The effect of remanence anisotropy on paleointensity estimates: a case study from the Archean Stillwater Complex

Paleomagnetism of Archean rocks potentially provides information about the early development of the Earth and of the geodynamo. Precambrian layered in trusive rocks are good candidates for paleomagnetic studies: such complexes are commonly relatively unaltered and may contain some single-domain magne tite 'armored' by silicate mineral grains. However, layered intrusives ofte n have a strong petrofabric that may result in a strong remanence anisotrop y. Magnetic anisotropy can have particularly disastrous consequences for pa leointensity experiments if the anisotropy is unrecognized and if its effec ts remain uncorrected. Here we examine the magnetic anisotropy of an anorth osite sample with a well-developed magmatic foliation. The effect of the sa mple's remanence fabric on paleointensity determinations is significant: pa leointensities estimated by the method of Thellier and Thellier range from 17 to 55 muT for specimens magnetized in a field of 25 muT. We describe a t echnique based on the remanence anisotropy tensor to correct paleointensity estimates for the effects of magnetic fabric and use it to estimate a pale ointensity for the Stillwater Complex (MT, USA) of similar to 32 muT (adjus ted for the effects of slow cooling). (C) 2000 Elsevier Science B.V. All ri ghts reserved.