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  • 1
    Publication Date: 2020-06-18
    Description: The Ontong Java and Manihiki oceanic plateaus are believed to have formed through high-degree melting of a mantle plume head. Boninite-like, low-Ti basement rocks at Manihiki, however, imply a more complex magma genesis compared with Ontong Java basement lavas that can be generated by ∼30% melting of a primitive mantle source. Here we show that the trace element and isotope compositions of low-Ti Manihiki rocks can best be explained by re-melting of an ultra-depleted source (possibly a common mantle component in the Ontong Java and Manihiki plume sources) re-enriched by ≤1% of an ocean-island-basalt-like melt component. Unlike boninites formed via hydrous flux melting of refractory mantle at subduction zones, these boninite-like intraplate rocks formed through adiabatic decompression melting of refractory plume material that has been metasomatized by ocean-island-basalt-like melts. Our results suggest that caution is required before assuming all Archaean boninites were formed in association with subduction processes.
    Type: Article , PeerReviewed
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  • 2
    Publication Date: 2020-02-06
    Description: In 2013, high-temperature vent fluids were sampled in the Nifonea vent field. This field is located within the caldera of a large shield-type volcano of the Vate Trough, a young extensional rift in the New Hebrides back-arc. Hydrothermal venting occurs as clear and black smoker fluids with temperatures up to 368 °C, the hottest temperatures measured so far in the western Pacific. The physico-chemical conditions place the fluids within the two-phase field of NaCl–H2O, and venting is dominated by vapour phase fluids with Cl concentrations as low as 25 mM. The fluid composition, which differs between the individual vent sites, is interpreted to reflect the specific geochemical fluid signature of a hydrothermal system in its initial, post-eruptive stage. The strong Cl depletion is accompanied by low alkali/Cl ratios compared to more evolved hydrothermal systems, and very high Fe/Cl ratios. The concentrations of REY (180 nM) and As (21 μM) in the most Cl-depleted fluid are among the highest reported so far for submarine hydrothermal fluids, whereas the inter-element REY fractionation is only minor. The fluid signature, which has been described here for the first time in a back-arc setting, is controlled by fast fluid passage through basaltic volcanic rocks, with extremely high water-rock ratios and only limited water-rock exchange, phase separation and segregation, and (at least) two-component fluid mixing. Metals and metalloids are unexpectedly mobile in the vapour phase fluids, and the strong enrichments of Fe, REY, and As highlight the metal transport capacity of low-salinity, low-density vapours at the specific physico-chemical conditions at Nifonea. One possible scenario is that the fluids boiled before the separated vapour phase continued to react with fresh glassy lavas. The mobilization of metals is likely to occur by leaching from fresh glass and grain boundaries and is supported by the high water/rock ratios. The enrichment of B and As is further controlled by their high volatility, whereas the strong enrichment of REY is also a consequence of the elevated concentrations in the host rocks. However, a direct contribution of metals such as As from magmatic degassing cannot be ruled out. The different fluid end-member composition of individual vent sites could be explained by mixing of vapour phase fluids with another fluid phase of different water/rock interaction history.
    Type: Article , PeerReviewed
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  • 3
    Publication Date: 2020-02-06
    Description: It was proposed to utilize siderite FeCO3 in mid to late Archaean Superior type banded as a proxy to constrain the CO2 partial pressure of Archaean atmospheres. Implicit in this proposition is that siderite was a primary carbonate mineral that crystallized directly from Fe2+ enriched Archaean seawater, in equilibrium with atmospheric CO2. To our knowledge that proposition has not been demonstrated to be valid. We test with water-gas exchange experiments under controlled CO2 partial pressures if siderite can be stabilized as a primary mineral in Fe2+ bearing seawater. Reduced seawater proxies enriched in Fe2+ and Mn2+ are equilibrated with reduced N2-CH4-CO2-H2 gas phases with variable CO2. The solid phases stabilized in Fe2+ enriched water compositions are amorphous ferrous iron hydroxy carbonates. Crystalline siderite FeCO3 is not found to be a stable phase. The phases precipitating from Mn2+ enriched water include crystalline rhodochrosite MnCO3 and possibly amorphous Mn-enriched phases. Based on these results we advise against using siderite in banded iron formations as a CO2 sensor for the Archaean atmosphere.
