The Aurora volcanic field,California-Nevada: oxygen fugacity constraints on the development of andesitic magma

 The Aurora volcanic field, located along the northeastern margin of Mono Lake in the Western Great Basin, has erupted a diverse suite of high-K and shoshonitic lava types, with 48 to 76 wt% SiO₂, over the last 3.6 million years. There is no correlation between the age and composition of the lavas. Three-quarters of the volcanic field consists of evolved (<4 wt% MgO) basaltic andesite and andesite lava cones and flows, the majority of which contain sparse, euhedral phenocrysts that are normally zoned; there is no evidence of mixed, hybrid magmas. The average eruption rate over this time period was ∼200 m³/km²/year, which is typical of continental arcs and an order of magnitude lower than that for the slow-spreading mid-Atlantic ridge. All of the Aurora lavas display a trace-element signature common to subduction-related magmas, as exemplified by Ba/Nb ratios between 52 and 151. Pre-eruptive water contents ranged from 1.5 wt% in plagioclase-rich two-pyroxene andesites to ∼6 wt% in a single hornblende lamprophyre and several biotite-hornblende andesites. Calculated oxygen fugacities fall within –0.4 and +2.4 log units of the Ni-NiO buffer. The Aurora potassic suite follows a classic, calc-alkaline trend in a plot of FeO^T/MgO vs SiO₂ and displays linear decreasing trends in FeO^T and TiO₂ with SiO₂ content, suggesting a prominent role for Fe-Ti oxides during differentiation. However, development of the calc-alkaline trend through fractional crystallization of titanomagnetite would have caused the residual liquid to become so depleted in ferric iron that its oxygen fugacity would have fallen several log units below that of the Ni-NiO buffer. Nor can fractionation of hornblende be invoked, since it has the same effect as titanomagnetite in depleting the residual liquid in ferric iron, together with a thermal stability limit that is lower than the eruption temperatures of several andesites (∼1040–1080°C; derived from two-pyroxene thermometry). Unless some progressive oxidation process occurs, fractionation of titanomagnetite or hornblende cannot explain a calc-alkaline trend in which all erupted lavas have oxygen fugacites ≥ the Ni-NiO buffer. In contrast to fractional crystallization, closed-system equilibrium crystallization will produce residual liquids with an oxygen fugacity that is similar to that of the initial melt. However, the eruption of nearly aphryic lavas argues against tapping from a magma chamber during equilibrium crystallization, a process that requires crystals to remain in contact with the liquid. A preferred model involves the accumulation of basaltic magmas at the mantle-crust interface, which solidify and are later remelted during repeated intrusion of basalt. As an end-member case, closed-system equilibrium crystallization of a basalt, followed by equilibrium partial melting of the gabbro will produce a calc-alkaline evolved liquid (namely, high SiO₂ and low FeO^T/MgO) with a relative f _{O 2} (corrected for the effect of changing temperature) that is similar to that of the initial basalt. Differentiation of the Aurora magmas by repeated partial melting of previous underplates in the lower crust rather than by crystal fractionation in large, stable magma chambers is consistent with the low eruption rate at the Aurora volcanic field. Received: 7 July 1995 / Accepted: 19 April 1996     

Ore paragenesis of recent hydrothermal deposits at the Cocos-Nazca plate boundary (Galápagos Rift) at 85‡ 51' and 85‡ 55' W: Complex massive sulfide mineralizations,non-sulfidic mineralizations and mineralized basalts

Complex massive sulfide ores (»black smokers«) as well as mineralized basalts and unconsolidated sediments were sampled during the German research expedition »Garimas 1« in the eastern portions of the Galapagos Rift at 85 50' W longitude. The massive sulfide ores are characterized by a porous, layered, compositional texture with typical colloidal and/or gel textures. Local occurrences of organism remains (worm tubes), which are embedded in the ore, were also found. The composition of the massive sulfide ores exhibits a considerable range, from iron sulfide ores (pyrite, melnikovite-pyrite, marcasite) to copper-rich ores (chalcopyrite), and/or zinc-dominated ores (sphalerite, wurtzite, schalenblende). Additional new types of hydrothermal mineralizations were found, which consist primarily of manganese hydroxide, limonite or Fe-rich smectite (Fe-rich »protosmectite«) together with limonite and opaline silica. Hydrothermally altered basalts may be characterized by significant sulfide contents (primarily pyrite) and, locally, by hematite. Sediments in the vicinity of the »black smokers« frequently contain fragments of complex massive sulfide ores. With increasing distance from the »black smokers« these massive sulfide fragments exhibit complete replacement and pseudomorphism by limonite. Some replacement and pseudomorphism of microorganisms in unconsolidated sediments by limonitic material is also observed. In places, the unconsolidated sediments contain manganese hydroxide mineralizations of hydrothermal origin.

