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Garnet xenocrysts from kimberlites provide unique insights into the composition, structure and evolution of the subcontinental lithospheric mantle (SCLM). For example, different metasomatic events in the SCLM are reflected in compositional differences between garnet xenocrysts. As mantle metasomatism largely controls the physical and chemical properties of the SCLM, it exerts first order control over the genesis of kimberlitic magmas and diamond formation. However, dating mantle lithologies and processes is complicated by high ambient temperatures that allow the equilibration of most isotopic systems up to the time of kimberlite eruption. As a consequence, the temporal connection between metasomatic events in the mantle and kimberlite genesis is commonly ambiguous.
In this study, we applied LA-ICPMS U-Pb dating to 43 harzburgitic, lherzolithic and megacrystic garnet xenocrysts from the ~376 Ma diamondiferous V. Grib kimberlite, Russia, in order to investigate the link between different types of mantle metasomatism and kimberlite genesis.
Our results indicate that, with two possible exceptions, only harzburgitic garnet overlaps in age with the kimberlite eruption, whereas lherzolitic and megacrystic garnet crystals are ~20 to 130 million years older. Furthermore, garnet U-Pb ages and Ni-in-garnet temperatures of ~820 to 1200 °C do not correlate. This, and the high closure temperature of U-Pb in garnet (≥900 °C) suggests that the garnet U-Pb ages indeed reflect metasomatic events in the SCLM. However, the U-Pb ages could also reflect cooling ages. In this case, the metasomatic events recorded in the garnet crystals must still have occurred up to ~130 million years prior to the eruption of the V. Grib kimberlite.
These findings have far-reaching implications for the genesis of (diamondiferous) kimberlites, as they clearly show that the time lag between metasomatic events in the SCLM, as recorded in kimberlitic garnet xenocrysts, and kimberlite eruption may extend to tens of millions of years.
The oxidation state of sulfur in slab fluids is controversial, with both dominantly oxidized and reduced species proposed. Here we use in situ X-ray absorption spectroscopy analysis of sulfur-in-apatite to monitor changes in the oxidation state of sulfur during high-P metasomatism by slab fluids in the subduction channel. Our samples include a 73 cm continuous transect of reaction zones between a metagabbroic eclogite block and serpentinite matrix from a mélange zone on the island of Syros, Greece. The block core consists of garnet, omphacite, phengite, paragonite, epidote-clinozoisite, and rutile. In this region, apatite is only observed as elongate inclusions in omphacite cores. From the core outwards micas are increasingly replaced by epidote-clinozoisite, garnets are smaller and more frequent, pyrite + bornite is observed as inclusions in recrystallized omphacite, and apatite is increasingly abundant in the matrix and inclusions in garnet. A major transition at 48 cm separates an assemblage of Ca-Na amphibole, omphacite, chlorite, pyrite, and apatite from the inner garnet-bearing eclogite assemblages. Omphacite disappears from the assemblage at ~56 cm and amphibole compositions sharply transition to tremolite at 59 cm. Finally, the assemblage tremolite + talc + pyrite is observed after ~70 cm.Apatites in the eclogite assemblages exclusively display S6+ peaks in their absorption spectra. This includes apatite inclusions in omphacite in the least altered lithology, as well as matrix apatite and isolated apatite inclusions in garnet in the outermost metasomatized eclogite zone. In the intermediate pyrite-rich (~1-5 vol %) amphibole + omphacite + chlorite zone, apatite displays a strong S1- absorption peak in most grains, with rare analyses showing mixed S1- and S6+. Finally, apatite in the outermost tremolite-bearing assemblages only displays a S6+ peak. The pyrite-rich zone at 48 cm occurs at the initial interface between the serpentinite matrix and eclogite block, characterized by a dramatic decrease in Na content and Mg#. Our data suggest that reduction of S6+ in infiltrating fluids to S1- in pyrite became focused as Fe diffused across the steep Mg# gradient, resulting in pyrite precipitation. In contrast, S reduction in the Mg-rich tremolite-dominant portions of the transect was limited by a lack of Fe, resulting in low modes of pyrite and fluid buffered S6+ in apatite. Finally, S6+-bearing apatite is also observed in reaction zone lithologies from elsewhere on Syros, suggesting our observations are not isolated.Two important conclusions are drawn from these data and observations: (1) In the case of Syros, slab fluids at eclogite-facies conditions carried oxidized S6+, and (2) The interaction of these fluids with eclogites composed of ferrous-Fe silicates resulted in extensive sulfide precipitation.