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  • 4
    Publication Date: 2018-01-12
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 5
    Publication Date: 2018-01-12
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  • 6
    Publication Date: 2021-02-08
    Description: Highlights • Widespread Holocene tephra KHG, from Kamchatka, is found as a cryptotephra in the NGRIP ice-core. • This is the first identification of tephra from the Kamchatka Peninsula in Greenland ice. • NGRIP KHG has an age of 7872 ± 50 a BP and improves age models for Kamchatka. • Existing 14C age estimates for the KHG eruption are too young. Abstract Contiguous sampling of Holocene ice from the NGRIP core, Greenland, has revealed a new rhyolitic cryptotephra that is geochemically identical to the KHG tephra, a widespread marker deposit originating from the Khangar volcano, Kamchatka. This is the first identification of tephra from the Kamchatka Peninsula in Greenland ice and the first finding of the KHG tephra outside Kamchatka. The NGRIP KHG has an age of 7872 ± 50 a BP 1950, and this date will help improve age models for Kamchatka, where existing age estimates of KHG are too young, thus highlighting the importance of locating long-range, low-concentration cryptotephra deposits in well-dated ice cores. In Greenland KHG is located close to the termination of the 8.2 ka BP cooling event that is also a climate feature in palaeo-records of Kamchatka. This tie-point therefore provides a unique opportunity to synchronise records of environmental change in distal locations.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 7
    Publication Date: 2021-02-08
    Description: The strong dependence of vanadium partitioning between olivine and silicate melt (DVOl-M) on redox conditions (fO2) can be used as sensitive oxybarometer in magmatic systems. Here we extend the experimental database on DVOl-M, obtained so far at high temperatures (mainly above 1250 °C), to lower temperatures that are typical for island-arc basalts. Crystallization experiments were performed using a composition from Mutnovsky volcano (Kamchatka), and the investigated temperature, pressure, and oxygen fugacity ranges were 1025–1150 °C, 0.1 and 0.3 GPa, and ΔQFM of –0.5 to +3.2, respectively. The water content in melts ranged from 0.6 to ∼6.5 wt% H2O. The data demonstrate a strong negative correlation between DVOl-M and oxygen fugacity, similar to the behavior observed previously at higher temperatures and in MgO-rich compositions. The correlation between DVOl-M and ΔQFM in the range from –0.5 to +3.2 is described for melts with MgO 〈 12 wt% and Na2O 〈 4 wt% at temperatures ≤1250 °C by the empirical equation: ΔQFM = −3.07−0.29+0.26 logDVOl-M – 3.34−0.49+0.40 with the standard error (SE) as a function of logDVOl-M: 2SE(ΔQFM) = –0.275logDVOl-M + 0.4. We suggest that this equation can be used as an oxybarometer, which is particularly well applicable to the hydrous island-arc magmas at relatively low temperature. Application of the equation to the composition of melt inclusions and their host olivine phenocrysts from basalts of Mutnovsky volcano, containing vanadium concentrations in the range of 250–370 and 4–6 ppm, respectively, reveals an oxygen fugacity in the range ΔQFM +1.9 to +2.3. The estimates are in a good agreement with olivine-spinel oxybarometry for Mutnovsky basalts and may be typical for moderately evolved island-arc magmas.
    Type: Article , PeerReviewed
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  • 8
    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.
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  • 9
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    In:  [Talk] In: 11. Workshop on Russian-German Cooperation: Laptev Sea System – The Transpolar System of the Arctic Ocean, 24.-26.01.2016, Kiel, Germany .
    Publication Date: 2017-01-10
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 10
    Publication Date: 2020-02-06
    Description: Tertiary rift-related intraplate basanites from the Batain basin of northeastern Oman have low SiO2 (〈 45.6 wt.%), high MgO (〉 9.73 wt.%) and moderate to high Cr and Ni contents (Cr 〉 261 ppm, Ni 〉 181 ppm), representing near primary magmas that have undergone fractionation of mainly olivine and magnetite. Rare earth element systematics and p-T estimates suggest that the alkaline rocks are generated by different degrees of partial melting (4–13%) of a spinel-peridotite lithospheric mantle containing residual amphibole. The alkaline rocks show restricted variations of 87Sr/86Sr and 143Nd/144Nd ranging from 0.70340 to 0.70405 and 0.51275 to 0.51284, respectively. Variations in Pb isotopes (206Pb/204Pb: 18.59–18.82, 207Pb/204Pb: 15.54–15.56, 208Pb/204Pb: 38.65–38.98) of the alkaline rocks fall in the range of most OIB. Trace element constraints together with Sr–Nd–Pb isotope composition indicate that assimilation through crustal material did not affect the lavas. Instead, trace element variations can be explained by melting of a lithospheric mantle source that was metasomatized by an OIB-type magma that was accumulated at the base of the lithosphere sometimes in the past. Although only an area of less than 1000 km2 was sampled, magmatic activity lasted for about 5.5 Ma with a virtually continuous activity from 40.7 ± 0.7 to 35.3 ± 0.6 Ma. During this period magma composition was nearly constant, i.e. the degree of melting and the nature of the tapped source did not change significantly over time.
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