ALBERT

Description: Fourier Transform infrared (FTIR) absorption spectra of hydroxyl were measured on olivine phenocrysts from hydrous basaltic melts that originated in island-arc tectonic settings. The basaltic melts encompass a wide range of silica activities from orthopyroxene-saturated hypersthene-normative to nepheline-normative compositions. The intensities and wavenumber placement of hydroxyl absorption bands correlate with the degree of silica saturation of the parent melt from which the olivine crystallized. Olivines from silica-undersaturated nepheline-normative melts absorb IR radiation in the wavenumber range 3430-3590 cm(-1) (Group 1). In contrast, olivines from orthopyroxene-saturated boninitic melts exhibit hydroxyl absorption bands in the wavenumber range 3285-3380 cm(-1) (Group 2). Olivines crystallized at intermediate silica activities exhibit a combination of the two groups of hydroxyl IR bands, where the proportion of Group 2 bands increases with increasing silica saturation of the parent melt. The positions of hydroxyl absorption peaks observed here for natural samples are consistent with previous measurements on experimentally annealed olivines. Thus protonation experiments can be employed to make spectroscopically dry olivine structures visible by IR, yielding information on the silica saturation of the parental magmas. Hydroxyl concentrations in the studied olivines were estimated to be 1-2 ppm, corresponding to an olivine-melt partition coefficient of similar to(1.0 +/- 0.3) x 10(-4).

Description: Discovery of seafloor volcanism west of Buldir Volcano, the westernmost emergent volcano in the Aleutian arc, demonstrates that surface expression of active Aleutian volcanism falls below sea level just west of 175·9°E longitude, but is otherwise continuous from mainland Alaska to Kamchatka. Lavas dredged from newly discovered seafloor volcanoes up to 300 km west of Buldir have end-member geochemical characteristics that provide new insights into the role of subducted basalt as a source component in Aleutian magmas. Western Aleutian seafloor lavas define a highly calc-alkaline series with 50–70% SiO2. Most samples have Mg-numbers [Mg# = Mg/(Mg + Fe)] greater than 0·60, with higher MgO and lower FeO* compared with average Aleutian volcanic rocks at all silica contents. Common basalts and basaltic andesites in the series are primitive, with average Mg# values of 0·67 (±0·02, n = 99, 1SD), and have Sr concentrations (423 ± 29 ppm, n = 99) and La/Yb ratios (4·5 ± 0·4, n = 29) that are typical of island arc basaltic lavas. A smaller group of basaltic samples is more evolved and geochemically more enriched, with higher and more variable Sr and La/Yb (average Mg# = 0·61 ± 0·1, n = 31; Sr = 882 ± 333 ppm, n = 31; La/Yb = 9·1 ± 0·9, n = 16). None of the geochemically enriched basalts or basaltic andesites has low Y (〈15 ppm) or Yb (〈1·5 ppm), so none show the influence of residual or cumulate garnet. In contrast, most western seafloor andesites, dacites and rhyodacites have higher Sr (〉1000 ppm) and are adakitic, with strongly fractionated trace element patterns (Sr/Y = 50–350, La/Yb = 8–35, Dy/Yb = 2·0–3·5) with low relative abundances of Nb and Ta (La/Ta 〉 100), consistent with an enhanced role for residual or cumulate garnet + rutile. All western seafloor lavas have uniformly radiogenic Hf and Nd isotopes, with εNd = 9·1 ± 0·3 (n = 31) and εHf = 14·5 ± 0·6 (n = 27). Lead isotopes are variable and decrease with increasing SiO2 from basalts with 206Pb/204Pb = 18·51 ± 0·05 (n = 11) to dacites and rhyodacites with 206Pb/204Pb = 18·43 ± 0·04 (n = 18). Western seafloor lavas form a steep trend in 207Pb/204Pb–206Pb/204Pb space, and are collinear with lavas from emergent Aleutian volcanoes, which mostly have 206Pb/204Pb 〉 18·6 and 207Pb/204Pb 〉 15·52. High MgO and Mg# relative to silica, flat to decreasing abundances of incompatible elements, and decreasing Pb isotope ratios with increasing SiO2 rule out an origin for the dacites and rhyodacites by fractional crystallization. The physical setting of some samples (erupted through Bering Sea oceanic lithosphere) rules out an origin for their garnet + rutile trace element signature by melting in the deep crust. Adakitic trace element patterns in the dacites and rhyodacites are therefore interpreted as the product of melting of mid-ocean ridge basalt (MORB) eclogite in the subducting oceanic crust. Western seafloor andesites, dacites and rhyodacites define a geochemical end-member that is isotopically like MORB, with strongly fractionated Ta/Hf, Ta/Nd, Ce/Pb, Yb/Nd and Sr/Y. This eclogite component appears to be present in lavas throughout the arc. Mass-balance modeling indicates that it may contribute 36–50% of the light rare earth elements and 18% of the Hf that is present in Aleutian volcanic rocks. Close juxtaposition of high-Mg# basalt, andesite and dacite implies widely variable temperatures in the western Aleutian mantle wedge. A conceptual model explaining this shows interaction of hydrous eclogite melts with mantle peridotite to produce buoyant diapirs of pyroxenite and pyroxenite melt. These diapirs reach the base of the crust and feed surface volcanism in the western Aleutians, but are diluted by extensive melting in a hotter mantle wedge in the eastern part of the arc.

