Tracing fluids during medium to ultra-high pressure metamorphism: insights by combined in situ oxygen isotopes and trace element analysis

Fluids are an essential component of tectonic and metamorphic processes such as subduction and crustal anatexis. Fluids are elusive to trace as they commonly escape high-pressure rocks. This study uses oxygen isotopes to identify fluid influxes in metamorphic rocks and tie them to geologic events, measuring δ18O in situ by ion microprobe in garnet, zircon, apatite, monazite and lawsonite. New method developments are presented for δ18O analyses by Sensitive High Resolution Ion MicroProbe: (i) in apatite, a precision of 0.2‰ (1σ) is achieved; diffusion modelling shows that apatite is expected to preserve oxygen isotope signatures from 400-450°C and below; (ii) a matrix correction scheme is derived for monazite oxygen isotope measurement, allowing a precision of 0.35‰ (1σ); (iii) rutile oxygen isotope measurements yield major orientation effects. Fluid pulses generated by prograde dehydration reactions are investigated in the ultra-high-pressure Dora Maira whiteschists, Italy. Rare-earth-element abundances indicate prograde monazite and zircon growth (pre-garnet 34.5±0.7 Ma, 6.4‰ and syn-garnet at 34.9±0.4 Ma, 6.2 to 6.7‰), which are linked to dehydration reactions using thermodynamic modelling. This allows drawing a P-T-time-fluid path that implies that (i) prograde subduction from 25 to 45 kbar occurred within a couple of My (minimum burial rate of 2 cm/yr); (ii) high-pressure fluids were of internal origin and metasomatism likely have occurred at a rifting phase before subduction. Four phases of fluid circulations are identified in the high-pressure low-temperature lawsonite eclogites and blueschists of the Tavşanlı zone, Turkey. The Halilbağı unit is an oceanic complex containing various sediments and serpentinite together with 222±5 Ma MORB and 123±3 Ma OIB lithologies (zircon U-Pb, whole-rock major and trace elements). The sequence was thoroughly altered and mechanically mixed at the seafloor and in the accretionary prism, leading to overall high whole-rock δ18O of 11.0-17.0‰ for metabasites. Garnet, apatite and lawsonite are zoned in δ18O in samples across the unit, with contrasts of 7‰ in a MORB eclogite (garnet core: 6.3‰, rim: 13‰) and 3‰ in an impure quartzite (apatite core: 19.5‰, rim: 17‰). Petrographic and trace element evidence allow identifying localised prograde-peak fluid influx, and homogenisation of oxygen isotopes and Sr/Pb at the start of retrogression by pervasive fluid circulation across the unit. Heavy δ18O signatures (WR: 11.8 to 13.6‰) were measured in Eoarchaean metasediments from the Isua supracrustal belt, Greenland. The sources of the sediments were mantle-derived boninites (mafic component) and andesites (felsic component, detrital/volcanic zircons dated at 3709 Ma,δ18O 5.3‰). Three garnet growth zones record high δ18O (9 to 10‰), in equilibrium with the whole-rock. Rare-earth-element and petrographic evidence allow identifying a higher-pressure signature in the high- δ18O garnet, which can be linked to a 3690-3660 Ma tectonic event. The elevated δ18O signature in the metasediments thus originated from surficial processes (e.g. weathering) before 3690Ma. Melting of such heavy-δ18O amphibolite-facies sediments could represent a source for early Archean high-δ18O magmas and zircons. The combination of oxygen isotope and trace element microanalysis in zoned minerals proves a powerful tool for uncovering multistage minor and major fluid infiltration events in metamorphic rocks

Les mesures de la ville

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L'étude du lien entre l'espace social et l'espace construit. Réflexion à propos de la refonte du centre de Lyon au XIXe siècle

L'histoire de l'espace construit des villes et sa compréhension sont un champ de recherche peu exploré. Cet article aborde la question pour le centre de Lyon au XIXe siècle. Le rapport entre la forme urbaine et architecturale produite et l'espace social est analysé. Des directions de recherche sur ce rapport sont esquissées.The history of the built space of the towns and its understanding are a poorly explored research field. This paper deals with this matter by the study of the center of Lyons in the XIXth century. The relation between the new architectural and urban shape and the social space is analysed. Further researches on this relation are drawn

Dating prograde fluid pulses during subduction by in situ U–Pb and oxygen isotope analysis

