Palaeointensity of the 1.3 billion-yr-old Gardar basalts, southern Greenland revisited: no evidence for onset of inner core growth

Abstract

SUMMARY The age of the inner core nucleation is a first-order problem in the thermal evolution of the Earth that can be addressed with palaeomagnetism. We conducted a palaeointensity study on the 1.3 Ga Gardar basalts from southern Greenland to investigate previously reported high ancient geomagnetic field intensities. Biggin et al. used the earlier result to identify nucleation of Earth's solid inner core at 1.3 Ga. We collected 106 samples from 39 flows from the lavas of the Eriksfjord Formation, sampling 17 of the lower flows, 8 of the middle flows and 14 of the upper flows. Rock magnetic analyses, including magnetic hysteresis, first-order reversal curves and magnetic susceptibility versus temperature measurements, suggest that the predominate magnetic mineral in the lower basalts is low Ti titanomagnetite, whereas the middle and upper flows have varying amounts of hematite. The magnetic hysteresis data suggest that magnetic grains range from multidomain to single domain in character, with an apparent dominance of pseudo-single behaviour. Thellier–Thellier double heating experiments using the IZZI methodology yielded vector endpoint diagrams and Arai plots showing two components of magnetization, one up to approximately 450 °C and the higher temperature component typically from 450°C up to 580°C, but sometimes to as high as 680°C. We attribute the lower temperature component, to partial overprinting by the nearby Ilimaussaq intrusion, and acquisition of viscous remanent magnetization. We use the Thellier autointerpreter assigning standard selection criteria vetted by cumulative distribution plots. This approach yields a palaeointensity of 6.5 ± 5.9 μT (1 SD) based on 27 samples from 13 flows and a nominal virtual dipole moment (VDM) of 1.72 × 1022 Am2. However, we cannot exclude the possibility of bias in this value related to chemical remanent magnetization (CRM) and multidomain effects. We isolate a conservative upper bound on palaeointensity as the highest palaeointensity result that is free of CRM effects. This yields a palaeointensity of ∼18 μT, and a VDM of ∼4.5 × 1022 Am2, which is a field strength similar to many other Proterozoic values. Thus, our analysis of the Gardar basalts supports the conclusion of Smirnov et al. that there is no palaeointensity signature of inner core growth 1.3 billion yr ago.