Do reduction spots predate finite strain? - A magnetic diagnosis of Cambrian slates in North Wales

The purple Cambrian slates of North Wales contain green-colored, irregular patches drawn out along cleavage and the more familiar ellipsoidal reductio n spots that are parallel to cleavage. Because parallelism of pre-strain el lipsoids could only be produced by extremely high strain, we reject the hyp othesis that these are strain markers. Here, we use magnetic methods to res olve this argument. The magnetic analysis reveals that the magnetic fabric of purple slate differs from the green variety of the slate only in the oxi dation state, and indicates that the reduction process postdates cleavage. This suggests that the shape of reduction spots reflects the anisotropy of diffusion during reduction, not finite strain and alignment of an initially ellipsoidal object in the slates. (C) 2001 Elsevier Science B.V All rights reserved.