, and low (La/Yb)N and high 87Sr/86Sr ratios of the crystalline schists of the lower unit are demonstrated to testify to their subduction nature and suggest that their protolithic volcanics were produced in the suprasubduction environment of the Ozernoi-Valaginskii (Achaivayam-Valaginskii) island volcanic arc of Campanian-Paleogene age. The green schists of the upper unit show features of depleted MOR tholeiitic melts and subduction melts, which cause the deep Ta-Nb minima, and low K/La and 87Sr/86Sr ratios suggesting that the green schists were formed in a marginal basin in front of the Ozernoi-Valaginskaya island arc. Recently obtained K-Ar ages in the Khavyven Highland vary from 32.4 to 39.3 Ma and indicate that the metamorphism of the protolithic rocks occurred in the Eocene under the effect of collision and accretion processes of the arc complexes of the Ozernoi-Valaginskii and Kronotskii island arcs with the Asian continent and the closure of forearc oceanic basins in front of them. The modern position of the collision suture that marks the fossil subduction zone of the Ozernoi-Valaginskii arc and is spatially restricted to the buried Khavyven uplift in the Central Kamchatka Depression, which is characterized by well-pronounced linear gravity anomalies.listelement.badge.dso-typeЭлемент, REGIONAL FEATURES OF PRIMARY ALKALINE MAGMAS OF THE ATLANTIC OCEAN(2007) Kogarko L.N.; Asavin A.M.In the framework of the GIS project on the geochemistry of Atlantic intraplate magmatism, primary high-magnesian melts were identified there and subdivided into five types: foidites, picrites, basanite-nephelinites, alkaline olivine basalts, and tholeiite. Their relative proportions were determined for both the Atlantic Ocean as a whole and individual magmatic centers. The compositional ranges and average compositions were calculated. It was shown that alkali rocks are predominant, but tholeiite melts account for about 25%. Among ocean-island volcanic rocks, differentiated varieties clearly dominate over primary melts (80 and 20%, respectively). Variations in the proportions of the distinguished types were applied to prepare a map for the petrochemical typification of Atlantic intraplate magmatism. Seven petrochemical zones were provisionally identified, first demonstrating the lateral petrochemical heterogeneity of intraplate sources of the Atlantic Ocean. In addition to the global heterogeneity, each large center of intraplate magmatism (archipelago or island chain) demonstrates local heterogeneities. The variations in the Na/K, Ti/Na, and Si/Ca ratios reflect significant magma generation depths (in the lower mantle) for intraplate magmatism. It was proposed that variations in the Ti/Na ratio in the high-magnesian melts are controlled by a change in the Na and Ti partition coefficients of pyroxene with increasing magma generation depth. A comparison between evolution of the oceanic and continental alkaline magmatism was conducted.listelement.badge.dso-typeЭлемент, HIGH-PRESSURE PARTIAL MELTING OF GABBRO AND ITS ROLE IN THE HAWAIIAN MAGMA SOURCE(2007) Yaxley G.M.; Sobolev A.V.We have conducted high-pressure experiments on a natural oceanic gabbro composition (Gb108). Our aim was to test recent proposals that Sr-enrichment in rare primitive melt inclusions from Mauna Loa, Hawaii, may have resulted from melting of garnet pyroxenite formed in the magma source regions by reaction of peridotite with siliceous, Sr-enriched partial melts of eclogite of gabbroic composition. Gb108 is a natural, Sr-enriched olivine gabbro, which has a strong positive Sr anomaly superimposed on an overall depleted incompatible trace element pattern, reflecting its origin as a plagioclase-rich cumulate. At high pressures it crystallises as a coesite eclogite assemblage, with the solidus between 1,300 and 1,350°C at 3.5 GPa and 1,450 and 1,500°C at 4.5 GPa. Clinopyroxenes contain 4–9% Ca-eskolaite component, which varies systematically with pressure and temperature. Garnets are almandine and grossular-rich. Low degree partial melts are highly siliceous in composition, resembling dacites. Coesite is eliminated between 50 and 100°C above the solidus. The whole-rock Sr-enrichment is primarily hosted by clinopyroxene. This phase dominates the mode (>75 wt%) at all investigated PT conditions, and is the major contributor to partial melts of this eclogite composition. Hence the partial melts have trace element patterns sub-parallel to those of clinopyroxene with ≈10× greater overall abundances and with strong positive Sr anomalies. Recent studies of primitive Hawaiian volcanics have suggested the incorporation into their source regions of eclogite, formerly gabbroic material recycled through the mantle at subduction zones. The models suggest that formerly gabbroic material, present as eclogite in the Hawaiian plume, partially