Further evidence for early lunar magnetism from troctolite 76535

Garrick-Bethell, I., Weiss, B. P., Shuster, D. L., Tikoo, S. M. and Tremblay, M. M. (2017) Further evidence for early lunar magnetism from troctolite 76535. Journal of Geophysical Research: Planets, 122(1), pp. 76-93. (doi: 10.1002/2016JE005154)

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Abstract

The earliest history of the lunar dynamo is largely unknown and has important implications for the thermal state of the Moon and the physics of dynamo generation. The lunar sample with the oldest known paleomagnetic record is the 4.25 billion year old (Ga) troctolite 76535. Previous studies of unoriented subsamples of 76535 found evidence for a dynamo field with a paleointensity of several tens of microteslas. However, the lack of mutual subsample orientation prevented a demonstration that the magnetization was unidirectional, a key property of thermoremanent magnetization. Here we report further alternating field demagnetization on three mutually oriented subsamples of 76535, as well as new pressure remanent magnetization experiments to help rule out shock magnetization. We also describe new 40Ar/39Ar thermochronometry and cosmogenic neon measurements that better constrain the rock's thermal history. Although the rock is unbrecciated, unshocked, and slowly cooled, its demagnetization behavior is not ideal due to spurious remanence acquisition. Despite this limitation, all three subsamples record a high coercivity magnetization oriented in nearly the same direction, implying that they were magnetized by a unidirectional field on the Moon. We find no evidence for shock remanence, and our thermochronometry calculations show no significant reheating events since 4249 ± 12 million years ago (Ma). We infer a field paleointensity of approximately 20–40 μT, supporting the previous conclusion that a lunar dynamo existed at 4.25 Ga. The timing of this field supports an early dynamo powered by thermal or thermochemical core convection and/or a mechanical dynamo but marginally excludes a dynamo delayed by thermal blanketing from radiogenic element-rich magma ocean cumulates.

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Tremblay, Dr Marissa
Authors: Garrick-Bethell, I., Weiss, B. P., Shuster, D. L., Tikoo, S. M., and Tremblay, M. M.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Journal of Geophysical Research: Planets
Publisher:Wiley
ISSN:2169-9097
ISSN (Online):2169-9100
Published Online:04 January 2017
Copyright Holders:Copyright © 2016 American Geophysical Union
First Published:First published in Journal of Geophysical Research: Planets 122(1): 76-93
Publisher Policy:Reproduced in accordance with the publisher copyright policy

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