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

Autor(es)

Hodel, F. ^{[1, 2]} ; Macouin, M. ^[1] ; Trindade, R. I. F. ^[3] ; Araujo, J. F. D. F. ^[4] ; Respaud, M. ^[5] ; Meunier, J. F. ^[6] ; Cassayre, L. ^[7] ; Rousse, S. ^[1] ; Drigo, L. ^[1] ; Schorne-Pinto, J. ^{[7, 8]}

Afiliação do(s) autor(es)

^[1] Univ Toulouse III Paul Sabatier, UPS, CNRS, OMP, Geosci Environm Toulouse, IRD, Toulouse - France

^[2] CNRS UFC, Lab ChronoEnvironm, UMR6249, Besancon - France

^[3] Univ Sao Paulo, Inst Astron Geofis & Ciencias Atmosfer, Sao Paulo - Brazil

^[4] Pontifical Catholic Univ Rio de Janeiro, Dept Phys, Rio De Janeiro - Brazil

^[5] Univ Toulouse, CNRS, CEMES, UPR 8011, Toulouse - France

^[6] Univ Toulouse, CNRS, LCC CNRS, Toulouse - France

^[7] Univ Toulouse, Lab Genie Chim, UPS, INPT, CNRS, Toulouse - France

^[8] Univ South Carolina, Gen Atom Ctr Nucl Technol, Columbia, SC 29208 - USA

• Tipo de documento: Artigo Científico
• Fonte: GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS; v. 21, n. 11 NOV 2020.
• Citações Web of Science: 0

Resumo

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 Mossbauer spectroscopy, to better constrain their composition. We identify Cr-magnetite by a Curie point of ca. 520 degrees C. We show that it is characterized by an n between 0.1 and 0.2 in the Fe-Cr spinel formula {[}Fe2+(Fe1-nCrn)(2)O-4], 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 degrees C followed by the destabilization of this maghemite starting at 300 degrees 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. (AU)

• Processo FAPESP: 16/06114-6 - O Sistema Terra e a evolução da vida durante o Neoproterozoico
• Beneficiário: Ricardo Ivan Ferreira da Trindade
• Modalidade de apoio: Auxílio à Pesquisa - Temático