Sulfur in the slab: a sulfur-isotopes and thermodynamic-modeling perspective from exhumed terranes
(2022)
Sulfur is a key element in the subduction zone-volcanic arc system; however, the mechanism(s) that recycle sulfur from the slab into the overlying volcanic arc are debated. Here we summarize recent advances in quantifying this component of the deep sulfur cycle. First, primary metamorphic or inherited sulfides in oceanic-type eclogites are only rarely observed as inclusions and are typically absent from the rock matrix. Additionally, sulfides are relatively common in rocks metasomatized at the slab-mantle interface by slab-derived fluids during exhumation. Combined, these two observations suggest that sulfur loss from subducted mafic crust is relatively efficient. Thermodynamic modeling in Perple_X using the Holland and Powell (2011) database combined with the Deep Earth Water model suggests that the efficiency and speciation of sulfur loss varies depending on the degree of seafloor alteration prior to subduction and the geothermal gradient of the slab. In relatively cold subduction zones, such as Honshu, slab-fluids derived from subducted mafic crust are predicted to exhibit elevated concentrations of HSO4-, SO42-, HSO3-, and CaSO4(aq), whereas hot subduction zones, such as Cascadia, are predicted to produce slab fluids enriched in HS- and H2S at lower pressures. The oxidation of sulfur expelled from subducted pyrite is balanced by the reduction of Fe3+ to Fe2+, consistent with the low Fe3+/SFe of exhumed eclogites relative to blueschists and altered oceanic crust. Where oxidized S-bearing fluids are produced, they are anticipated to interact with more reduced rocks at the slab-mantle interface and within the mantle wedge, resulting in sulfide precipitation and significant isotopic fractionation. The δ34S values of slab fluids are estimated to fall between -11 and +8 ‰. Rayleigh fractionation during progressive fluid-rock interaction results in fractionations of tens of per mil as oxidized species are depleted and sulfides are precipitated, resulting in δ34S values of sulfides that easily span the -21.7 to +13.9 ‰ range observed in metasomatic sulfides in exhumed high-pressure rocks. However, in subduction zones where reduced species prevail, the S isotopic signature of slab fluids is expected to reflect their source and will exhibit a narrower range in δ34S values. As a result, the δ34S values measured in arc magmas may not always be a reliable indicator of the contribution of different components of the slab, such as sediments vs. AOC. Additionally, the impact of S recycling on the oxygen fugacity of arc magmas is expected to vary both spatially and temporally throughout Earth history.
The Ceboruco is a 2280 m high stratovolcano located in Nayarit State, Mexico. Despite its last eruption which occurred in 1870, it is the most active volcano in the area, showing volcanicearthquake activity together with ongoing vapor emissions. The magnetotelluric survey was carried out in November 2016. It was part of a geothermal project (CeMIEGeo-P24) and focused on the determination of the electrical conductivity distribution in the subsurface of the volcano.
The Magnetotelluric Apparent Resistivity Tensor, as introduced by Brown (2016), can be decomposed into an amplitude and a phase tensor. The fundamental physics behind those new tensors were presented in Hering et al. (2019), using canonical models in 1-D (isotropic and anisotropic) and 2-D resistivity environments. Here, the tensors are introduced for a high-quality data set, where their interpretational benefits become very obvious. Additionally, results from an isotropic 3-D inversion are presented and compared to an alternative 3-D anisotropic forward model.
