An assessment of the reliability of palaeointensity results obtained from the Cretaceous aged Suhongtu section, Inner Mongolia, China

Abstract

Here we present microwave palaeointensity results from 89 sister samples from the study of Zhu et al. [Zhu, R., Pan, Y., He, H., Qin, H., Ren, S., 2008. Palaeomagnetism and 40Ar/ 39Ar age from a Cretaceous volcanic sequence, Inner Mongolia, China: implications for the field variation during the Cretaceous normal superchron. Phys. Earth Planet. Int., 169, 59–75] who carried out Thellier palaeointensity analysis as part of their integrated palaeomagnetic and 40Ar/ 39Ar dating study of Cretaceous lava from the Suhungtu section, Inner Mongolia, China. Additionally, a comprehensive rock magnetic investigation has been carried out in order to determine the mineralogy and hence the validity of assuming that the remanence is a thermal remanent magnetisation (TRM). The microwave results are of apparent high quality and give flow mean palaeointensity estimates ranging from 13 to 49 μT corresponding to virtual dipole moment (VDM) estimates ranging from 2.5 to 8.9 × 10 22 Am 2, and an overall mean VDM of 5.5 ± 1.9 × 10 22 Am 2 for the 24 flows (aged 110.6 ± 0.1 Ma). When the microwave results (using the perpendicular applied field method with partial microwave thermal remanence (pT MRM) and pT MRM tail checks) are compared to those obtained with the Thellier method (Coe version with pTRM but not tail checks, and heating in argon atmosphere) differences are seen at the sample, flow and palaeomagnetic unit level however, the overall means and spread in palaeointensity estimates are consistent. Some discrepancy is due to the differing sized sample sets and sample inhomogeneity but discrepancy is also interpreted to be due to the differing protocols, methodology, plus the subjectivity in interpretation. Considering only those results that are consistent to within 20% the spread in palaeointensity estimates remains. There is substantial rock magnetic evidence from progressive heating in air and argon experiments (both showing irreversible thermomagnetic behaviour) as well as looking at samples under the scanning electron microscope to suggest that maghaemite is present (albeit to varying degrees) in many of the samples. Alteration therefore occurred in nature and it is likely that the remanence will have been affected to differing degrees potentially causing underestimates in palaeointensity. Maghaemite is interpreted to be a remanence carrier where a component of remanence remains after heating to 580 °C and loss of pTRM acquisition capacity is found on heating. No correlation was found between the estimated palaeointensity (microwave or Thellier datasets) and amount of high temperature remanence or any rock magnetic parameter. This could suggest that the palaeointensity estimates are reliable or as seems likely more than one factor (such as methodology, protocol, interpretation, chemical remanent magnetisation (CRM) contamination and the geomagnetic field) are influencing the palaeointensity estimates. In an attempt to remove the influence of palaeointensity protocol and methodology the palaeointensity datasets were reduced to consist only of those results mutually consistent to 20% ( N = 22). No obvious correlation was again seen between the inferred maghaemite contribution and palaeointensity estimate. There is therefore no clear evidence for the biasing of results due to CRM contamination (assuming a simple relationship between biasing and maghaemite contribution) and it can thus be inferred that the geomagnetic field was a major influencing factor. Further studies are however needed to fully elucidate the extent and result of each possible influencing factor and ultimately the reliability of palaeointensity estimates from rocks of this type.