Nanogoethite as a Potential Indicator of Remagnetization in Red Beds

Abstract

Red beds are well‐known for recording stable natural remanent magnetization (NRM). However, discriminating primary NRM from secondary remanence in red beds is difficult. The Paleogene Nangqian red beds in eastern Tibetan Plateau variably record an overprint related to nearby magmatism and thus provide a great opportunity to characterize remagnetization in red beds. Through comprehensive rock magnetic, Mössbauer spectroscopic, and petrographic analyses, we find that remagnetization was controlled by temperature. Remagnetized red beds contain abundant authigenic hematite and goethite, with some larger grains unblocking at the Néel temperatures and the remainder (nanoparticles) unblocking at lower temperatures. In contrast, red beds retaining primary NRM are characterized by dominance of detrital hematite and magnetite, presence of fine‐grained authigenic hematite, and absence of authigenic goethite and magnetite. High temperature behaviors of NRM/susceptibility are indicative of remagnetization, but the presence of goethite appears to be a more sensitive criterion for diagnosing remagnetization in red beds.

Remagnetization of Red Beds on the Tibetan Plateau: Mechanism and Diagnosis

“…The normal polarity component from the Nangqian red beds is an overprint, which is indistinguishable in direction from the primary NRM of the igneous rocks in the basin (Roperch et al, 2017). The common true mean direction test between the overprint in red beds and primary NRM in igneous rocks is Class C. The high‐temperature component of Type B samples is also contaminated by the overprint; only the intermediate high temperature component of Type C samples represents a primary NRM (Huang, Jackson, Dekkers, Solheid, et al, 2019). The convex thermal decay curves of Type C samples are similar to those of the Gongjue red beds, whereas Type A samples have linear‐convex thermal decay curves, and Type B samples are characterized by concave thermal decay curves with a sharp decrease in magnetization above 650°C (Figure 2h).…”

“…The Gongjue red beds record a DRM (Li et al, 2020; Tong et al, 2017; Zhang et al, 2018), and red beds in the Shanglaxiu and Jinggu basins are remagnetized (Li et al, 2017; Roperch et al, 2017). The situation in the Nangqian Basin is more complicated: most red beds in the Nangqian Basin are remagnetized, but some retain a DRM (Huang, Jackson, Dekkers, Solheid, et al, 2019). Specifically, we analyze the thermal demagnetization behaviors of the NRM in all of these red beds, apply comprehensive rock magnetic experiments, conduct Mössbauer spectroscopy analysis, and carry out scanning electron microscopy (SEM) examinations with associated energy‐dispersive X‐ray spectrometry (EDS) analysis.…”

“…Uncertainty still exists, however, in the use of this criterion because paleomagnetic studies on natural samples reveal that the unblocking temperature spectra of the CRM can overlap with those of the DRM and extend up to the Néel temperature of hematite (Jiang et al, 2017;Swanson-Hysell et al, 2019). In a recent study of the Paleogene red beds in the Nangqian Basin (eastern Tibetan Plateau), we have identified that remagnetized red beds contain large amounts of nanogoethite, whereas goethite is not present in red beds retaining the DRM (Huang, Jackson, Dekkers, Solheid, et al, 2019). The presence of abundant nanoscaled goethite seems like a sensitive criterion for diagnosing remagnetization in red beds.…”

Eocene Rotation of the Northeastern Central Tibetan Plateau Indicating Stepwise Compressions and Eastward Extrusions

“…Stepwise thermal demagnetization and κ-T curves from representative samples show that the dominant magnetic carrier of the characteristic remnant magnetization (ChRM) directions is hematite at unblocking temperature of 690°C (Figures S2 and S3) and with high coercivity ( Figures S4 and S5), which is detrital origin of the thermal demagnetization models ( Figure S6) (Jiang et al, 2015(Jiang et al, , 2017, with no observations of metasomatism and alteration in electron microprobe analysis (BSE and EDS) (Figures S7). Stable magnetic direction can be obtained from the high temperature component (HTC) between 600°C and 690°C in the red beds of the NB (Huang et al, 2019;Zhang et al, 2018). To avoid the reset of the orientation of the chemical remnant magnetization (CRM), which is isolated before 600°C (Huang et al, 2019), the ChRM direction was obtained from the HTC component.…”

“…Stable magnetic direction can be obtained from the high temperature component (HTC) between 600°C and 690°C in the red beds of the NB (Huang et al, 2019;Zhang et al, 2018). To avoid the reset of the orientation of the chemical remnant magnetization (CRM), which is isolated before 600°C (Huang et al, 2019), the ChRM direction was obtained from the HTC component. The angular difference between the average normal and reverse declinations from the ChRM directions after tilt-corrected are close to 180°and these directions are more clustered in the stratigraphic coordinates than in the geographic coordinates ( Figures S8b and 8d), and the reverse and normal polarity directions overlap each other and the average values of the three axes after reversal are very close to the average values of the nonreversal data sets ( Figures S8e and 8f), indicating positive reversal test and bootstrap reversals test within the 95% confidence bounds from the two sets of data overlapping in all three components (Tauxe, 1998).…”