Abstract
Recent paleomagnetic studies of two Main Group pallasites, the Imilac and Esquel, have found evidence for a strong, late-stage magnetic field on the parent body. It has been hypothesized that this magnetic field was generated by a core dynamo, driven by compositional convection during core solidification. Cooling models suggest that the onset of core solidification occurred ∼200 Ma after planetary accretion. Prior to core solidification, a core dynamo may have been generated by thermal convection; however a thermal dynamo is predicted to be short-lived, with a duration of ∼10 Ma to ∼40 Ma after planetary accretion. These models predict, therefore, a period of quiescence between the thermally driven dynamo and the compositionally driven dynamo, when no core dynamo should be active. To test this hypothesis, we have measured the magnetic remanence recorded by the Marjalahti and Brenham pallasites, which based on cooling-rate data locked in any magnetic field signals present ∼95 Ma to ∼135 Ma after planetary accretion, before core solidification began. The cloudy zone, a region of nanoscale tetrataenite islands within a Fe-rich matrix was imaged using X-ray photoemission electron microscopy. The recovered distribution of magnetisation within the cloudy zone suggests that the Marjalahti and Brenham experienced a very weak magnetic field, which may have been induced by a crustal remanence, consistent with the predicted lack of an active core dynamo at this time. We show that the transition from a quiescent period to an active, compositionally driven dynamo has a distinctive paleomagnetic signature, which may be a crucial tool for constraining the time of core solidification on differentiated bodies, including Earth.
Highlights
Paleomagnetic studies of meteorites provide evidence that core dynamos were a widespread feature of planetesimals during the early solar system (Weiss and Elkins-Tanton, 2013; Scheinberg et al, in press)
Time-resolved paleomagnetic records suggest the Main Group (MG) parent body experienced an intense, latestage magnetic field, which is attributed to an active core dynamo driven by compositional convection during inner core solidification (Nimmo, 2009)
The cloudy zone displays contrasting magnetic behaviour; magnetic domains are clearly observed to be clustered around regions of tetrataenite islands and Fe-rich matrix (Fig. 2)
Summary
Paleomagnetic studies of meteorites provide evidence that core dynamos were a widespread feature of planetesimals during the early solar system (Weiss and Elkins-Tanton, 2013; Scheinberg et al, in press). Thermal modelling suggests an active thermal dynamo would require a magma ocean to generate the required heat flux out of the core This is predicted to have acted at most for 10–40 Ma after accretion (Elkins-Tanton et al, 2011), leading to a quiescent period after the shut down of the thermal dynamo and before the onset of core solidification. Numerical modelling and simple energy balance arguments suggest that a small planetary body with radius
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