Report

Evidence for a Dynamo in the Main Group Pallasite Parent Body

Science  16 Nov 2012:
Vol. 338, Issue 6109, pp. 939-942
DOI: 10.1126/science.1223932

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Magnetic Pallasites

The origin of pallasite meteorites seems to defy explanation because their main constituents—iron and olivine—should have segregated into layers inside their parent body. The generally accepted model suggests that they formed at the coremantle boundary of an asteroid. Tarduno et al. (p. 939; see the Perspective by Weiss) measured a remnant magnetization in olivine crystals of two pallasite meteorites and conclude that a dynamo must have operated in their parent body, providing further evidence that some asteroids were capable of dynamo generation. The data, together with thermal modeling, suggest that some pallasites could have formed when liquid FeNi from the core of an impacting asteroid was injected into the mantle of a large protoplanet.

Abstract

Understanding the origin of pallasites, stony-iron meteorites made mainly of olivine crystals and FeNi metal, has been a vexing problem since their discovery. Here, we show that pallasite olivines host minute magnetic inclusions that have favorable magnetic recording properties. Our paleointensity measurements indicate strong paleomagnetic fields, suggesting dynamo action in the pallasite parent body. We use these data and thermal modeling to suggest that some pallasites formed when liquid FeNi from the core of an impactor was injected as dikes into the shallow mantle of a ~200-kilometer-radius protoplanet. The protoplanet remained intact for at least several tens of millions of years after the olivine-metal mixing event.

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