Palaeointensity just at the onset of the Cretaceous normal superchron

Abstract

A combined palaeomagnetic and geochronologic study has been conducted on a basaltic lava sequence of five flows at Jianchang in Liaoning Province, northeastern China. Radiometric ⁴⁰Ar/³⁹Ar dating indicates that the volcanism occurred at about 119 Ma within the marine anomaly C34n in Cretaceous normal superchron (CNS). Rock magnetic investigations show that pseudo-single domain (PSD) or a mixture of single domain and multi-domain grain titanium-poor titanomagnetite is dominant in the studied lava flows. Both stepwise thermal and alternating field demagnetizations isolate the well-defined normal characteristic remanent magnetization (ChRM) in five independent lava flows with a mean direction of D/I = 11.1°/57.2° (α₉₅ = 5.7°). Palaeointensity was determined using the Coe modified Thellier method with systematic partial thermoremanent magnetization (pTRM) checks. All together, 61 samples from 286 samples with positive pTRM and tail checks yield palaeointensities ranging between 10.1 and 73.6 μT. To further check the effects of thermal alteration on the palaeointensity results, a new ratio (M_rs,480°C/M_rs,25°C, where M_rs,25°C and M_rs,480°C are the room temperature saturation isothermal remanent magnetizations for the raw sample and the product after a 480 °C thermal treatment, respectively) is proposed. Results show that the ratio changes significantly even for those samples with positive pTRM check. We further put forward that palaeointensity results are acceptable with ratio between 0.9 and 1.1. With this new criteria, 17 samples show consistent palaeointensity results with an average of 20.0 ± 3.2 μT. This value corresponds to the virtual dipole magnetic moments (VDM) of (3.5 ± 0.6) × 10²² Am², which is about half of the value of present field. This finding, combined with other available results, suggests that magnetic field of the Earth just at the onset of the CNS is characterized by a weak palaeointensity.

Introduction

The long-term variations in the strength of the geomagnetic field provide important constraints on the chemical–physical processes of the Earth’s interior (Prévot and Perrin, 1992, Coe et al., 2000). Especially, the intensity of the geomagnetic field during the Cretaceous normal superchron (CNS) is crucial to understand the geodynamo. Glatzmaier et al. (1999) have suggested that the Earth’s dipole moment correlates negatively with the geomagnetic reversal rate, which implies that the geomagnetic field during the CNS should be strong. However, there exist arguments about the measured palaeointensity data, e.g., high palaeointensities obtained from plagioclase single crystals (Tarduno et al., 2001, Tarduno et al., 2002) versus low palaeointensities from whole rocks and submarine basalt glass (SBG) (e.g., Prévot et al., 1990, Pick and Tauxe, 1993, Tanaka and Kono, 2002, Riisager et al., 2003, Zhu et al., 2004a, Zhu et al., 2004b, Zhu et al., 2008, Pan et al., 2004, Zhao et al., 2004, Shi et al., 2005). Although it has been reported that palaeointensities near the onset and the termination of the CNS are only half of the present day field strength (Pick and Tauxe, 1993), debates still remain due to the insufficient palaeointensity data (Goguitchaichvili et al., 2002a, Ruiz et al., 2006). Therefore, the palaeointensity during the geomagnetic field switching from a reversing to a non-reversing state at the onset of the CNS is particularly interesting. In this paper, we present the first palaeointensity results obtained from Jianchang basalts, NE China, erupted at ∼119 Ma.