Magnetic Properties through the Archean/Paleoproterozoic Transition from the Pilbara Craton, Western Australia: Bio-environmental Implications

Friday, 18 December 2015
Poster Hall (Moscone South)
Aude Isambert1, Julie H Carlut1, Helene Bouquerel1, Ernesto Pecoits1,2, Pascal Philippot1, Emmanuelle Vennin3, Magali Ader1, Christophe Thomazo3, Jean François Buoncristiani3, Frank Baton1, Anne Lise Le Huen1, Elodie Muller1, Damien Deldicque4 and Marie Catherine Sforna5, (1)Institut de Physique du Globe de Paris, Paris, France, (2)Universidade Federal de Minas Gerais Instituto de Geociências, Departamento de Geologia, Belo Horizonte, Brazil, (3)Université de Bourgogne, UMR CNRS 6282 Biogéosciences, Dijon, France, (4)Ecole Normale Supérieure Paris, Geosciences, Paris, France, (5)Università degli Studi di Modena e Reggio Emilia, Modena, Italy
The origin of iron oxides in Archean and Paleoproterozoic Banded Iron Formations is still a matter of debate. We report here low and high temperature magnetic properties, susceptibility and saturation magnetization results coupled with scanning microscope, transmission electron microscopy, Raman observations and microprobe analyses along a 60 meters section, which encompasses the uppermost Archean Boolgeeda Iron Formation and its transition into the lower Paleoproterozoic Kungarra Formation in the Pilbara Craton, Western Australia. With the exception of two volcanoclastic intervals characterized by low susceptibility and magnetization, nearly pure magnetite is identified as the main magnetic carrier in all iron-rich layers including hematite-bearing jasper beds. The relative magnetic contribution of magnetite and hematite throughout the section is evidenced by IRM acquisition curves. We observed a sharp decrease in magnetization at the Archean-Proterozoic transition and a general trend in the Verwey temperature. Two populations of magnetically distinct magnetites are reported from a 2 meter-thick interval lying within the late Archean section of the core. Each population shows a specific Verwey transition temperature: one around 120-124K and the other in the range of 105-110K. The two Verwey transitions are interpreted to reflect two distinct stoichiometry and likely two stages of magnetite crystallization. The 120-124K transition is attributed to nearly pure stoichiometric magnetite, whereas SEM, TEM and microprobe observations suggest that the lower temperature transition is related to chemically impure silician magnetite. Microbial-induced partial substitution of iron by silicon is suggested here. This is supported by an increase in Total Organic Carbon (TOC) in the same interval and Raman spectroscopy data showing a close association of organic carbon with magnetite.