ALBERT

Description: Data on the evolution of Earth's magnetic field intensity are important for understanding the geodynamo and planetary evolution. However, the paleomagnetic record in rocks may be adversely affected by many physical processes, which must be taken into account when analysing the palaeointensity database. This is especially important in the light of an ongoing debate regarding core thermal conductivity values, and how these relate to the Precambrian geodynamo. Here, we demonstrate that several data sets in the Precambrian palaeointensity database overestimate the true paleofield strength due to the presence of non-ideal carriers of palaeointensity signals and/or viscous re-magnetizations. When the palaeointensity overestimates are removed, the Precambrian database does not indicate a robust change in geomagnetic field intensity during the Mesoproterozoic. These findings call into question the recent claim that the solid inner core formed in the Mesoproterozoic, hence constraining the thermal conductivity in the core to ‘moderate’ values. Instead, our analyses indicate that the presently available palaeointensity data are insufficient in number and quality to constrain the timing of solid inner core formation, or the outstanding problem of core thermal conductivity. Very young or very old inner core ages (and attendant high or low core thermal conductivity values) are consistent with the presently known history of Earth's field strength. More promising available data sets that reflect long-term core structure are geomagnetic reversal rate and field morphology. The latter suggests changes that may reflect differences in Archean to Proterozoic core stratification, whereas the former suggest an interval of geodynamo hyperactivity at ca. 550 Ma.

Description: 〈span〉〈div〉Summary〈/div〉The age of the inner core nucleation is a first-order problem in the thermal evolution of the Earth that can be addressed with paleomagnetism. We conducted a paleointensity study on the 1.3 Ga Gardar basalts from southern Greenland to investigate previously reported high ancient geomagnetic field intensities. Biggin 〈span〉et al.〈/span〉 (2015) 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 (FORCs), 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 magnetic grains range from multi-domain to single domain in character, with an apparent dominance of pseudo-single behavior. 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 auto-interpreter (Shaar & Tauxe, 2013) assigning standard selection criteria vetted by cumulative distribution plots. This approach yields a paleointensity of 6.5 μT ± 5.9 μT (1 SD) based on 27 samples from 13 flows and a nominal virtual dipole moment (VDM) of 1.72 × 10〈sup〉22〈/sup〉 Am〈sup〉2〈/sup〉. However, we cannot exclude the possibility of bias in this value related to chemical remanent magnetization (CRM) and multi-domain effects. We isolate a conservative upper bound on paleointensity as the highest paleointensity result that is free of CRM effects. This yields a paleointensity of ∼18 μT, and a VDM of ∼4.5 × 10〈sup〉22〈/sup〉 Am〈sup〉2〈/sup〉, which is a field strength similar to many other Proterozoic values. Thus, our analysis of the Gardar basalts supports the conclusion of Smirnov 〈span〉et al.〈/span〉 (2016) that there is no paleointensity signature of inner core growth 1.3 billion years ago.〈/span〉