| Full text | |
| Author(s): |
Bellon, Ualisson Donardelli
;
Trindade, Ricardo Ivan Ferreira
;
Williams, Wyn
;
Galante, Douglas
;
Sant'Anna, Lucy Gomes
;
Pescarini, Thales
Total Authors: 6
|
| Document type: | Journal article |
| Source: | JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH; v. 129, n. 10, p. 33-pg., 2024-10-01. |
| Abstract | |
The Neoproterozoic carbonate rocks of the Araras Group (Amazon Craton) and the Sete-Lagoas and Salitre Formations (S & atilde;o Francisco Craton) share a statistically indistinguishable single-polarity (reversed) characteristic direction. This direction is associated with paleomagnetic poles that do not align with the expected directions for primary detrital remanence. We employ a combination of classical rock magnetic properties and micro imaging/chemical analysis (in thin sections) using synchrotron radiation to examine these remagnetized carbonate rocks. Magnetic data indicate that most samples lack the anomalous hysteresis properties typically associated with carbonate remagnetization (except for distorted loops). Through a combination of Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS), X-ray Fluorescence (XRF), and X-ray Absorption Spectroscopy (XAS), we identified subhedral/anhedral magnetite, or spherical grains with a core-shell structure of magnetite surrounded by maghemite. These grains are within the pseudo-single domain size range, as do most of the iron sulfides, and are spatially associated with potassium-bearing aluminosilicates. While fluid percolation and organic matter maturation play a role, smectite-illitization appears to be crucial for the growth of these phases. X-ray diffraction analysis, in addition, identifies these silicates as predominantly highly crystalline illite, suggesting exposure to epizone temperatures. These temperatures were likely reached during the final stages of the Gondwana assembly (Cambrian), but remanence was only locked in afterward, in successive cooling events during the Early Middle Ordovician. This is supported by the carbonates' paleomagnetic pole positions compared to Gondwana's apparent polar wander path, and the absence of reversals, contrasting with the high reversal frequency of the Late Ediacaran/Cambrian. Carbonate rocks serve as important records of ancient climates and host magnetic minerals capable of documenting the evolution of Earth's magnetic field. Nevertheless, their original magnetization, acquired during sedimentation, is frequently supplanted by a secondary magnetization, originating from various geological processes altering local thermochemical stability. For carbonate rocks, this remagnetization process is often associated with a "magnetic fingerprint." In South America, carbonate rocks from different sedimentary basins (hundreds of kilometers apart) exhibit statistically similar magnetic components that deviate from their formation origin, indicating secondary processes. This multidisciplinary study integrates classical paleomagnetic analysis with micro-chemical and imaging analysis to comprehend the geological phenomena responsible for remagnetizing such an extensive continental area. We demonstrate that not all remagnetized samples display the anticipated magnetic fingerprint. Highly detailed chemical analysis confirms the presence of nanoscopic magnetic minerals spatially correlated with clay minerals indicating that while organic matter transformation may play a significant role, clay mineral transformation is a key phenomenon governing remagnetization in these rocks. However, our data also supports the notion that these rocks underwent heating during the final assembly of the ancient continental landmass, Gondwana, and a sequential cooling event is suggested as locking their magnetization in the state observed today. Remagnetized Neoproteorozoic carbonates in Brazil may not consistently display anomalous hysteresis parameters Synchrotron-based analysis revealed pseudo-single domain-sized magnetite spatially correlated with aluminosilicates (smectite-illite) The Late Cambrian assembly of Gondwana thermally reset the carbonates' remanence, eventually blocking it (470-460 Ma) during gradual cooling (AU) | |
| FAPESP's process: | 21/00861-2 - Acquisition and chemical remagnetization in carbonates: an evaluation by micromagnetic modeling, laboratory simulations and case studies |
| Grantee: | Ualisson Donardelli Bellon |
| Support Opportunities: | Scholarships in Brazil - Doctorate |
| FAPESP's process: | 22/14100-6 - Exploring micromagnetic modelling of highly-detailed three-dimensional finite element meshes: a new pathway to study the properties of remanence carriers in remagnetized carbonates |
| Grantee: | Ualisson Donardelli Bellon |
| Support Opportunities: | Scholarships abroad - Research Internship - Doctorate |
| FAPESP's process: | 16/06114-6 - The Neoproterozoic Earth System and the rise of biological complexity |
| Grantee: | Ricardo Ivan Ferreira da Trindade |
| Support Opportunities: | Research Projects - Thematic Grants |