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° ( α 95 = 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) × 10 22 Am 2, 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.