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

Hodel, Florent and Macouin, Mélina and Trindade, Ricardo Ivan Ferrreira and Araujo, Jefferson F.D.F. and Respaud, Marc and Meunier, Jean-François and Cassayre, Laurent and Rousse, Sonia and Drigo, Loïc and Schorne Pinto, Juliano Magnetic Properties of Ferritchromite and Cr‐Magnetite and Monitoring of Cr‐Spinels Alteration in Ultramafic and Mafic Rocks. (2020) Geochemistry, Geophysics, Geosystems, 21 (11). ISSN 1525-2027

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Official URL: https://doi.org/10.1029/2020GC009227

Abstract

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.

Item Type:Article
HAL Id:hal-03116531
Audience (journal):International peer-reviewed journal
Uncontrolled Keywords:
Institution:French research institutions > Centre National d'Études Spatiales - CNES (FRANCE)
French research institutions > Centre National de la Recherche Scientifique - CNRS (FRANCE)
Université de Toulouse > Institut National Polytechnique de Toulouse - Toulouse INP (FRANCE)
French research institutions > Institut de Recherche pour le Développement - IRD (FRANCE)
Université de Toulouse > Université Toulouse III - Paul Sabatier - UT3 (FRANCE)
Other partners > Pontifícia Universidade Católica do Rio de Janeiro - PUC (BRAZIL)
Other partners > Université Bourgogne Franche-Comté - UBFC (FRANCE)
Other partners > Universidade de São Paulo - USP (BRAZIL)
Laboratory name:
Funders:
Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP, Brazil) - nstitut national des scienc-es de l'Univers (INSU, CNRS, France) - nseignement supérieur et de la Recherche - MENESR, (France)
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Deposited On:20 Jan 2021 12:32

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