Keywords High-pressure fluids · Whiteschists · U–Pb dating · Oxygen isotopes · Ion microprobe · Metasomatism Introduction The subduction of crustal material to mantle depths and its chemical modification during burial and exhumation contribute to element recycling in the mantle and the formation of new crust through arc magmatism. Crustal rocks that Abstract The Dora-Maira whiteschists derive from metasomatically altered granites that experienced ultrahighpressure metamorphism at ~750 °C and 40 kbar during the Alpine orogeny. In order to investigate the P–T–time–fluid evolution of the whiteschists, we obtained U–Pb ages from zircon and monazite and combined those with trace element composition and oxygen isotopes of the accessory minerals and coexisting garnet. Zircon cores are the only remnants of the granitic protolith and still preserve a Permian age, magmatic trace element compositions and δ18O of ~10 ‰. Thermodynamic modelling of Si-rich and Si-poor whiteschist compositions shows that there are two main fluid pulses during prograde subduction between 20 and 40 kbar. In Si-poor samples, the breakdown of chlorite to garnet + fluid occurs at ~22 kbar. A first zircon rim directly overgrowing the cores has inclusions of prograde phlogopite and HREE-enriched patterns indicating zircon growth at the onset of garnet formation. A second main fluid pulse is documented close to peak metamorphic conditions in both Si-rich and Si-poor whiteschist when talc + kyanite react to garnet + coesite + fluid. A second metamorphic overgrowth on zircon with HREE depletion was observed in the Si-poor whiteschists, whereas a single metamorphic overgrowth capturing phengite and talc inclusions was observed in the Si-rich whiteschists. Garnet rims, zircon rims and monazite are in chemical and isotopic equilibrium for oxygen, demonstrating that they all formed at peak metamorphism at 35 Ma as constrained by the age of monazite (34.7 ± 0.4 Ma) and zircon rims (35.1 ± 0.8 Ma). The prograde zircon rim in Si-poor whiteschists has an age that is within error indistinguishable from the age of peak metamorphic conditions, consistent with a minimum rate of subduction of 2 cm/year for the Dora-Maira unit. Oxygen isotope values for zircon rims, monazite and garnet are equal within error at 6.4 ± 0.4 ‰, which is in line with closed-system equilibrium fractionation during prograde to peak temperatures. The resulting equilibrium Δ18Ozircon-monazite at 700 ± 20 °C is 0.1 ± 0.7 ‰. The in situ oxygen isotope data argue against an externally derived input of fluids into the whiteschists. Instead, fluidassisted zircon and monazite recrystallisation can be linked to internal dehydration reactions during prograde subduction. We propose that the major metasomatic event affecting the granite protolith was related to hydrothermal seafloor alteration post-dating Jurassic rifting, well before the onset of Alpine subduction

Lyon, das fontes escritas ao SIG histórico – método e exemplos de aplicação

The Lyons historical GIS was developed from the end of the 1990s whit the goal to reach a new un­derstanding of the transformation of urban and social spaces by spatializing data at the buildings scale. We thought that by such a jump in precision of a factor 100, from a subdivision by 36 quarters to one by 3,500 buildings in the modern period, the mapping would lead to new perspectives and results in urban history. This involved working through two centuries of archival records, taxes registers, censuses, building permits, property changes in order to create critically researched data bases followed by vector GIS layers. It was necessary develop a method to reveal the implicit spatiality of these written sources, to establish a mapping topography, allowed by a careful and geometrically checked reconstruction of the city’s plot pattern together with its variation befo­re 1800, and to take into account the space trans­formation, studied at the actual scale of indivi­dual investments, owing to the building permits checked with the still extant constructions, and the administrative record.The Lyons historical GIS was developed from the end of the 1990s whit the goal to reach a new un­derstanding of the transformation of urban and social spaces by spatializing data at the buildings scale. We thought that by such a jump in precision of a factor 100, from a subdivision by 36 quarters to one by 3,500 buildings in the modern period, the mapping would lead to new perspectives and results in urban history. This involved working through two centuries of archival records, taxes registers, censuses, building permits, property changes in order to create critically researched data bases followed by vector GIS layers. It was necessary develop a method to reveal the implicit spatiality of these written sources, to establish a mapping topography, allowed by a careful and geometrically checked reconstruction of the city’s plot pattern together with its variation befo­re 1800, and to take into account the space trans­formation, studied at the actual scale of indivi­dual investments, owing to the building permits checked with the still extant constructions, and the administrative record.O SIG histórico de Lyon foi desenvol­vido a partir do final da década de 1990, com o objetivo de chegar a um novo entendimento da transformação dos espaços urbanos e sociais pela espacialização dos dados à escala dos edifí­cios. Pensamos que, por tal salto na precisão de um fator 100, de uma subdivisão por 36 bairros para uma por 3.500 edifícios no período moderno, o mapeamento levaria a novas perspectivas e no­vos resultados a história urbana. Isso envolveu o trabalho ao longo de dois séculos de documentos de arquivo, registros de impostos, censos, licenças de construção, alterações de propriedade, a fim de criar bases de dados criticamente pesquisados, seguidos por layers de SIG vetorizados. Foi neces­sário desenvolver um método para revelar a espa­cialidade implícita dessas fontes escritas, estabe­lecer um mapeamento da topografia, permitido pela reconstrução cuidadosa do padrão de trama da cidade, juntamente com sua variação antes de 1800, e levar em conta a transformação do espaço, estudado na escala real de investimentos indivi­duais, através das licenças de construção verifi­cadas com as construções ainda existentes, e o re­gistro administrativo.

