Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


  • NSF Public Access
  • Search Results
  • Strongly Negative Low‐Field Variation of Magnetic Susceptibility: Rock Magnetic Character of the Basement‐Cover Interface of Northeastern Oklahoma
Title: Strongly Negative Low‐Field Variation of Magnetic Susceptibility: Rock Magnetic Character of the Basement‐Cover Interface of Northeastern Oklahoma
Abstract Some rock and soil samples exhibit significant loss of magnetic susceptibility (χ) with increasing applied field amplitude even at relatively low (10–100s of A/m) fields, a behavior which remains unexplained. Exceptionally strong negative field‐dependence of susceptibility (χHD) is present in sandstones and altered intermediate‐felsic igneous rocks in several cores from the northeastern Oklahoma subsurface. These same rocks also show elevated frequency‐dependence of susceptibility (χFD), with reasonable correlation ofχHDtoχFD, and frequency‐dependentχHD. Results from multiple characterization methods indicate that strongly negativeχHDin these rocks is linked to a yet‐unidentified phase which begins the approach to magnetic saturation in low fields (<1 mT/800 A/m), shows elevatedχFDto low temperatures, is unstable at high temperatures, possesses significant anisotropy of magnetic susceptibility, and becomes paramagnetic above ∼83°C. Clear associations with fluid alteration features indicate that this material may be highly relevant to rock alteration, diagenetic, and environmental studies.  more » « less
Award ID(s):
2153786
PAR ID:
10554909
Author(s) / Creator(s):
; ; ; ;
Publisher / Repository:
American Geophysical Union
Date Published:
Journal Name:
Journal of Geophysical Research: Solid Earth
Volume:
129
Issue:
8
ISSN:
2169-9313
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; ; ; ; ; ; ; ; et al (, Geochemistry, Geophysics, Geosystems)
    Abstract The past ∼200 million years of Earth's geomagnetic field behavior have been recorded within oceanic basalts, many of which are only accessible via scientific ocean drilling. Obtaining the best possible paleomagnetic measurements from such valuable samples requires an a priori understanding of their magnetic mineralogies when choosing the most appropriate protocol for stepwise demagnetization experiments (either alternating field or thermal). Here, we present a quick, and non‐destructive method that utilizes the amplitude‐dependence of magnetic susceptibility to screen submarine basalts prior to choosing a demagnetization protocol, whenever conducting a pilot study or other detailed rock‐magnetic characterization is not possible. We demonstrate this method using samples acquired during International Ocean Discovery Program Expedition 391. Our approach is rooted in the observation that amplitude‐dependent magnetic susceptibility is observed in basalt samples whose dominant magnetic carrier is multidomain titanomagnetite (∼TM60–65, (Ti0.60–0.65Fe0.35–0.40)Fe2O4). Samples with low Ti contents within titanomagnetite or samples that have experienced a high degree of oxidative weathering do not display appreciable amplitude dependence. Due to their low Curie temperatures, basalts that possess amplitude‐dependence should ideally be demagnetized either using alternating fields or via finely‐spaced thermal demagnetization heating steps below 300°C. Our screening method can enhance the success rate of paleomagnetic studies of oceanic basalt samples. 
    more » « less
  2. ; ; ; ; ; (, Geochemistry, Geophysics, Geosystems)
    Abstract Interpreting the paleomagnetic records of altered rocks, especially those from Earth's earliest history, is complicated by metamorphic overprints and recrystallization of ferromagnetic minerals. However, these records may be as valuable as a primary signal if the timing and mechanism of alteration‐related remagnetizations can be ascertained. We illustrate the success of this approach in the case of seafloor hydrothermal alteration by integrating simple rock magnetic and magnetic microscopy data with petrography, hyperspectral imagery, aeromagnetic surveys, field mapping, and geochronology of Paleoarchean basalts from North Pole Dome located in the East Pilbara Craton, Western Australia. We identify 12 hydrothermal episodes during the deposition of the stratigraphy between ∼3490 and 3350 Ma. These episodes produced stratabound zones of hydrothermal alteration with predictable facies successions of mineral assemblages reflecting sub‐seafloor gradients in fluid temperature, pH, composition, and water/rock ratios. Rock magnetic data and magnetic microscopy pinpoint the secondary ferromagnetic minerals within each alteration assemblage, revealing a specific single‐domain magnetite population within leucoxenes (titanite and anatase after primary titanomagnetites) that always accompanies low‐water/rock alteration in fluids buffered to pH equilibrium with the host basalts. Highly uniform magnetic properties indicate that once formed, these magnetites remain unchanged upon further exposure to rock buffered fluids, stabilizing them against later alteration events and making them durable paleofield recorders. The altered basalts hosting this magnetite have unique and consistent appearances, mineralogy, IR absorption features, aeromagnetic signatures, and magnetic properties across all hydrothermal systems studied here, highlighting how integrating these data sets can identify and interpret this alteration style in future paleomagnetic investigations. 
