Pallasite paleomagnetism: Quiescence of a core dynamo

Nichols, Claire I. O. and Bryson, James F. J. and Herrero-Albillos, Julia and Kronast, Florian and Nimmo, Francis and Harrison, Richard J. (2016) Pallasite paleomagnetism: Quiescence of a core dynamo. Earth and Planetary Science Letters, 441. pp. 103-112. ISSN 0012-821X DOI 10.1016/j.epsl.2016.02.037

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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.

Item Type: Article
Uncontrolled Keywords: 2016AREP; IA70; weekly list
Subjects: 03 - Mineral Sciences
Divisions: 03 - Mineral Sciences
07 - Gold Open Access
Journal or Publication Title: Earth and Planetary Science Letters
Volume: 441
Page Range: pp. 103-112
Identification Number: 10.1016/j.epsl.2016.02.037
Depositing User: Sarah Humbert
Date Deposited: 21 Apr 2016 17:00
Last Modified: 22 Apr 2016 09:03

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