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Zusammenfassung Anlä\lich der deutschen Forschungsfahrt »Garimas 1« wurden im östlichen Teil des Galápagos-Riftes bei 85 50' W vor allem komplexe massive Sulfiderze (»Schwarze Raucher« — »Black Smokers«) gewonnen, daneben au\erdem vererzte Basalte sowie unverf estigte Sedimente. Die massiven Sulfiderze sind durch einen porösen, lagig bis krustigschaligen Aufbau gekennzeichnet und weisen charakteristische Kolloidalgefüge bzw. Geltexturen auf. Stellenweise liegen Organismenreste (z. B. Wurmröhren), eingebettet in Erz, vor. Die Zusammensetzung der jeweiligen massiven Sulfiderze schwankt erheblich und reicht von praktisch nur aus Eisensulfid (Pyrit, Melnikovitpyrit, Markasit) beste-henden Bildungen bis zu solchen, die reich an Kupfersulfid (Kupferkies) und/oder Zinksulfid (Zinkblende, Wurtzit, Schalenblende) sind. Als weitere und neue Typen hydrothermaler Bildungen bzw. von »Schwarzen Rauchern« konnten hydrothermale Mineralisationen nachgewiesen werden, die vorherrschend entweder aus Manganhydroxid, aus Limonit oder aus eisenreichem Montmorillonit (Fe-reicher »Protosmektit«) mit Limonit und amorphem SiO₂ bestehen. Die alterierten Basalte können stellenweise durch eine reichliche Sulfidführung (vor allem Pyrit) ausgezeichnet sein, wobei gelegentlich auch Eisenglanz beibricht. Die Sedimente aus der Nachbarschaft der »Schwarzen Raucher« enthalten öfters zahlreiche Bruchstücke der komplexen Massivsulfiderze. Diese Sulfiderz-Bruchstücke lassen häufig mit zunehmender Entfernung von den »Schwarzen Rauchern« starke bis völlige Verdrängung und Pseudomorphosierung durch Limonit erkennen. Die in diesen Lockersedimenten vorliegenden rezenten Mikroorganismen zeigen stellenweise Verdrängung und Pseudomorphosierung durch Limonit. In den Lockersedimenten treten Ausscheidungen von Manganhydroxid hydrothermaler Herkunft auf.

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Résumé Dans la partie est de la dorsale des Galapagos, par 85 50' de longitude ouest, l'expédition allemande »GARIMAS I« a récolté des minerais sulfurés complexes massifs (minerais de »fumeurs noirs«), ainsi que des basaltes minéralisés et des échantillons de sédiments meubles. Les sulfures massifs présentent un aspect poreux lité à écailleux et montrent des structures colloÏdales caractéristiques. Localement des traces d'organismes (p. ex. des trous de vers) sont inclues dans le minéral. Les sulfures massifs présentent diverses compositions depuis des formations constituées uniquement de sulfures de Fe (pyrite, pyrite »colloÏdale«, marcasite) jusqu'à d'autres qui sont riches en sulfures de cuivre (chalcopyrite) et/ou de zinc (blende, wurtzite, blende zonaire). De plus, d'autres minéralisations hydrothermales, nouvelles dans l'environnement des fumeurs noirs, ont été mises en évidence: elles sont constituées soit d'oxydes et hydroxydes de Mn, soit de limonite ou de montmorillonite riche en fer (»protosmectite« ferreuse) avec de la limonite et silice amorphe. Les basaltes hydrothermalisés peuvent montrer localement une teneur élevée en sulfure (surtout de la pyrite) éventuellement accompagné d'hématite. Les sédiments recueillis au voisinage des fumeurs noirs renferment souvent de nombreux fragments de sulfures complexes massifs. Au fur et à mesure qu'on s'éloigne des fumeurs, ces fragments sont progressivement et mÊme complètement pseudomorphosés en limonite. Cette pseudomorphose limonitique affecte également, par endroits, les micro-organismes récents inclus dans les sédiments meubles. Dans ces derniers ont lieu des formations d'hydroxydes de manganèse d'origine hydrothermale.

