Abstract
Abstract We firstly present the results of a detailed palaeointensity study performed on 54 samples from 9 volcanic units of late Archaean age (2724-2772 Ma) from the Pilbara Craton, Western Australia. These results were severely affected by magnetomineralogical alteration occurring during the laboratory heating process necessitating the application of a correction procedure. The correction allowed results from three lavas to pass strict selection criteria but we deem that only one of these exhibits sufficient internal consistency to be considered moderately reliable. It yields a virtual dipole moment of 47±6 ZAm2which is 60% of the present-day value. We combine this determination with a filtered dataset from the updated IAGA (International Association of Geomagnetism and Aeronomy) palaeointensity database, PINT08. Directional secular variation has recently been shown to have changed fundamentally since the Archaean, probably as a consequence of inner core growth since that time. However, here we argue that it is still unclear whether this evolution was accompanied by a single long timescale change in average poloidal field intensity. While the distribution of Precambrian palaeointensity determinations as a whole is significantly lower than that for the last 300 Myr, we show that this finding largely reflects data from the Proterozoic aeon. The distribution of more ancient measurements from the late Archaean-earliest Proterozoic is indistinguishable from that of the last 300 Myr which might suggest that a ‘Proterozoic dipole low’ period existed between two periods of higher field intensity. Were this pattern of long-term geomagnetic intensity variation to be supported by the addition of new data in the future, then it could indicate a related three-stage evolution in core dynamics, namely: vigorous thermal convection caused by high core-mantle heat flux early in the Earth’s history, weaker thermal convection later as the heat flux fell, and finally, strong compositional convection since the inner core nucleated.
Highlights
Geomagnetic field behaviour varies on timescales much longer than those associated with outer core convection
We firstly report on a palaeointensity study performed on late Archaean lavas from the Nullagine and Mount Jope Supersequences, Pilbara Craton, Western Australia
The samples faithfully record the direction of the palaeomagnetic field but, in spite of our pre-selection based on the results from thermomagnetic analysis, the results of the palaeointensity experiments were severely affected by alteration caused by the laboratory heating
Summary
Geomagnetic field behaviour varies on timescales much longer than those associated with outer core convection. More strictly filtered dataset and concluded that it was fundamentally different during the late Archaean and early Proterozoic relative to more recent times (both the period 0–5 Ma and the period 0–195 Ma). They argued that, consistent with some dynamo simulations (Roberts and Glatzmaier, 2001; Coe and Glatzmaier, 2006), this was probably due to the growth of the inner core between these periods acting to destabilise the geodynamo by changing the geometry of the outer core
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
