Young inner core inferred from Ediacaran ultra-low geomagnetic field intensity

• Published in: Nature geoscience 2019-02, Vol.12 (2), p.143-147
• Main Authors: Bono, Richard K., Tarduno, John A., Nimmo, Francis, Cottrell, Rory D.
• Format: Article
• Language: English
• Subjects: 704/2151/123; 704/2151/412; Crystals; Data; Dipole moments; Dynamo theory; Earth; Earth and Environmental Science; Earth core; Earth Sciences; Earth System Sciences; Field strength; Geochemistry; Geology; Geomagnetic field; Geomagnetism; Geophysics/Geodesy; Magnetism; Palaeomagnetism; Paleomagnetism; Plagioclase; Predictions; Thermal conductivity
• ISSN: 1752-0894; 1752-0908
• DOI: 10.1038/s41561-018-0288-0

An enduring mystery about Earth has been the age of its solid inner core. Plausible yet contrasting core thermal conductivity values lead to inner core growth initiation ages that span 2 billion years, from ~0.5 to >2.5 billion years ago. Palaeomagnetic data provide a direct probe of past core conditions, but heretofore field strength data were lacking for the youngest predicted inner core onset ages. Here we present palaeointensity data from the Ediacaran (~565 million years old) Sept-Îles intrusive suite measured on single plagioclase and clinopyroxene crystals that hosted single-domain magnetic inclusions. These data indicate a time-averaged dipole moment of ~0.7 × 10 22  A m 2 , the lowest value yet reported for the geodynamo from extant rocks and more than ten times smaller than the strength of the present-day field. Palaeomagnetic directional studies of these crystals define two polarities with an unusually high angular dispersion ( S  = ~26°) at a low latitude. Together with 14 other directional data sets that suggest a hyper-reversal frequency, these extraordinary low field strengths suggest an anomalous field behaviour, consistent with predictions of geodynamo simulations, high thermal conductivities and an Ediacaran onset age of inner core growth. A late onset of inner-core growth is inferred from ultra-low palaeomagnetic field strengths about 565 million years ago, as measured in magnetic inclusions in Ediacaran crystals.