No nebular magnetization in the Allende CV carbonaceous chondrite

Magnetic fields in the solar nebula may have played a central role in mass and angular momentum transport in the protosolar disk and facilitated the accretion of the first planetesimals. Thought to be key evidence for this hypothesis is the high unblocking-temperature, randomly oriented magnetization in chondrules in the Allende CV carbonaceous chondrite. However, it has recently been realized that most of the ferromagnetic minerals in Allende are products of secondary processes on the parent planetesimal. Here we reevaluate the pre-accretional magnetism hypothesis for Allende using new paleomagnetic analyses of chondrules including the first measurements of mutually oriented subsamples from within individual chondrules. We confirm that Allende chondrules carry a high-temperature component of magnetization that is randomly oriented among chondrules. However, we find that subsamples of individual chondrules are also non-unidirectionally magnetized. Therefore, the high-temperature magnetization in Allende chondrules is not a record of nebular magnetic fields and is instead best explained by remagnetization during metasomatism in a <8 mu T magnetic field. This low field intensity suggests that any core dynamo on the CV parent body decayed before the end of metasomatism, likely <40 My after the formation of calcium aluminum-rich inclusions (CAIs). Despite widespread practice, the magnetization in Allende should not be used to constrain magnetic fields in the protosolar nebula. (C) 2014 Elsevier B.V. All rights reserved.