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  • 1
    Keywords: Forschungsbericht
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (18 Seiten, 1.019,00 KB) , Illustrationen, Karten
    Language: German
    Note: Förderkennzeichen BMBF 03G0258C , Verbundnummer 01177085 , Unterschiede zwischen dem gedruckten Dokument und der elektronischen Ressource können nicht ausgeschlossen werden
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  • 2
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (454 Blatt = 29 MB) , Illustrationen
    Series Statement: GEOMAR Report N.S. 30
    Language: English
    Note: Zusammenfassung in deutscher und englischer Sprache
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  • 3
    Keywords: Forschungsbericht
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (16 Seiten, 959,99 KB) , Illustration
    Language: German
    Note: Förderkennzeichen BMBF 03G0233A , Vornamen der Autoren und Mitarbeiter ermittelt , Unterschiede zwischen dem gedruckten Dokument und der elektronischen Ressource können nicht ausgeschlossen werden
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  • 4
    Keywords: Forschungsbericht ; Beringmeer ; Aleuten ; Vulkanismus ; Tektonik
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (17 Seiten, 689,52 KB)
    Language: German
    Note: Förderkennzeichen BMBF 03G0249A , Unterschiede zwischen dem gedruckten Dokument und der elektronischen Ressource können nicht ausgeschlossen werden
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  • 5
    Publication Date: 2020-02-06
    Description: Highlights • An eclogite-melt component (slab melt) is present in volcanic rocks throughout the Aleutian arc. • Fluids that drive slab melting are produced by dehydration of serpentinite in the subducting plate. • Slab melting encompasses a large section of mafic oceanic crust unaffected by seawater alteration. • The subducting plate beneath the Aleutian arc is hotter than indicated by most thermal models. Abstract High Mg# andesites and dacites (Mg# = molar Mg/Mg + Fe) from western Aleutian seafloor volcanoes carry high concentrations of Sr (〉1000 ppm) that is unradiogenic (87Sr/86Sr 〈 0.7029) compared to lavas from emergent volcanoes throughout the arc (200–800 ppm Sr, 87Sr/86Sr 〉0.7030). Data patterns in plots of 87Sr/86Sr vs Y/Sr and Nd/Sr imply the existence of an eclogite-melt source component – formed by partial melting of MORB eclogite in the subducting Pacific Plate – which is most clearly expressed in the compositions of western Aleutian andesites and dacites (Nd/Sr and Y/Sr 〈 0.02) and which dominates the source budget for Sr in volcanic rocks throughout the arc. When viewed in combination with inversely correlated εNdεNd and 87Sr/86Sr, these patterns rule out aqueous fluids as an important source of Sr because mixtures of fluids from altered oceanic crust with depleted mantle and sediment produce compositions with 87Sr/86Sr higher than in common Aleutian rocks. The unradiogenic nature of Sr in the western Aleutian andesite–dacite end-member may be understood if H2O required to drive melting of the subducting oceanic crust is transported in fluids containing little Sr. Mass balance demonstrates that such fluids may be produced by dewatering of serpentinite in the mantle section of the subducting plate. If the eclogite-melt source component is present throughout the Aleutian arc, melting of the subducting plate must extend into minimally altered parts of the sheeted dike section or upper gabbros, at depths 〉2 km below the paleo-seafloor. Oxygen isotopes in western Aleutian seafloor lavas, which fall within a narrow range of MORB-like values (δ18O=5.1–5.7δ18O=5.1–5.7), are also consistent with this model. These results indicate that the subducting Pacific lithosphere beneath the Aleutian arc is significantly hotter than indicated my most thermal models.
    Type: Article , PeerReviewed
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  • 6
    Publication Date: 2021-02-08
    Description: Olivine-hosted inclusions of silicate and sulfide melts, Cr-spinel and pyroxene were studied to estimate magma composition, temperature, pressure, and fO2 at the onset and during the silicate-sulfide immiscibility in modern arc basalt from Tolbachik volcano, Kamchatka arc. We demonstrate that the olivine phenocrysts hosting sulfide and silicate melt inclusions belong to the same population. The compositions of the silicate melt inclusions in most primitive olivine (88–91 mol% Fo) represent moderately oxidized (~ QFM + 1.1) high-MgO (up to 12–12.6 wt%) and high CaO/Al2O3 (0.8–1.2) melt that has abundances and ratios of the lithophile trace elements typical of island arc magmas. The initial volatile contents in parental Tolbachik magma are estimated from the melt inclusions and mass-balance considerations to be at least 4.9 wt% H2O, 2600 ppm S, 1100 ppm Cl, 550 ppm F, and 1200 ppm CO2. These data are used to calculate the temperature (~ 1220 °C) and minimum pressure (3 kbar) at which the beginning of crystallization and exsolution of sulfide melt took place. The presence of anhydrite, especially ubiquitous in the crystallized silicate melt associated with sulfide globules, suggest that much higher sulfur abundances prior to degassing and sulfate immiscibility and/or crystallization should be expected. We tentatively considered hydrothermal accumulations of sulfur (elemental, sulfate and sulfide) in the volcanic conduit responsible for local contamination and oversaturation of the Tolbachik magma in sulfur and related sulfide immiscibility. Coexisting sulfide and sulfate can be also interpreted in favor of the magmatic sulfide oxidation and related generation of S-rich fluids. Such fluids are expected to accumulate metals released from decomposed sulfide melts and supply significant epithermal mineralization, including native gold.