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Shock effects on hydrous minerals and implications for carbonaceous meteorites

New infrared absorption spectra, thermo-gravimetric analyses and optical-and scanning electron microscopy of shock-recovered specimens of antigorite serpentine (Mg₃Si₂O₅(OH)₄) from the pressure range between 25 to 59 GPa are reported. The infrared spectra show systematic changes in absorption peaks related to structural and molecular surface absorbed water. H₂O absorption peaks increase at the expense of OH peaks with increasing shock pressure. Changes in SiO bond vibrational modes with increasing shock pressure parallel those seen for other, non-hydrous minerals. Thermogravimetric analyses of shock-recovered samples determine the amount of shock-induced water loss. For samples shocked in vented assemblies, the data define a relation between shock-induced water loss versus shock pressure. Results for samples shocked in sealed assemblies demonstrate a dependence of water loss on shock pressure and target confinement. For the vented assembly samples, a linear relation between shock pressure and both the length of dehydration interval and the effective activation energy for releasing post-shock structural water in antigorite is found. Optical and scanning electron miscroscopy of shocked antigorite reveal a number of textures thought to be unique to shock loading of volatile-bearing minerals. Gas bubbles, which probably are the result of shock-released H₂O appear to be injected into zones of partial melting. This process may produce the vesicular dark veins which are distributed throughout heavily shocked samples. The present observations suggest several criteria which may constrain possible shock histories of the hydrous matrix phases of carbonaceous condrites. A model is proposed for explaining hydrous alteration processes occurring on carbonaceous chondrite parent bodies in the course of their accretion. We speculate that shock loading of hydrous minerals would release and redistribute free water in the regoliths of carbonaceous chondrite parent bodies giving rise to the observed hydrous alterations.     

The Barents Sea impact study (BASIS): methodology and first results

Global and climate changes are subject to scientific, societal and political debates. Recent observational evidence and results of global climate models have identified the circumpolar North as a region particularly susceptible to future climate change. To understand and assess the consequences of these changes for environmental and societal components of the European Arctic, the Barents Sea Impact Study (BASIS), an EU-funded integrated regional impact study (IRIS) has been carried out (ENV4-CT-97-0637). In common with global integrated assessments (IAs), IRISs also take a holistic view on climate change and its impact. Contrary to IAs, however, IRISs adopt a regional to sub-regional spatial scale. BASIS was carried out by an interdisciplinary team of specialists from 13 institutions in 6 countries. Major results pertain to impacts of possible climate change on marine and terrestrial ecosystems, freshwater hydrology, marine trace gas budgets, forestry and fishery. However, in this paper we focus on the major methodological aspects of an IRIS in general and on methods applied in BASIS in particular.     

Species selection and assessment of eco-engineering effects of seedlings for biogeomorphological landscape experiments

Landscape experiments of fluvial environments such as rivers and deltas are often conducted with live seedlings to investigate effects of biogeomorphological interactions on morphology and stratigraphy. However, such experiments have been limited to a single species, usually alfalfa (Medicago sativa), whereas important environments in nature have many different vegetation types and eco-engineering effects. Landscape experimentation would therefore benefit from a larger choice of tested plant species. For the purpose of experimental design our objective was to identify fast-germinating and fast-growing species and determine their sensitivity to flow conditions during and after settling, their maximum growth, hydraulic resistance and added bank strength. We tested germination time and seedling growth rate of 18 candidate species with readily available seeds that are fast growing and occur at waterlines, plus Medicago sativa as a control. We selected five species that germinate and develop within days and measured properties and eco-engineering effects depending on plant age and density, targeting typical experimental conditions of 0–0.3 m/s flow velocity and 0–30 mm water depth. Tested eco-engineering effects include bank strength and flow resistance. We found that Rumex hydrolapathum can represent riparian trees. The much smaller Veronica beccabunga and Lotus pedunculatus can represent grass and saltmarsh species as they grow in dense patches with high flow resistance but are readily erodible. Sorghum bicolor grows into tall, straight shoots, which add significantly to bank strength, but adds little flow resistance and may represent sparse hardwood trees. Medicago sativa also grows densely under water, suggesting a use for mangroves and perhaps peat. In stronger and deeper flows the application of all species changes accordingly. These species can now be used in a range of landscape experiments to investigate combined effects on living landscape patterns and possible facilitation between species. The testing and treatment methodology can be applied to new species and other laboratory conditions. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd. © 2019 The Authors Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd © 2019 The Authors Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.     