Description: The paper presents the very first data on concentrations of major and trace elements; Sr, Nd, and Pb isotopic ratios of rocks; and the composition of olivine phenocrysts of 38-Ma basalts recovered by Hole 513a (DSDP Leg 71) in the South Atlantic. The bulk-rock samples and the chilled glasses are mildly magnesian (7–8 wt % MgO) and bear elevated FeO and low Na2O concentrations, as is typical of MORB of the TOR-1 type. Olivine phenocrysts (Fo84.5–88) in these rocks contain concentrations of trace elements (Ni, Mn, Cr, and Zn) that are typical of classic MORB, which are produced by partial melting of mantle peridotite. The rocks are strongly depleted in incompatible elements [(La/Sm)n ~ 0.6] but have elevated Ba/Nb, K/Nb, and Pb/Ce ratios and Cu, Ag, and Au concentrations that are 1.5–4 times higher than in typical depleted MORB (N-MORB) and in most rift basalts in the South Atlantic. Isotope compositions of the basalts (average ratios 206Pb/204Pb ~ 18.0; 207Pb/204Pb ~ 15.6, 208Pb/204Pb ~ 38.0, 143Nd/144Nd ~ 0.5130, and 87Sr/86Sr ~ 0.7040) are close to those in modern tholeiites from the southern MAR segment (SMAR) north of the Agulhas Fracture Zone. The data indicate that the magmas were derived from a strongly depleted mantle source that contained a minor (~3%) admixture of an enriched component, which is discernible in the magmas of the Discovery hotspot. The composition of the source, which is more depleted than DM, and the high degrees of melting of this source explain why the basalts from DSDP Hole 513a are enriched in chalcophile elements. It is believed that spreading magmatism at 45°–48° S in SMAR as far back as 40 Ma was already affected by the Discovery hotspot. This hotspot might be related to the Tristan plume system, and its origin and long-lasting influence on spreading magmatism in the South Atlantic are regarded as evidence of the extensive effect of the Tristan plume.

Description: Geochemical study of volcaniclastic material and radiocarbon dating of charred plant debris from Holocene deposits of the Guram site, which is located in vicinity of Vetrovoi Isthmus on Iturup Island, demonstrate that an explosive eruption (VEI 4-5) occurred there about 2000 years ago. The geochemical and age similarity with the tephra of marker layer CKr that was distinguished on Iturup, Urup, Simushir, Rasshua, and Matua islands of the Kuril Island Arc led to the conclusion that this eruption is possibly a source of this tephra. The data presented are proposed as a motivation for revision of the volcanic hazard on Iturup Island.

Description: The paper presents petrographic, mineralogical, and geochemical data on dunites, pyroxenites, peridotites, and gabbroids of the Kamchatsky Mys ophiolite. These data were acquired to distinguish cogenetic assemblages of igneous rocks, gain an insight into their geodynamic settings, and test various criteria of genetic links between the different magmatic rocks of ophiolites. The ultramafic and mafic rocks are shown to belong to two series, which differ in the compositions of the primary minerals, bulk rocks, and estimated trapped melts. The rocks of these series are found out to have been produced by geochemically different melts in different geodynamic settings, and during different episodes of mantle magmatism. The rocks of the high-Ti series (gabbro of the Olenegorsk massif, dunite and melanogabbro xenoliths in them, and vein gabbro in these xenoliths) crystallized from N-MORB melts in an oceanic spreading center. The rocks of the low-Ti series (dunite, pyroxenite, and gabbro veins in the residual spinel peridotites of the Mount Soldatskaya massif, as well as pyroxenite, peridotite, and gabbro alluvium and diluvium in the central and western parts of the peninsula) crystallized from water-rich boninite melts in relation to initial subduction magmatism. Taken into account the absence of boninite lavas from the Kamchatsky Mys ophiolite, the plutonic ultramafic rocks (including the rocks of the veins) might be the only evidence of subduction boninitic magmatism in the ophiolites. It was demonstrated that conclusions about the geodynamic settings of plutonic ultramafic and mafic rocks and recognition of cogenetic relations of these rocks with spatially associated basalts are more reliable when derived from the compositions of the trapped melts, which are estimated from their bulk geochemistry and primary mineral compositions, than when they are based on the mineral compositions only.