    Type: Article , PeerReviewed
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  • 7
    Publication Date: 2021-02-08
    Description: Primitive olivine-hosted melt inclusions provide information concerning the pre-eruptive volatile contents of silicate melts, but compositional changes associated with post-entrapment processes (PEP) sometimes complicate their interpretation. In particular, crystallization of the host phase along the wall of the melt inclusion and diffusion of H+ through the host promote CO2 and potentially S or other volatiles to exsolve from the melt into a separate fluid phase. Experimental rehomogenization and analysis of MI, or a combination of Raman spectroscopy, numerical modeling, and mass balance calculations are potentially effective methods to account for PEP and restore the original volatile contents of melt inclusions. In order to compare these different approaches, we studied melt inclusions from a suite of samples from Klyuchevskoy volcano (Kamchatka Arc) for which volatile compositions have been determined using experimental rehydration, Raman spectroscopy, and numerical modeling. The maximum CO2 contents of melt inclusions are in agreement (~3600-4000 ppm), regardless of the method used to correct for CO2 in the bubble, but significantly more uncertainty is observed using mass balance calculations. This uncertainty is largely due to the lack of precision associated with the petrographic method of determining bubble volumes and may also be related to the presence of daughter minerals at the glass-bubble interface.
    Type: Article , PeerReviewed
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  • 8
    Publication Date: 2021-02-08
    Description: Highlights • First comprehensive data set of the seamounts from the Walvis Ridge. • The seamounts are 20–40 Myr younger than the age progressive Walvis Ridge basement. • The composition of the seamounts extends from the St. Helena HIMU to EMORB. • The seamounts are derived from a distinct source compared to the Walvis Ridge. • The temporal change from EM I to HIMU could reflect the compositional heterogeneities of the LLSVP. Abstract Volcanic activity at many oceanic volcanoes, ridges and plateaus often reawakens after hiatuses of up to several million years. Compared to the earlier magmatic phases, this late-stage (rejuvenated/post-erosional) volcanism is commonly characterized by a distinct geochemical composition. Late-stage volcanism raises two hitherto unanswered questions: Why does volcanism restart after an extended hiatus and what is the origin of this volcanism? Here we present the first 40Ar/39Ar age and comprehensive trace element and Sr–Nd–Pb–Hf isotopic data from seamounts located on and adjacent to the Walvis Ridge in the South Atlantic ocean basin. The Walvis Ridge is the oldest submarine part of the Tristan-Gough hotspot track and is famous as the original type locality for the enriched mantle one (EM I) end member. Consistent with the bathymetric data, the age data indicates that most of these seamounts are 20–40 Myr younger than the underlying or nearby Walvis Ridge basement. The trace element and isotope data reveal a distinct compositional range from the EM I-type basement. The composition of the seamounts extend from the St. Helena HIMU (high time-integrated 238U/204Pb mantle with radiogenic Pb isotope ratios) end member to an enriched (E) Mid-Ocean-Ridge Basalt (MORB) type composition, reflecting a two-component mixing trend on all isotope diagrams. The EMORB end member could have been generated through mixing of Walvis Ridge EM I with normal (N) MORB source mantle, reflecting interaction of Tristan-Gough (EM I-type) plume melts with the upper mantle. The long volcanic quiescence and the HIMU-like geochemical signature of the seamounts are unusual for classical hotspot related late-stage volcanism, indicating that these seamounts are not related to the Tristan-Gough hotspot volcanism. Two volcanic arrays in southwestern Africa (Gibeon-Dicker Willem and Western Cape province) display similar ages to the late-stage Walvis seamounts and also have HIMU-like compositions, suggesting a larger-scale event at ∼77–49 Ma. We propose that the EM I-like mantle plumes rise from the edges of the African Large Low Shear Velocity Province (LLSVP; Tristan-Gough, Discovery and Shona hotspot), whereas the HIMU-dominated intraplate lavas (St. Helena, Gibeon-Dicker Willem and Western Cape province) and the late-stage Walvis seamounts tap material from internal portions of the African LLSVP, suggesting possible lateral and/or vertical chemical zonation of the African LLSVP.