Des unités pertinentes pour mesurer la ville concrète

L’espace urbain peut être décomposé en unités permettant une opération de quantification. Ces unités doivent avoir un sens univoque distinct de celles généralement utilisées comme l’immeuble, le type, voire la parcelle. De plus, elles doivent pouvoir être identifiables sur le terrain. Parmi ces unités, l’« unité de construction » et l’« aménagement urbain », aux échelles de l’architecture et de l’urbanisme, ont une correspondance étroite dans les sources écrites, en particulier, pour la première, avec les autorisations de construire. Cet article définit ces unités et expose leur mise en œuvre dans les cas de Rouen et de Lyon. dans cette dernière ville, un SIG a été développé à partir de la série des actes d’alignements entre 1617 et 1763. La décomposition en « aménagements urbains » permet une compréhension fine de la formation de l’espace urbain lyonnais.Pertinents Units of Measure of the concrete City. Urban space can be separated in units allowing quantification. To do so, the units must have a univocal meaning, which is not the case with « building », « type » or even « property-plot », generally used to describe the urban fabric. They must be also easy to recognize in the field. Two of them, « building-unit » and « urban-development », at different scales, have direct correspondents in written records, especially the former in the building permits. This paper aims to define these units and exposes their adequacy to studying Rouen and Lyon. A GIS has been developed on Lyon to locate precisely the building permits from 1617 to 1763 on a reconstructed map of the city, and past urban developments on the present day plan

La formation à la mixité scolaire à la mesure du genre

La mixité scolaire est inaccomplie car elle a été installée de manière pragmatique et doit être (re)fondée dans une perspective d’égalité - altérité. La nécessaire conscientisation du genre et de la socialisation différenciée garçons - filles implique l’identité personnelle et professionnelle des enseignants dans le cadre d’une formation transdisciplinaire à l’éduquer - évaluer - enseigner - orienter qui vise à réduire l’asymétrie.Coeducation has only been partially implemented, because the concern has been purely practical ; it must be (re)established to take into account equality and difference. The necessary gender and male-female socialisation awareness implicates personal and professional engagement of teachers, via a transdisciplinary training course to educate, evaluate, teach and guide, in order to reduce the unbalance

Ordre textuel et ordre spatial à Lyon à l’époque moderne

Les données des sources écrites comportent très souvent des indications spatiales implicites, notamment dans les rôles de taxes, qui ont nécessité des visites de terrain pour constituer l’information. La spatialisation de ces données implique de comprendre la logique de visite des agents chargés des rôles, et donc l’organisation administrative des enquêtes et de la gestion de l’espace dans l’Ancien Régime. Elle implique aussi une confrontation aux données spatiales explicites comme les plans de censive et les immeubles eux-mêmes. Une fois réalisé, ce travail permet de localiser, grâce à un système d’adressage des propriétaires, toutes les informations comportant une mention minimale de localisation ou placées dans des séquences d’enregistrement correspondant à des séquences de visite. De nombreuses sources deviennent alors mobilisables non plus de façon généralisée, à l’échelle d’un quartier ou d’une rue, mais à celle de l’immeuble, soit avec un grain de précision très fin ouvrant à des questionnements nouveaux.Written sources often include implicit spatial indications, notably in the tax rolls that implied a visit on the ground to collect the needed information. It is possible, following a method developed here, to locate this information at the scale of its registering, generally the property unit or a house. This implies a good understanding of the visit practiced by local civil officers and of the administration of the city regarding the quarters (pennonages in Lyon). This needs also a close examination of the contemporary property map production and of the subsisting buildings, historical sources themselves, explicitly spatial. Once realized, this location, a sort of addressing system based on the known proprietors of the buildings, makes it possible to locate all information bearing a minimal spatial component or part of a visit sequence. Many sources are usable in that new way and one has no more to do as often in the past, i.e. generalize at the scale of a district or even a street, but can study features at their finest scale, the building. This new precision opens new questions

Les mesures de la ville

La ville est un objet historique complexe. D’un point de vue empirique ou théorique, ses réalités changent au fil du temps et des sociétés, tout autant que ses définitions et ses appropriations. À travers sa mesure, la ville abrite un enjeu multiforme pour les sociétés anciennes, que ce soit pour son gouvernement ou pour son économie ordinaire, sans parler d’un enjeu de qualification par les historiens, archéologues, géographes et autres chercheurs en sciences sociales qui s’en emparent aujou..

Les mesures de la ville

La ville est un objet historique complexe. D’un point de vue empirique ou théorique, ses réalités changent au fil du temps et des sociétés, tout autant que ses définitions et ses appropriations. À travers sa mesure, la ville abrite un enjeu multiforme pour les sociétés anciennes, que ce soit pour son gouvernement ou pour son économie ordinaire, sans parler d’un enjeu de qualification par les historiens, archéologues, géographes et autres chercheurs en sciences sociales qui s’en emparent aujou..