    more » « less
  3. ; ; ; ; (, Journal of Geophysical Research: Solid Earth)
    Abstract Mafic rocks are volumetrically and rheologically significant components of the mid‐to lower continental crust, yet tools to study their fabrics have not been well developed. We examine amphibolites exhumed from mid‐to lower crustal levels in a gneiss dome (Entia dome, central Australia) that display various strengths of mineral lineation and foliation associated with different deformation geometries. Combining petrofabric analysis (electron backscatter diffraction, EBSD) with magnetic fabric analysis (Anisotropy of magnetic susceptibility (AMS), we quantify relationships between AMS‐derived fabrics and crystallographic‐preferred alignment of fabric‐defining amphiboles. We combine single‐crystal AMS data with EBSD data to model amphibole textures and their expected magnetic anisotropy. We formulate a new EBSD‐derived petrofabric index,CAindex, and correlate it with the calculated AMS shape parameterU.CAindexvalues can then be estimated for natural samples using measuredUvalues, leveraging both rapid but texturally low‐resolution AMS and texturally‐resolved but time‐ and analytically‐onerous petrofabric analyses to interpret petrofabrics from magnetic fabric data. In the Entia dome, we identify amphibole c‐fibers (L‐tectonite) in the high‐strain core of the dome, which reflect constrictional strains. In contrast, a‐fibers (S‐tectonites) are dominant near the dome margins and indicate flattening strains. Fabrics measured in different structural subdomains agree well with 2D and 3D numerical models of finite strain distribution in domal structures. Combining textural modeling, AMS measurements, and EBSD analyses allows investigation of previously unexploited records of ductile deformation and flow in amphibole‐bearing rocks. These results can be applied to a wide range of field‐based studies of tectonic and magnetic processes. 
    more » « less
  4. ; ; ; ; ; ; ; (, Journal of Physics: Materials)
    Abstract The crystal structure, electron energy-loss spectroscopy (EELS), heat capacity, and anisotropic magnetic and resistivity measurements are reported for Sn flux grown single crystals of orthorhombic Pr2Co3Ge5(U2Co3Si5-type,Ibam). Our findings show thato-Pr2Co3Ge5hosts nearly trivalent Pr ions, as evidenced by EELS and fits to temperature dependent magnetic susceptibility measurements. Complex magnetic ordering with a partially spin-polarized state emerges nearTsp= 32 K, with a spin reconfiguration transition nearTM= 15 K. Heat capacity measurements show that the phase transitions appear as broad peaks in the vicinity ofTspandTM. The magnetic entropy further reveals that crystal electric field splitting lifts the Hund’s rule degeneracy at low temperatures. Taken together, these measurements show that Pr2Co3Ge5is an environment for complexfstate magnetism with potential strongly correlated electron states. 
    more » « less
  5. ; (, Geophysical Journal International)
    SUMMARY Accurate absolute palaeointensity is essential for understanding dynamo processes on the Earth and other planetary bodies. Although great efforts have been made to propose techniques to obtain magnetic field strength from rock samples, such as Thellier-series methods, the amount of high-fidelity palaeointensities remains limited. One primary reason for this is the thermal alteration of samples that pervasively occurred during palaeointensity experiments. In this study, we developed a comprehensive rock magnetic experiment, termed thermal rock magnetic cycling (TRMC), that can utilize measurements of critical rock magnetic properties at elevated temperatures during multiple heating-cooling cycles to track thermal changes in bulk samples and individual magnetic components with different Curie temperatures in samples for palaeointensity interpretations. We demonstrate this method on a Galapagos lava sample, GA 84.6. The results for this specimen revealed that GA 84.6v underwent thermophysical alteration throughout the TRMC experiment, resulting in changes in its remanence carrying capacity. These findings were then used to interpret the palaeointensity results of specimen GA 84.6c, which revealed that the two-slope Arai plot yielded two linear segments with distinct palaeointensity values that were both biased by thermophysical alteration. To further test the TRMC method, we selected another historical lava sample (HS 2) from Mt Lassen, detecting slight thermal-physical changes after heating the specimen HS 2–8C to a target temperature of 400 °C. We also isolated a stable magnetic component with a Curie temperature below 400 °C using the TRMC method, which may provide a more reliable palaeointensity estimate of 51 μT. By providing a method for tracking thermal alteration independent of palaeointensity experiments, the TRMC method can explore subtle, unrecognizable thermal alteration processes in less detailed palaeointensity measurements, which can help to assess the thermal stability of the measured samples and interpret the changes in the TRM unblocking spectrum and palaeointensity estimates, facilitating the acquisition of more reliable records for constrain the formation of the inner core and the evolution of Earth's magnetic field. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Text Encoded Conversion
  • XML
  • Save / Share this Record
  • Send to Email