    Type: Article , PeerReviewed
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  • 9
    Publication Date: 2021-02-08
    Description: Highlights • Melt inclusions from southern Payenia have highly variable element enrichment • Magmas formed by mixing of asthenospheric high Nb/U and lithospheric low Nb/U melts • Low Nb/U type inclusions are similar in composition to alkaline lamprophyres • Low Nb/U melts were formed by fractionation of high Nb/U melts in the SCLM • The percolative fractional crystallization involved cpx, rutile and apatite Abstract We present major and trace element compositions of melt inclusions from three alkali basalts from the Río Colorado volcanic field in the Payenia backarc province, Argentina. Modeling of diffusion profiles around the inclusions showed that most inclusions equilibrated 〈14 days after formation, indicating a short crustal residence time for the magmas and nearly direct ascent through the crust. Despite overlapping host rock isotopic compositions, the inclusions show a large variation in their degree of enrichment, and display trends that we interpret as mixing between asthenospheric OIB-type low K2O-high Nb/U melts and enriched high K2O-low Nb/U lithospheric mantle melts similar in composition to alkaline lamprophyres. The low Nb/U magmas are excessively enriched in the elements Cs, Rb, Ba, Th, U, K, Pb and Cl relative to Nb, Ta and REEs. The enriched low Nb/U components are interpreted to have formed by percolative fractional crystallization of asthenospheric high Nb/U melts in the lithospheric mantle involving crystallization of clinopyroxene, apatite and rutile. The residual fluid-rich melts either mixed directly with new batches of high Nb/U melts or metasomatized and veined the lithospheric mantle which later re-melted during continued volcanism. The major element compositions of the high K2O-low Nb/U components are distinct for the whole rocks and melt inclusions, and most enriched inclusions have lower SiO2 and higher TiO2 contents indicating derivation by melting of amphibole-bearing veins. In contrast, most wr low Nb/U basalts have higher SiO2 and lower TiO2 and were most likely formed by melting of pyroxenitic veins or peridotitic metasomatized lithospheric mantle.
    Type: Article , PeerReviewed
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  • 10
    Publication Date: 2021-02-08
    Description: Highlights • First data on the composition of deep crust and primitive rocks of high-Ti magmatic series of Manihiki Plateau • High potential mantle temperature for Manihiki sources (〉1460oC) suggests a lower mantle plume origin • EM1 signature in high-Ti Manihiki basalts could originate from recycled lower continental crust or re-fertilized SCLM • The presence of refractory mantle in the Manihiki plume explains 30% lower crustal thickness compared to OJP • Manihiki and OJP could have been formed from a geochemically zoned plume or from two spatially separated mantle plumes Abstract Geochemical studies revealed two major (high- and low-Ti) magmatic series composing the Manihiki Plateau in the Western Pacific. Here we report new geochemical data (major and trace element and Sr-Nd-Pb isotope compositions) of the Manihiki rocks. The rocks belong to the previously rarely sampled high-Ti Manihiki series and represent a section of deep crust of the plateau. The rocks were collected by remotely operated vehicle ROV Kiel 6000 during R/V SONNE SO225 expedition from a tectonic block at a stretched and faulted boundary between the Northern and Western Manihiki sub-plateaus. Additional data is presented on samples obtained by dredging during the same cruise. Judging from the age of stratigraphically higher lavas, most samples must be ≥125 Ma old. They comprise fully crystalline microdolerites, aphyric and Ol-Px-Pl-phyric basalts and breccias metamorphosed under greenschist to amphibolite facies with peak metamorphic temperatures of 636–677 °C and pressures of 2.0–2.7 kbar. A single sample of hornblende gabbro was also recovered and likely represents a late stage intrusion. Despite strong metamorphism, the samples from the ROV profile reveal only minor to moderate chemical alteration and their initial compositions are well preserved. The rocks are relatively primitive with MgO up to 13 wt%, range from enriched to depleted in LREE (LaN/SmN = 0.7–1.1), exhibit variable but mostly depleted Nb contents (Nb/Nb* = 0.8–1.3) and display only a narrow range in isotope compositions with strong EM1 characteristics (εNd (t) = 1.8–3.6, 206Pb/204Pb (t) = 17.9–18.1, 207Pb/204Pb (t) = 15.49–15.53, 208Pb/204Pb (t) = 38.08–38.42). The parental magmas are interpreted to originate from a thermochemical plume with a potential mantle temperature 〉1460 °C. The trace element and isotope EM1 signature of the high-Ti rocks reflects the presence of recycled lower continental crust material or re-fertilized subcontinental lithospheric mantle in the plume source. A highly refractory mantle was the primary source of the low-Ti basalts and could also contribute to the origin of high-Ti basalts. On average a more depleted mantle source for the Manihiki rocks can explain ~30% lower crustal thickness of this plateau compared to Ontong Java Plateau, which was mainly formed by melting of similarly hot but more fertile mantle. The presently available data suggest that the sources of Ontong Java and Manihiki Plateaus were compositionally different and could represent two large domains of a single plume or two contemporaneous but separate plumes.
    Type: Article , PeerReviewed
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