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

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

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

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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

Deciphering the impact of diagenesis overprint on negative δ<SUP>13</SUP>C excursions using rock magnetism: Case study of Ediacaran carbonates, Yangjiaping section, South China

International audienceRock magnetism is used here to investigate the genesis of one of the puzzling negative carbon isotopic excursions of the Neoproterozoic in the Yangtze platform (South China). A detailed characterization of the magnetic mineralogy, which includes low-temperature and high-field magnetometry and classical magnetic measurement (ARM, IRM, susceptibility), was therefore performed along upper Doushantuo and lower Dengying Formations outcropping in the Yangjiaping section. The derived magnetic parameters show variations that can be interpreted as variations in magnetic grains size and in oxide contents. They show that the magnetic content is significantly reduced in samples presenting negative δ13Ccalcite values. We interpret this as a result of magnetite dissolution and secondary carbonate precipitation during early diagenesis bacterial sulfate reduction. Combined with C and O isotopic data, paleomagnetic techniques thus show that the upper Doushantuo-lower Dengying negative excursion of the Yangjiaping section is largely due to diagenesis, although the preservation of a genuine δ13C excursion of lower magnitude from +7‰ down to 0‰, instead of down to -9‰ as usually considered, cannot be ruled out. A corrected δ13Ccarbonate chemostratigraphic curve is therefore proposed. The unambiguous identification of a strong diagenetic component for this excursion casts doubts on the primary nature of other potentially time equivalent negative excursions of the Yangtze platform and thus to its correlation to negative excursions in other cratons (i.e. Shuram excursion). More generally, this study illustrates the potential of magnetic mineralogy characterization, a low cost, time efficient and non-destructive technique, as screening tool for diagenetic overprints of δ13C and δ18O

Cartographie de la pollution métallique des sédiments fluviatiles parisiens par analyse magnétique

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Environmental magnetism and magnetic mapping of urban metallic pollution (Paris, France)

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