Hydrothermally-induced changes in mineralogy and magnetic properties of oxidized A-type granites

The changes in magnetic mineralogy due to the hydrothermal alteration of A-type granitic rocks have been thoroughly\ud investigated in samples fromthe granite of Tana (Corsica, France), and compared with other A-type granites:\ud Meruoca (NE Brazil), Bushveld (South Africa), Mount Scott (Wichita Mountains, Oklahoma, USA) and the\ud stratoid hypersolvus granites of Madagascar. The altered red-colored samples and their non-altered equivalents\ud were magnetically characterized by means of magnetic susceptibility measurements, hysteresis loops, remanent\ud coercivity spectra, and Lowrie test. It is shown that hydrothermalization in magnetite-bearing granites is related\ud to the formation of fine-grained magnetite and hematite, and to coeval depletion in the content of primary lowcoercive\ud coarse-grained magnetite. These mineralogical changes give typical rock magnetic signatures, namely\ud lower susceptibilitymagnitudes and anisotropy degrees, prolateAMS (anisotropy ofmagnetic susceptibility) fabrics\ud and increased coercivities. Optical microscopy and SEM (scanning electronic microscopy) images suggest\ud that the orientation of the secondary magnetic minerals is related to fluid-pathways and micro-fractures formed\ud during the hydrothermal event and therefore may be unrelated to magma emplacement and crystallization fabrics.\ud Changes inmagnetic mineralogy and grain-size distribution have also to be considered for any paleomagnetic\ud and iron isotope studies in granites.INSU-3F (Failles, fractures, flux) 2008 projec

Magnetic Properties of Ferritchromite and Cr‐Magnetite and Monitoring of Cr‐Spinels Alteration in Ultramafic and Mafic Rocks

Spinel is a ubiquitous mineral in mafic/ultramafic rocks. Spinel cores chemistry is extensively used as a petrogenetic proxy while their alteration phases, ferritchromite, and Cr‐magnetite, are used as metamorphic grade indicators. However, the magnetic properties and composition of these phases are still ill‐defined and no consensus exists concerning the metamorphic conditions involved in their formation. Here, we use the magnetic properties of these Cr‐spinel alteration phases, via field‐dependent parameters and observations with a magnetic microscope coupled with mineral chemistry and Mössbauer spectroscopy, to better constrain their composition. We identify Cr‐magnetite by a Curie point of ca. 520°C. We show that it is characterized by an n between 0.1 and 0.2 in the Fe‐Cr spinel formula [Fe2+(Fe1−n Cr n)2 O4], which corresponds to 6–13 wt.% of Cr2O3. The abundance of Cr‐magnetite indicates a strong alteration of Cr‐spinels that could reflect a significant hydrothermal activity rather than a high metamorphism grade. Normalized variation curves of the magnetic susceptibility during heating allow a relative quantification of the contributions of different magnetic phases to the magnetic susceptibility. This highlights a link between ferritchromite destabilization into maghemite at ca. 130°C followed by the destabilization of this maghemite starting at 300°C. We identify specific covariation trends between these two magnetic species characterizing different alteration processes. This study opens the door to magnetic monitoring of the Cr‐spinel alteration state in mafic and ultramafic rocks. It constitutes a new, fast, and weakly destructive way to study the petrological history of both terrestrial and extraterrestrial rocks

Moroccan Anti-Atlas ophiolites: Timing and melting processes in an intra-oceanic arc-back-arc environment

peer reviewe

The role of the arts in a community-based participatory research project

International audienceRecycling metals such as iron and lead appears crucial for sustainable development, but metal recycling often results in poor quality of life for people living near recycling sites. Transdisciplinary approaches, involving researchers from the physical, natural, and social sciences and humanities working with non-academic partners, are now recognized as essential for tackling such a challenging Anthropocene issue. However, collaboration and understanding between partners are often hampered by the specific modes of communication and concepts used by each community. We present here the beneficial role of the arts in a community-based participatory research project addressing the impacts of metal recycling activities in West Africa, particularly in terms of air pollution. In Senegal (Africa), the town of Sébikotane has become a rapidly growing urban center with three recycling plants (steel and lead batteries). The project aims to produce, jointly evaluate, and share knowledge on air quality in this urban area in transition, far from official measurements. The team includes researchers from geosciences, aerology, anthropology, literature, and botany. Artists, the city council, an NGO, and citizens are officially and actively involved in the project as non-academic partners. We will show how the different artistic productions helped to implement and share knowledge along the project, especially the participatory part, and promoted efficient communication between all partners. Forum theater, live sketching, literature, and design provided effective artistic means to translate concepts and share knowledge. The collaboration between artists and scientists facilitated the elaboration of ethical rules governing the project's stance

