ALBERT

Description: Olivine major and trace element compositions from 12 basalts from the southern Payenia volcanic province in Argentina have been analyzed by electron microprobe and laser ablation inductively coupled plasma mass spectrometry. The olivines have high Fe/Mn and low Ca/Fe and many fall at the end of the global olivine array, indicating that they were formed from a pyroxene-rich source distinct from typical mantle peridotite. The olivines with the highest Fe/Mn have higher Zn/Fe, Zn and Co and lower Co/Fe than the olivines with lower Fe/Mn, also suggesting contributions from a pyroxene-rich source. Together with whole-rock radiogenic isotopes and elemental concentrations, the samples indicate mixing between two mantle sources: (1) a pyroxene-rich source with EM-1 ocean island basalt type trace element and isotope characteristics; (2) a peridotitic source with more radiogenic Pb that was metasomatized by subduction-zone fluids and/or melts. The increasing contributions from the pyroxene-rich source in the southern Payenia basalts are correlated with an increasing Fe-enrichment, which caused the olivines to have lower forsterite contents at a given Ni content. Al-in-olivine crystallization temperatures measured on olivine–spinel pairs are between 1155 and 1243°C and indicate that the magmas formed at normal upper mantle (asthenospheric) temperatures of ~1350°C. The pyroxene-rich material is interpreted to have been brought up from the deeper parts of the upper mantle by vigorous asthenospheric upwelling caused by break-off of the Nazca slab south of Payenia during the Pliocene and roll-back of the subducting slab beneath Payenia. The pyroxene-rich mantle mixed with peridotitic metasomatized South Atlantic mantle in the mantle wedge beneath Payenia.

Description: The volatile contents of olivine-hosted (Fo89–71) melt inclusion glasses in rapidly quenched mafic tephras from volcanic front volcanoes of the Central American Volcanic Arc (CAVA) in Guatemala, Nicaragua, and Costa Rica, were analyzed by secondary ion mass spectrometry (SIMS) in order to derive the minimum eruptive output of CO2, along with H2O, Cl, and S. Details of the analytical method are provided that establish melt inclusion CO2 analyses with the Cameca ims6f at the Helmholtz Centre Potsdam. The highest CO2 concentrations (up to 1800 μg/g) are observed in Nicaraguan samples, while melt inclusions from Guatemala and Costa Rica have CO2 contents between 50 and 500 μg/g. CO2 does not positively covary with sediment/slab fluid tracers such as Ba/La, Ba/Th, or U/La. Instead, the highest CO2 concentrations occur in the inclusions with the most depleted incompatible element compositions and low H2O, approaching the composition of mid-ocean ridge basalts (MORBs), whereas the most H2O-rich inclusions are relatively CO2-poor (

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% SiO 2 . 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 Nd = 9·1 ± 0·3 ( n = 31) and Hf = 14·5 ± 0·6 ( n = 27). Lead isotopes are variable and decrease with increasing SiO 2 from basalts with 206 Pb/ 204 Pb = 18·51 ± 0·05 ( n = 11) to dacites and rhyodacites with 206 Pb/ 204 Pb = 18·43 ± 0·04 ( n = 18). Western seafloor lavas form a steep trend in 207 Pb/ 204 Pb– 206 Pb/ 204 Pb space, and are collinear with lavas from emergent Aleutian volcanoes, which mostly have 206 Pb/ 204 Pb 〉 18·6 and 207 Pb/ 204 Pb 〉 15·52. High MgO and Mg# relative to silica, flat to decreasing abundances of incompatible elements, and decreasing Pb isotope ratios with increasing SiO 2 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: Bowers Ridge is an ~700 km long arcuate ridge behind the Central Aleutian Arc in the Bering Sea. The lack of age and geochemical data for the ridge has hampered the development of geodynamic models for the evolution of the North Pacific and the Aleutian–Bering Sea region. Here we present the first geochemical and 40 Ar/ 39 Ar age data for the volcanic basement of Bowers Ridge and a seamount from the western end of the ridge sampled during R/V Sonne cruise SO201-1b. The northern Bowers Ridge basement (26–32 Ma) consists of mafic to intermediate calc-alkaline rocks with adakite-like (Sr/Y = 33–53, La N /Yb N = 3.3–7.8), high field strength element (HFSE)–depleted (e.g., Nb N /La N = 0.07–0.31) trace element patterns and Sr-Nd-Pb isotope compositions within the Western Aleutian Arc array, implying magma generation above an obliquely subducting slab. The seamount samples (22–24 Ma) are HFSE-rich alkaline olivine basalts (La N /Yb N = 3.3–3.9, Nb N /La N = 1.0–1.4) with minor arc-type trace element signatures (Pb N /Ce N = 1.4–1.6, K N /Nb N = 1.7–1.9) but with Pacific mid-oceanic-ridge basalt (MORB)–like isotopic compositions, pointing to an origin by small-degree decompression melting from slightly subduction-modified mantle. The geochemistry of the recovered rocks can be explained by highly oblique subduction along the northern part of Bowers Ridge in its present-day configuration, consistent with an in-situ origin of Bowers Ridge as a Cenozoic island arc.

Description: The morphology, specific surface area, and sorption properties toward Sr-90 radionuclides in seawater of a sorption material based on manganese oxide have been investigated. The material was fabricated through interaction of KMnO 4 and H 2 O 2 with subsequent annealing at 500 °C. The sorbent is characterized with the mechanical strength sufficient for using under dynamic sorption conditions: here, the efficiency of Sr-90 removal at feeding of 150 bed volumes exceeds 95 %. The values of Sr-90 distribution coefficients are equal to 0.8–1.2×10 3 ml/g in the real seawater and to 1.6-1.8 ×10 3 ml/g in the simulated one.