Datensatz

Zusammenfassung

In the past decade there has been increased multidisciplinary interest in the onset time of inner core nucleation (ICN). A major step forward toward understanding ICN from paleomagnetism was the discovery by Bono et al. (Nature Geoscience, 2019) of ultralow fields in Ediacaran-age (565 Ma) anorthosites of the Sept Îles layered mafic sequence of northern Quebec, Canada. Independent thermal and numerical dynamo modeling (Driscoll, GRL, 2016) predicted the weak-field state prior to ICN. Bono et al. (2019) combined the unusual geomagnetic behavior (a hyper-reversal state) from global studies together with the Sept Îles ultra-low field value to suggest ICN near 565 Ma. We consider whether the internally generated field did not just decline to ultra-low values but might have effectively disappeared for some time interval of the Ediacaran Period. We study whole rock paleointensity values from Ediacaran-age dikes sampled from Ontario, Canada. We find Arai plots with extreme two-slope behavior. The low temperature slope appears to be a thermo-viscous overprint, whereas the high temperature slope, if interpreted as a viable paleointensity measure, records ultra-low fields. The whole rocks almost certainly contain some large multidomain grains. Here we investigate the possibility that the high temperature slope is a field overestimate, reflecting not the true ambient field but rather the dominant viscous overprint that has contaminated higher unblocking temperatures. If correct, it is further possible that this overprint might be concealing even lower ambient fields, or a complete collapse of the dynamo. We will discuss our plans to experimentally assess this hypothesis.