Chemical evolution of metamorphic fluids in the Central Alps,Switzerland: insight from LA‐ICPMS analysis of fluid inclusions

The chemical evolution of fluids in Alpine fissure veins (open cavities with large free‐standing crystals) has been studied by combination of fluid inclusion petrography, microthermometry, LA‐ICPMS microanalysis, and thermodynamic modeling. The quartz vein systems cover a metamorphic cross section through the Central Alps (Switzerland), ranging from subgreenschist‐ to amphibolite‐facies conditions. Fluid compositions change from aqueous inclusions in subgreenschist‐ and greenschist‐facies rocks to aqueous–carbonic inclusions in amphibolite‐facies rocks. The fluid composition is constant for each vein, across several fluid inclusion generations that record the growth history of the quartz crystals. Chemical solute geothermometry, fluid inclusion isochores, and constraints from fluid–mineral equilibria modeling were used to reconstruct the pressure–temperature conditions of the Alpine fissure veins and to compare them with the metamorphic path of their host rocks. The data demonstrate that fluids in the Aar massif were trapped close to the metamorphic peak whereas the fluids in the Penninic nappes record early cooling, consistent with retrograde alteration. The good agreement between the fluid–mineral equilibria modeling and observed fluid compositions and host‐rock mineralogy suggests that the fluid inclusions were entrapped under rock‐buffered conditions. The molar Cl/Br ratios of the fluid inclusions are below the seawater value and would require unrealistically high degrees of evaporation and subsequent dilution if they were derived from seawater. The halogen data may thus be better explained by interaction between metamorphic fluids and organic matter or graphite in metasedimentary rocks. The volatile content (CO₂, sulfur) in the fluid inclusions increases systematically as function of the metamorphic grade, suggesting that the fluids have been produced by prograde devolatilization reactions. Only the fluids in the highest grade rocks were partly modified by retrograde fluid–rock interactions, and all major element compositions reflect equilibration with the local host rocks during the earliest stages of postmetamorphic uplift.     

CANDID DISCUSSIONS ABOUT FIELDWORK: The New Ethnographer pre-field training workshop

In this conference review, the author reviews the first The New Ethnographer (TNE) pre-fieldwork training workshop for doctoral students that took place over two days at the London School of Economics and Political Science in February 2020. TNE's approach to pre-fieldwork training runs counter to institutionalized approaches of risk management: it acknowledges fieldwork's inherent messiness and ethnographers’ emotional and embodied entanglements, while inviting participants to rethink the language of ‘risk’ and ‘danger’. TNE's contribution lies in facilitating frank and experimental conversations between early career researchers, first via its digital platform, and now through scenario-based learning in the classroom.     

Geochemical impacts to prehistoric iron-rich siliceous artifacts in the nearshore coastal zone

In the Middle Atlantic region of the United States, landscapes drowned by sea level rise contain scores of prehistoric archaeological sites. These sites provide archaeologists with a rare opportunity to investigate various geologic processes. During the transition from a terrestrial to an offshore setting, the materials associated with an archaeological site are exposed to a series of geochemical processes inherent to the formation of tidal marsh. The duration of the geochemical exposure to tidal marsh is largely dependent on the rate of marine transgression. Here we describe the stages associated with the sulfidization and sulfuricization scheme and the impact to iron-rich lithic artifacts from naturally drowned archaeological sites. Sulfidization and sulfuricization should also impact buried archaeological materials as a result of anthropogenic dredge spoil dumping and the creation of man-made tidal marshes along modern coastlines. Our results indicate that the surfaces of an iron-rich artifact, as well as its interior are visually and geochemically altered by prolonged exposure to the anaerobic conditions of a tidal marsh. Not only should researchers be cautious about making lithic material identification on artifacts found within coastal tidal marsh areas, but museum curators should be aware of the damaging impact of long-term aerobic storage. The geochemical tidal marsh scheme that has altered or corroded iron-rich lithic artifacts in the nearshore zone is an expression of a process that has impacted numerous earlier prehistoric sites currently located on the continental shelf or beneath the coastal estuaries around the world.     

Gegen die Resignation Kenianische Jugendclubs gehen in die Offensive

Abstrakt Obdachlosigkeit, Kriminalit?t, Hunger, Krankheiten, verunreinigtes Wasser und Seuchen sind nur einige Alltagsprobleme fast einer Milliarde Slumbewohner weltweit. Unter ihnen sind zahlreiche Kinder und Jugendliche, die in den Slumgebieten scheinbar ohne Zukunftsperspektiven aufwachsen. Viele haben ihre Familie durch AIDS verloren oder sind selbst infiziert. Ohne Arbeit oder Ausbildungsplatz verbringen sie ihren Alltag auf den Stra?en der Slums. Die extremen Lebensbedingungen erh?hen das gesellschaftliche Konfliktpotenzial erheblich; nicht selten ist der Einstieg der Kinder und Jugendlichen in die Kriminalit?t die Folge. Auch in Kenia ist die Lage auf den ersten Blick trostlos: Der Staat zieht sich aus seiner Verantwortung mehr und mehr zurück, Betroffene sind sich selbst überlassen. Zu ergründen, wie die Jugendlichen selbst L?sungen für ihre Probleme suchen und welche Perspektiven sich ihnen bieten, war Ziel einer Studie von 15 Studierenden der Universit?t Trier im Jahr 2004 unter der Leitung von Dr. Johannes Michael Nebe. Im Rahmen einer mehrw?chigen Feldarbeit in verschiedenen Slumgebieten Nairobis – n?mlich in Kibera, Dandora, Korogocho, Kariobangi und Mathare – konnten die Studierenden ihr literaturbezogenes Hintergrundwissen über die Probleme in Slumgebieten praktisch vertiefen. Unterstützt von der Deutschen Stiftung für Weltbev?lkerung trafen sie vor Ort auf selbst organisierte Jugendclubs und „Community Based Organisation“ (CBOs). Letztere werden von der lokalen Bev?lkerung einer Gemeinde selbst gegründet, organisieren und verwalten sich selbst. Sie zielen dabei auf die Befriedigung von speziellen Bedürfnissen der Bev?lkerung ab. Ihre Arbeit ist nicht an Profit, sondern an der Entwicklung der Gemeinde orientiert (UN-Habitat 2004; WELTBANK 2004). Die ersten CBOs wurden Anfang der 90er Jahre gegründet.