ALBERT

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: 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: 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.