ALBERT

Description: This review is intended to highlight recent exciting advances in the study of distal (〉100 km from the source) tephra and cryptotephra deposits and their potential application for volcanology. Geochemical correlations of tephra between proximal and distal locations have extended the geographical distribution of tephra over tens of millions square kilometers. Such correlations embark on the potential to reappraise volume and magnitude estimates of known eruptions. Cryptotephra investigations in marine, lake and ice-core records also give rise to continuous chronicles of large explosive eruptions many of which were hitherto unknown. Tephra preservation within distal ice sheets and varved lake sediments permit precise dating of parent eruptions and provide new insight into the frequency of eruptions. Recent advances in analytical methods permit an examination of magmatic processes and the evolution of the whole volcanic belts at distances of hundreds and thousands of kilometers from source. Distal tephrochronology has much to offer volcanology and has the potential to significantly contribute to our understanding of sizes, recurrence intervals and geochemical make-up of the large explosive eruptions.

Description: In 1935, A.N. Zavaritskii described high-magnesian [MgO/(MgO + FeO*) = 0.73–0.82] basalts and picrites, which are unique in Kamchatka and were found on Avachinsky volcano and eventually named ava- chites. This paper systematizes data on the composition of these rocks and presents the results of their detailed mineralogical examination on an electron microprobe (EMPA) and with the use of secondary-ion mass spec- trometry (SIMS) and vibrational IR spectrometry. The results thus obtained suggest that avachites are of volca- nic genesis and were produced by the crystallization of olivine (Fo91–80), clinopyroxene (Mg# = 92.5–73 mol %), and spinel [Mg# = 18–59 mol %, Cr/(Cr + Al) = 0.82–0.55] from a basaltic (SiO2 ≤ 52 wt %, MgO ~ 13 wt %) parental melt, whose composition was intermediate between those of island-arc ankaramites and high-Ca bon- inites. The high-Mg chemistry of the rocks (MgO = 14–20 wt %) is explained by the accumulation of olivine and pyroxene phenocrysts in an evolved basaltic melt (MgO ~ 5 wt %), which composes groundmass of ava- chites. The minerals crystallized under pressures of 1.0–0.1 GPa, at temperatures of ≤1380–1050°C, and an oxy- gen fugacity of ∆QFM = 0.5–2.0. The results of the IR spectroscopy of the olivine suggest that the parental magmas contained ≥0.5 wt % ç2é. The parental magmas of avachites were derived at high degrees (〉20%) of the partial melting of a mantle source that was depleted more strongly than the source of mid-oceanic ridge basalts (MORB) and was metasomatized by a fluid or melt rich in LREE, Th, Ba, K, and Sr. The typical basaltic andes- ites of Avachinsky volcano can be genetically related to avachites and could be produced by olivine and pyrox- ene crystallization from parental melts of similar composition but under pressures varying within a narrower range. The composition of the associated primitive basalts indicates that the parental melts of the volcano were heterogeneous, and magmas of ankaramite composition could contribute to the genesis of volcanic series in Kamchatka.

Description: Modern models for the development and evolution of the geochemical heterogeneity in the Earth's mantle and the genesis of mantle magmas attach much importance to the processes of interaction between deepseated rocks and metasomatic fluids, which are able, when occurring under mantle conditions, to dissolve significant amounts of major and trace elements (see, for example, [1]). Fluorine is one of the major anions of natural fluids and also one of the principal complex-forming ligands of several metals. To evaluate the possible role of fluorine in the processes of mantle magma genesis and to identify the source of this element in natural magmas in various geodynamic environments, it is necessary to know the fluorine concentration in primitive mantle melts. These data are still relatively scarce, particularly for low alkaline magmas [2-4]. The fluorine concentrations in magmas from suprasubduction zones, whose genesis is largely controlled by the interaction between mantle rocks and fluids, remain poorly known and need further refinement. Data presented in this paper are among the first to characterize the concentrations of fluorine in primitive magmas of suprasubduction zones. These data were obtained by secondary ion mass spectrometry of chill glasses from the lava complex of the Troodos ophiolites in Cyprus. Along with information on the concentrations of major and trace elements, H2O, and Cl in the glasses, our results make it possible to utilize the example of the Troodos ophiolites to characterize the main regularities in the geochemistry of fluorine during the origin of magmas above subduction zones and to assay the contributions of various components that participated in the processes of mantle melting. These data are among the first to demonstrate that subduction-related melts became enriched in F relative to LREE.