Mud volcanoes and gas hydrates in the Anaximander mountains (Eastern Mediterranean Sea)

Detailed multibeam, sedimentological, and geophysical surveys provide ample new data to confirm that the Anaximander Mountains (Eastern Mediterranean) are an important area for active mud volcanism and gas hydrate formation. More than 3000 km of multibeam track length was acquired during two recent missions and 80 gravity and box cores were recovered. Morphology and backscatter data of the study area have better resolution than previous surveys, and very detailed morphology maps have been made of the known targeted mud volcanoes (Amsterdam, Kazan and Kula), especially the Amsterdam “crater” and the related mud breccia flows. Gas hydrates collected repeatedly from a large area of Amsterdam mud volcano at a sub-bottom depth of around 0.3–1.5 m resemble compacted snow and have a rather flaky form. New gas hydrate sites were found at Amsterdam mud volcano, including the mud flow sloping off to the south. Gas hydrates sampled for the first time at Kazan mud volcano are dispersed throughout the core samples deeper than 0.3 m and display a ‘rice’-like appearance. Relative chronology and AMS dating of interbedded pelagic sediments (Late Holocene hemipelagic, sapropel layer S1 and ash layers) within the mud flows indicate that successive eruptions of Kula mud volcano have a periodicity of about 5–10 kyrs. New mud volcanoes identified on the basis of multibeam backscatter intensity were sampled, documented as active and named “Athina” and “Thessaloniki”. Gas hydrates were sampled also in Thessaloniki mud volcano, the shallowest (1264 m) among all the active Mediterranean sites, at the boundary of the gas hydrate stability zone. Biostratigraphical analyses of mud breccia clasts indicated that the source of the subsurface sedimentary sequences consists of Late Cretaceous limestones, Paleocene siliciclastic rocks, Eocene biogenic limestones and Miocene mudstones. Rough estimations of the total capacity of the Anaximander mud volcanoes in methane gas are 2.56–6.40 km³.     

Measurement of a Peak in the Cosmic Microwave Background Power Spectrum from the North American Test Flight of Boomerang

We describe a measurement of the angular power spectrum of anisotropies in the cosmic microwave background (CMB) at scales of 0&fdg;3 to 5 degrees from the North American test flight of the Boomerang experiment. Boomerang is a balloon-borne telescope with a bolometric receiver designed to map CMB anisotropies on a long-duration balloon flight. During a 6 hr test flight of a prototype system in 1997, we mapped more than 200 deg(2) at high Galactic latitudes in two bands centered at 90 and 150 GHz with a resolution of 26&arcmin; and 16&farcm;5 FWHM, respectively. Analysis of the maps gives a power spectrum with a peak at angular scales of 1 degrees with an amplitude 70 μK(CMB).     

Siliceous productivity changes in Gulf of Ancud sediments (42°S, 72°W), southern Chile,over the last ∼150 years

We evaluated changes in siliceous export production and the source of organic matter preserved in sediment core MD07-3109H recovered from the Gulf of Ancud, Chiloé Inner Sea (42°S, 72°W, water column depth: 328 m), southern Chile. We analyzed the abundance of siliceous microfossils (diatoms, silicoflagellates, sponge spicules, Chrysophyte cysts, phytoliths), geochemical proxies (weight percent silicon %Si_OPAL, organic carbon, total nitrogen, C/N molar), and sediment stable isotopes (δ¹³C_org, δ¹⁵N). Chronology based on ²¹⁰Pb and ¹⁴C provided an accumulated age of 144 years at the base of the core.Sediments of core MD07-3109H are predominantly marine in origin, averaging δ¹³C_org=–20.75‰±0.82, δ¹⁵N=8.7±0.35‰, and C/N=8.76±0.36. Marine diatoms compose 94% of the total assemblage of siliceous microfossils. Our record of productivity based on the mass accumulation rates of organic carbon, total nitrogen, Si_OPAL, and total diatoms showed high values between 1863 and 1869 AD followed by a declining trend until 1921 AD, a transition period from 1921 to 1959 AD with fluctuating values, and a clear decreasing pattern from 1960 AD to the present. This marked reduction in productivity was associated with decreased precipitation and Puelo River streamflow (41°S), as well as a warmer and more stratified water column, especially since the 1980s.