Participatory science at school: The Black Carbon educational experiment

International audienceBlack carbon (BC), also known as soot, is a fine particulate matter consisting of carbon particles produced from incomplete combustion of carbon-based fuels. BC contributes significantly to the air quality degradation and has various environmental impacts. BC is a short-lived climate agent and contributes to global warming.Measuring BC using citizen science involves simple but precise methods to estimate its presence in the environment. The Black Carbon EDUcational program (BC-EDU) is an initiative supported by the Caliph'air project. Calisph’Air is an outreach activity of the Centre National d’Etudes Spatiales in the frame of the CALIPSO space research program. BC-EDU actively involves middle school teachers and students. The BC-EDU serves as an educational tool, raising students' awareness of pollution issues by studying soot particles in the ambient atmosphere.BC atmospheric concentrations are measured daily using a light absorption technique on air filters thanks to a small aethalometer specifically designed for the students (manufactured by Staneo). The total BC deposit on the air filters is finally analyzed using a thermo-optical method (Sunset OC/EC analyzer).We present the campaigns carried out with 8 middle schools in 2018 and new experimentations on indoor and outdoor BC concentrations in 2024 in France. We evaluate the students and teachers engagement in such a research-led initiative and the impact of crowd-based BC monitoring for understanding BC sources and transport in and around middle schools

Magnétisme environnemental et cartographie des aérosols urbains dans la ville de Paris

International audienc

Cartographie et traçage magnétique de la pollution métallique des sédiments fluviatiles parisiens

International audienc

The Ediacaran sedimentary architecture and carbonate productivity in the Atar cliffs, Adrar, Mauritania: Palaeoenvironments, chemostratigraphy and diagenesis

International audienceThe Neoproterozoic strata of the Taoudeni Basin (West African Craton) display a single glaciogenic deposit, late Cryogenian in age, and included within a stratigraphic-marker ‘Triad' of tillite, carbonate, and bedded chert. This work is focused on the analysis of the sedimentary architecture and episodic carbonate productivity related to its transgressive deglaciation recorded in the Atar cliffs of Adrar, Mauritania. It offers a joint chemo-, event-, and sequence-stratigraphic analysis that allows us to control problems of stratigraphic completeness and time resolution, necessary for regional and worldwide stratigraphic correlation. In the Atar cliffs, the ‘cap carbonate sequence' is underlain by a composite unconformity that comprises the Pan-African-I discordance and an inherited palaeorelief associated with the glaciation. The transgressive sequence began with accumulation of a sheet-like apron onlapped by a toe-of-slope to basin transition with (cap) carbonates, bedded cherts and distal turbidites (Tichilît-el-Beïda shales). The carbonate strata of the Atar ‘cap carbonate sequence' have recorded both dolomitization and dedolomitization processes destroying the original cement textures. Carbon isotope values from the sheet-like apron range from 0 to −0.6‰, were directly controlled by the reworking of the underlying Atar strata, and do not reflect primary variations in seawater composition. The general pattern of δ13C fluctuations within the overlying incomplete ‘cap carbonate sequence' is consistent with other post-late Cryogenian strata, except in the initial shift from slightly to moderately negative values recognized elsewhere that is not preserved in the study area. Carbon isotope values in the preserved upper part of the cap carbonate record a pronounced rise in δ13C from −6.2‰ to highly enriched values (+3.7‰), followed by the final demise of carbonate productivity. The ‘cap carbonate sequence' is topped by an erosive unconformity succeeded by the recovery of carbonate accumulation represented by an onlapping dolostone deposited in coastal to foreshore environments. There, the carbon isotope values reflect the return to moderately negative values ranging from −0.9 to −2.0‰. The palaeogeographic development of a palaeorelief associated with the end of glacial conditions and deposition of the onlapping Atar cap carbonate is related to the presence of interbedded felsic bentonites and phenocrysts, and reflects the intermittent influence of a neighbouring explosive volcanism