In November 2016, magnetotelluric (MT) data were collected at the Ceboruco Volcano in cooperation with the Centro de Sismología y Volcanología de Occidente (SisVoc, Universidad de Guadalajara, Mexico). The Ceboruco is a 2280 m high stratovolcano, located in Nayarit State, Mexico. It is placed in the central part of the Tepic-Zacoalco Rift (TZR), which constitutes the north-western end of the Trans-Mexican Volcanic Belt. Together with Chapala and Colima (in the Jalisco Block), they form the triple rift system developed as a consequence of the ongoing subduction of the Rivera and Cocos oceanic plates beneath the North American continental crust. Although its last eruption occurred in 1870, it is the most active volcano in the area, showing volcanic-earthquake activity together with ongoing vapor emissions. The survey was part of a geothermal project (CeMIEGeo-P24) and focused on the determination of electrical conductivity properties to characterize the deep structure and the geothermal potential of the Volcano. Frequency dependent magnetotelluric response functions were calculated from 25 broadband MT stations, which covered an area of 10 x 10 km2 including its crater, calderas and foreland. The results were interpreted using anisotropic 3-D forward modelling and isotropic 3-D inversion approaches, considering strong topographical effects. The final resistivity model implies a highly conductive layer, reaching from near-surface to approximately 2 km depth, which might be related to a hydrothermal system. Here, mineralized fluids and clay minerals can cause high conductivities around 1 S/m. For longer periods, the principal axes of the MT response tensors (phase tensor, apparent resistivity tensor) are in good agreement with the strike direction of the underlying rift system. However, they are not rendered by the isotropic inversion. Thus the data suggest an anisotropic electrical conductivity at greater depth with its principal axis determined by the response tensors.
Zum Themenfeld "Diversität und Vielfalt" diskutierten im Rahmen des 8. Treffens des Nachwuchsnetzwerkes "Stadt, Raum, Architektur" Wissenschaftler_innen aus den Sozial-, Geistes- und Raumwissenschaften an den Instituten für Humangeographie, Kulturanthropologie und Europäische Ethnologie der Goethe-Universität Frankfurt am Main am 9. und 10. November 2012. Vor dem Hintergrund aktueller Debatten um die Konzeptualisierung von sowie den praktischen Umgang mit soziokultureller Vielfalt fand ein produktiver Austausch aus den Perspektiven der Stadtplanung, der Architekturwissenschaft sowie der sozial- und kulturwissenschaftlichen Stadt- und Raumforschung statt. Die Ergebnisse dieser interdisziplinären Auseinandersetzung hinsichtlich einer globalen Diskursverschiebung von "Multikulturalismus" zu "Diversität" und der Adaption entsprechender Strategien in Politik, Wirtschaft und Gesellschaft werden in diesem Tagungsbericht anhand theoretischer Ansätze zu "Super-Diversity", Kosmopolitismus und Transnationalismus diskutiert. Empirisch werden insbesondere Fragen zu Standortmarketing, Integrationspolitiken und der Verräumlichung von Diversität sowie konkreter Praktiken der Segregation, Marginalisierung und Aushandlung von Differenz aufgegriffen. Abschließend wird die Frage nach Konflikten und Potenzialen einer "neuen Diversität" aus stadtplanerischer, dekolonialer und poststrukturalistischer Perspektive diskutiert.
The 99th Annual Meeting of the Geologische Vereinigung (GV) and International Conference on Earth Control on Planetary Life and Environment, held in October 2009 at the Geosciences Centre of the Georg-August-Universität Göttingen, brings together researchers from all fields of Earth Sciences and beyond to shape an attractive interdisciplinary program on the geological history of Planet Earth and its control over and interaction with biological evolution, development of habitats, environmental and climate change as well as history and culture of Homo sapiens. This volume contains the abstracts of invited keynote lectures as well as all oral and poster presentations.