Description: The composition and crystallization conditions of the parental melts of avachites were elucidated by studying melt inclusions in olivine (Fo85.8–90.7) phenocrysts. The melt inclusions captured during the crys- tallization of primitive magmas subsequently reequilibrated with their host minerals and became partly recrys- tallized and decrepitated. The diffusion-controlled reequilibration of the melt inclusions with the olivine occurred at temperatures close to ~1100°ë and was associated with the crystallization of daughter phases: oli- vine, high-Ca pyroxene, and spinel. The composition of the pyroxene and spinel in the inclusions evolved toward extremely high Al contents, which is atypical of pyroxene in the rocks and was controlled by plagioclase absence from the daughter phase assemblage of the inclusions. Magma decompression induced the partial decrepitation of the melt inclusions, a process that was associated with the escape of fluid components (ëé2 and ç2é) and variable amounts of the residual silicate material from the inclusions. The initial compositions of the melt inclusions, which were reconstructed using techniques of experimental homogenization and mod- eling, show broad ranges in the contents of major and trace elements. Compared with the composition of the rocks, the compositions of inclusions in the olivine Fo 〉 90% are higher in CaO, Al2O3, and Na2O at lower concentrations of SiO2. Their geochemical characteristics are identical to those of low-Si ankaramite melts occurring in many island arcs. The carbonatite metasomatism of the arc mantle, the derivation of nepheline- normative ankaramite magmas, and the significant crustal contamination of these magmas during their fraction- ation can be spread more widely than is currently assumed in models for island-arc petrogenesis. The evolution of the avachite primitive magmas was controlled by the crystallization of early olivine, high-Ca pyroxene, spinel, and, perhaps, the assimilation of crustal rocks in the magmatic chambers at different depths (from 5 to 30 km). During two (or more) crystallization stages, olivine–pyroxene cumulates were produced, remobilized, and transported to the surface by the differentiated hypersthene-normative magmas. Avachites are hybrid cumu- lative rocks, which were produced in a long-lived open magmatic system.

Description: Cretaceous rocks comparable to mid-oceanic ridge and oceanic island basalts were described in ophiolite association of the Cape Kamchatskii, Eastern Kamchatka (Fedorchuk et al., 1989). New data on the composition of these basalts, their spinels, and melt inclusions in them are presented in this paper. Spinel from olivine–plagioclase basalts corresponds in composition to that of mid-oceanic ridge tholeiites (Mg# = 0.57−0.82, Cr# = 0.33–0.55, TiO2 = 0.03–0.85 wt %). High-K alkali basalts include less magnesian and chro- mian (Mg# = 0.57–0.70, Cr# = 0.23–0.30), but Ti-rich (TiO2 = 0.60–0.86 wt %) spinel. Low Cr# and very low Fe3+ contents of spinel distinguish the studied rocks from island-arc basalts of Eastern Kamchatka. Melt inclu- sions in spinel from plagioclase–olivine basalts have tholeiitic low-alkali (Na2O = 0.8–2.5 wt %, K2O = 0.01−0.09 wt %), high-Ca (CaO = 13–15 wt %), and low-Ti (TiO2 = 0.2–1.2 wt %) composition. Chlorine con- centrations are extremely low in the melt inclusions (Cl 〈 0.005 wt %), while S contents are high (S = 0.12 ± 0.03 wt %). Concentrations of incompatible elements in inclusions (La = 0.3–1.2 ppm, Sr = 27–70 ppm, Zr = 13–21 ppm) are systematically lower than in typical oceanic tholeiites. The melts are selectively enriched in K, Sr, and Ba relative to REE in the N-MORB-normalized trace-element spectra. Participation of plume com- ponent in the formation of the Late Cretaceous rift-related basalts of the Pacific Ocean, which are incorporated now in ophiolite association of the Cape Kamchatskii, is inferred from indicative rock assemblage and compo- sition of primitive melts and spinel.

Description: The main goal of the study is to establish the spatial and temporal distribution of pyroclastic material from large explosive eruptions of the volcanoes of Kamchatka, the Kuril, and Aleutian Islands to create a generalized tephrochronological model and reveal patterns of explosive activity in this region. This paper presents new data on the composition of volcanic ash (tephra) found in the Pleistocene deposits of the northwestern Pacific from the eastern slope of the Detroit Rise (northwestern part of the Imperial Ridge), 450–550 km east of the Kamchatka Peninsula. Eleven layers and lenses of tephra aged from 28 to 245 ka, which were previously unknown, were studied in the core Lv63-4-2. Their stratigraphic position and age were determined based on age models developed in this study. Based on the geochemical composition of volcanic glass (determined using an electron microprobe), seven layers were correlated with tephra from several cores in the northwestern Pacific and the Bering Sea. The obtained results supplement the information on large explosive eruptions of volcanoes in the region and their periods of activity. They also allow the development of a generalized tephrochronological model of Quaternary deposits, which is necessary for stratigraphic correlation, and of paleooceanological and paleogeographic reconstructions.