melted earlier than surrounding peridotite (i.e. at higher pressure) because of the lower solidus temperature of eclogite compared with peridotite. This produced highly siliceous melts which reacted with surrounding peridotite producing hybrid pyroxene + garnet lithologies. The Sr-enriched nature of the formerly plagioclase-rich gabbro was present in the siliceous partial melts, as demonstrated by these experiments, and was transferred to the reactive pyroxenite. These in turn partially melted, producing Sr-enriched picritic liquids which mixed with normal picritic partial melts of peridotite before eruption. On rare occasions these mixed, relatively Sr-rich melts were trapped as melt inclusions in primitive olivine phenocrysts.listelement.badge.dso-typeЭлемент, DECOMPRESSION MECHANISM OF FERRIC IRON REDUCTION IN TEKTITE MELTS DURING THEIR FORMATION IN THE IMPACT PROCESS(2007) Lukanin O.A.; Kadik A.A.The analysis of available data on the Fe3+/Fe2+ ratio of impact-produced glasses showed that tektites and some other types of impact glasses are reduced compared with the precursor target material. Possible reasons for the change in the degree of iron oxidation in the impact process are still debatable. Based on the analysis of redox reactions in relatively simple systems with iron in different oxidation states (Fe-O and SiO2-FeO-Fe2O3) and the available data on the influence of temperature, oxygen partial pressure (pO2), and total pressure (P tot) on the Fe3+/Fe2+ ratio of silicate melts, a model was proposed suggesting that the lower Fe3+/Fe2+ values of tektites formed in the impact process compared with the initial target material could be related to the characteristics of oxygen regime during the decompression stage following shock compression. One of the main prerequisites for the occurrence of reduction reactions involving iron and other elements is the attainment of high temperatures (>1800–2000°C) at a certain stage of decompression, providing the complete melting and partial evaporation of the material. When the vapor pressure in the system becomes equal to the total pressure during adiabatic decompression, a further decrease in P tot will be inevitably accompanied by a decrease in pO2 and, correspondingly, partial reduction of Fe3+ to Fe2+ in the melt. The reactions of decompression reduction occur under closed-system conditions and do not require oxygen removal from the system. The higher the temperature and Fe3+/Fe2+ ratio of the melt, the more extensive iron reduction can be observed during the final stages of decompression. If the temperatures attained during decompression after an impact event are sufficient (>2500–3000°C) for the complete evaporation of the material, the melt produced during subsequent condensation must be significantly more reduced than the initial material. The final stage of the impact process is characterized by a catastrophic expansion of the explosion cloud, condensation, and rapid cooling. During this stage, the system is already not closed. The quenched glasses of this stage record the redox state of earlier melts. In addition, they can contain microinclusions of the products of nonequilibrium vapor condensation with iron compounds of different oxidation states, including metallic iron and iron oxides (wüstite, magnetite, and hematite).listelement.badge.dso-typeЭлемент, INFLUENCE OF SALINE DEPOSITS ON THE CONDITIONS OF PETROLEUM GENERATION IN THE ROCKS UNDERLYING THE SALT COMPLEX OF THE NORTHERN PART OF THE PRECASPIAN BASIN(2007) Galushkin Yu.I.; Yakovlev G.E.Numerous studies on the potential of hydrocarbon generation by the rocks of the sedimentary cover of the northern Precaspian Basin are based either on the interpolation of measurements from a relatively sparse boreholes network or on the numerical estimations of the history of development of the hydrocarbon potential under the assumption of a steady temperature gradient both with depth in the sedimentary cover and with time during basin evolution. By the example of sedimentary sections from the northeastern part of the Precaspian Basin, variations in thermal history and petroleum formation conditions were numerically analyzed for the rocks underlying the salt complex of the basin. Variations in the temperature gradient and thermal properties of the rocks with depth and time were accounted for in the modeling. Numerical reconstructions of the thermal and maturation history of sedimentary sections from the basin were used to estimate the influence of evaporite sequences on the thermal history, the maturation level of organic matter, and the hydrocarbon generation potential of the subsalt complex of the basin. The calculations showed that this influence could be significant, but there remained an uncertainty related to the absence of reliable data on the time and rate of salt diapir formation.