Description: Kharchinsky and Zarechny volcanoes and the Kharchinsky Lake zone of monogenic cones are unique eruptive centers of magnesian lavas located above the northern margin of the Pacific plate subducting beneath Kamchatka. This paper presents new geochemical data on the composition of rocks (55 samples) and minerals (over 900 analyses of olivine, pyroxenes, amphibole, and plagioclase) of these centers analyzed by XRF and LA-ICP-MS (rocks) and electron microprobe (minerals). Most of the studied rocks are magnesian (Mg# = 60–75 mol %) medium-K basalts and basaltic andesites. Moderate-magnesian (Mg# = 52–59 mol %) basaltic andesites are present among the monogenic cones of Kharchinsky Lake. The rare rock varieties include the high-K basalts–basaltic andesites of dikes in the center of Kharchinsky volcano and the magnesian andesites (Mg# = 58–61 mol %) of the extrusions of Zarechnу volcano. The distribution of trace-element contents in these samples demonstrates enrichment in large-ion lithophile elements and light REEs at depletion in high field strength elements and heavy REEs, as is typical of arc rocks. The high-K basalts and basaltic andesites show anomalous enrichment in Ba (〉1000 ppm), Th (〉3.8 ppm), U (〉1.8 ppm), Sr (〉 800 ppm, Sr/Y 〉 50), and light REE (La 〉 20 ppm), and their compositions are close to those of low-Si adakites. The basalts and basaltic andesites contain phenocrysts of high-Mg olivine (up to Fo92.6) and clinopyroxene (Mg # up to 91 mol %). The rocks show petrographic and geochemical evidence of fractional crystallization, along with the processes of mineral accumulation and magma mixing. Some of the olivine phenocrysts show high NiO contents (up to 5000 ppm) and an elevated Fe/Mn ratio (up to 80), which were interpreted as evidence of the participation of a pyroxenite source in the magma generation processes. The use of the Ca/Fe and Ni/Mg ratios allowed us to distinguish the composition fields and evolution trends of olivine associated with different sources: peridotite and pyroxenite, which were formed by a reaction between mantle-wedge peridotites and high-Si melts of the subducted oceanic crust. The new data are consistent with other lines of evidence of melting of the subducted Pacific plate edge beneath the northern part of the Central Kamchatka Depression at the Kurile–Kamchatka and Aleutian subduction zone junction and testify to a significant heterogeneity of the mantle in this area.

Description: The role and conditions of liquid immiscibility or crystallization of sulfide phase during evolution of subduction-related magmas remains a debated topic, which bears relevance to the genesis of porphyry copper deposits and evolution of the continental crust. We studied rare volcanic rocks with inclusions of magmatic sulfides in olivine—the basalts of Medvezhya Mount in the Avachinsky group of volcanoes. The rocks belong to primitive (Mg# = 66 mol %) middle-K island-arc olivine basalts. Olivine with normal zoning predominates (~98%) among phenocrysts. The olivine compositions are typical for Kamchatka basalts, except for an unusual trend of increase of MnO content from 0.20 to 0.55 wt % and decrease of Fe/Mn from 60 to 35 with a change of olivine composition from Fo87.8 to Fo78.2. Olivine of this group contains numerous inclusions of spinel-group minerals varying in composition from chromium spinel to magnesian magnetite. Olivine phenocrysts with sulfide inclusions are characterized by the absence of or weak reverse zoning and reduced contents of Ca, Ni, Mn, Cr, and Al. The estimated crystallization temperatures are 1036–1241°C for olivine of the prevailing type and 1010–1062°C for sulfide-bearing olivine. The data suggest that crystallization of the main olivine population occurred under relatively shallow conditions and was accompanied by strong magma oxidation. On the contrary, the zoning pattern and compositional features of sulfide-bearing olivine suggest its xenogenic origin and the probable crystallization under deep-crustal conditions from low-temperature water-rich and/or low-Ca magmas. The results obtained confirm the possibility of saturation of oxidized island-arc magmas with sulfide phase at lower crustal conditions, but show that this process is rare and not typical for low-pressure crystallization stage.