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

Paleomagnetism of the ~860 Ma Manso dyke swarm, West Africa: implications for the assembly of Rodinia

The West African Craton (WAC) is one of the major cratons in the Rodinia jigsaw puzzle (~1000?750Ma). In the Rodinian models, the position of West Africa is mainly constrained by the assumptionthat it had been a partner of Amazonia since the Paleoproterozoic. Unfortunately, nopaleomagnetic data are available for these cratons when the Rodina supercontinent is consideredtectonically stable (~1000-750 Ma). Thus, every new reliable paleomagnetic pole for the WestAfrican Craton during the Neoproterozoic times is of paramount importance to constrain itsposition and testing the Rodinia models. In this study we present a combined paleomagnetic andgeochronological investigation for the Manso dyke swarm in the Leo-Man Shield, southern WestAfrica (Ghana). The ~860 Ma emplacement age for the NNW-trending Manso dykes is thus wellconstrainedby two new U-Pb apatite ages of 857.2 ± 8.5 Ma and 855 ± 16 Ma, in agreement withbaddeleyite data. Remanence of these coarse-to-fine grained dolerite dykes is carried by stablesingle to pseudo-single domain (SD-PSD) magnetite. A positive baked-contact test, associated to apositive reversal test (Class-C), support the primary remanence obtained for these dykes (13 sites).Moreover, our new paleomagnetic dataset satisfy all the seven R-criteria (R=7). The ~860 MaManso pole can thus be considered as the first key Tonian paleomagnetic pole for West Africa. Wepropose that the West Africa-Baltica-Amazonia-Congo-São Francisco were associated in a longlivedWABAMGO juxtaposition (~1100?800 Ma).Fil: Antonio, Paul Yves Jean. National Research Institute of Science and Technology. Centre de Montpellier; FranciaFil: Baratoux, Lenka. Université Paul Sabatier; FranciaFil: Ferreira Trindade, Ricardo Ivan. Universidade de Sao Paulo; BrasilFil: Rousse, Sonia. Université Paul Sabatier; FranciaFil: Ayite, Anani. University Of Ghana; GhanaFil: Lana, Cristiano. Universidade Federal de Ouro Preto; BrasilFil: Macouin, Melina. Université Paul Sabatier; FranciaFil: Kobby Adu, Emmanuel Williams. University Of Ghana; GhanaFil: Sanchez, Caroline. Université Paul Sabatier; FranciaFil: Silva, Marco. Universidade Federal de Ouro Preto; BrasilFil: Firmin, Anne Sophie. Université Paul Sabatier; FranciaFil: Martínez Dopico, Carmen Irene. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotópica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; ArgentinaFil: Proietti, Arnaud. Centre de Microcaractérisation Raimond Castaing; FranciaFil: Amponsah, Prince Ofori. University Of Ghana; GhanaFil: Asamoah Sakyi, Patrick. University Of Ghana; GhanaEGU General Assembly 2021ViennaAustriaEuropean Geosciences Unio

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

Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation

The Earth’s inner core grows by the freezing of liquid iron at its surface. The point in history at which this process initiated marks a step-change in the thermal evolution of the planet. Recent computational and experimental studies1,2,3,4,5 have presented radically differing estimates of the thermal conductivity of the Earth’s core, resulting in estimates of the timing of inner-core nucleation ranging from less than half a billion to nearly two billion years ago. Recent inner-core nucleation (high thermal conductivity) requires high outer-core temperatures in the early Earth that complicate models of thermal evolution. The nucleation of the core leads to a different convective regime6 and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner-core nucleation to be estimated directly. Previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements, by the lack of an effective means of assessing their reliability, and by shorter-timescale geomagnetic variations. Here we examine results from an expanded Precambrian database of palaeomagnetic intensity measurements7 selected using a new set of reliability criteria8. Our analysis provides intensity-based support for the dominant dipolarity of the time-averaged Precambrian field, a crucial requirement for palaeomagnetic reconstructions of continents. We also present firm evidence for the existence of very long-term variations in geomagnetic strength. The most prominent and robust transition in the record is an increase in both average field strength and variability that is observed to occur between a billion and 1.5 billion years ago. This observation is most readily explained by the nucleation of the inner core occurring during this interval9; the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the Earth

Distribution of Burkholderia pseudomallei within a 300-cm deep soil profile: implications for environmental sampling.

The environmental distribution of Burkholderia pseudomallei, the causative agent of melioidosis, remains poorly understood. B. pseudomallei is known to have the ability to occupy a variety of environmental niches, particularly in soil. This paper provides novel information about a putative association of soil biogeochemical heterogeneity and the vertical distribution of B. pseudomallei. We investigated (1) the distribution of B. pseudomallei along a 300-cm deep soil profile together with the variation of a range of soil physico-chemical properties; (2) whether correlations between the distribution of B. pseudomallei and soil physico-chemical properties exist and (3) when they exist, what such correlations indicate with regards to the environmental conditions conducive to the occurrence of B. pseudomallei in soils. Unexpectedly, the highest concentrations of B. pseudomallei were observed between 100 and 200 cm below the soil surface. Our results indicate that unravelling the environmental conditions favorable to B. pseudomallei entails considering many aspects of the actual complexity of soil. Important recommendations regarding environmental sampling for B. pseudomallei can be drawn from this work, in particular that collecting samples down to the water table is of foremost importance, as groundwater persistence appears to be a controlling factor of the occurrence of B. pseudomallei in soil