A Time‐Resolved Paleomagnetic Record of Main Group Pallasites: Evidence for a Large‐Cored, Thin‐Mantled Parent Body

Claire I O Nichols,Julia Herrero‐Albillos,Richard J Harrison,Rory D Cottrell,John A Tarduno,Florian Kronast,Benjamin P Weiss,Roger R Fu,James F J Bryson

doi:10.1029/2021je006900

Abstract

AbstractSeveral paleomagnetic studies have been conducted on five Main Group pallasites: Brenham, Marjalahti, Springwater, Imilac, and Esquel. These pallasites have distinct cooling histories, meaning that their paleomagnetic records may have been acquired at different times during the thermal evolution of their parent body. Here, we compile new and existing data to present the most complete time‐resolved paleomagnetic record for a planetesimal, which includes a period of quiescence prior to core solidification as well as dynamo activity generated by compositional convection during core solidification. We present new paleomagnetic data for the Springwater pallasite, which constrains the timing of core solidification. Our results suggest that in order to generate the observed strong paleointensities (∼65–95 μT), the pallasites must have been relatively close to the dynamo source. Our thermal and dynamo models predict that the Main Group pallasites originate from a planetesimal with a large core (>200 km) and a thin mantle (<70 km).

Highlights

The Main Group (MG) pallasites are all thought to originate from the same planetesimal (Greenwood et al, 2006)
We compile new and existing data to present the most complete timeresolved paleomagnetic record for a planetesimal, which includes a period of quiescence prior to core solidification as well as dynamo activity generated by compositional convection during core solidification
We present new paleomagnetic data for the Springwater pallasite, which constrains the timing of core solidification

Summary

Introduction

The Main Group (MG) pallasites are all thought to originate from the same planetesimal (Greenwood et al, 2006). Tarduno et al (2012) found the first evidence for dynamo activity recorded by magnetic inclusions in olivine crystals from the Imilac and Esquel pallasites. This observation was supported by a paleomagnetic study of the cloudy zone in the Imilac and Esquel pallasites (Bryson et al, 2015). An active thermal dynamo is predicted to have only lasted for a maximum of ∼40 Myr after accretion (Bryson, Neufeld, et al, 2019; Dodds et al, 2021; Elkins-Tanton et al, 2011) This measured magnetic remanence has instead been attributed to compositional convection resulting from core solidification. Dynamo initiation and the onset of core solidification was predicted to occur between the time at which Brenham and Marjalahti, and subsequently Imilac and Esquel, acquired their paleomagnetic records

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Conclusion