Electron microscope and microprobe analysis of chlorite–mica stacks in the Wenlock turbidites, mid Wales, U.K.

Abstract Chlorite–mica stacks in the Wenlock turbidites have been studied using backscattered electron microscopy and electron microprobe analysis, combined with thin-section work and bulk rock chemical analysis. The stacks occur in fine sandstones and silt–mud turbidites and range in length from < 30 μm to 1.5 mm. They consist of interlayered packets of Fe-rich chlorite and mica. Combined textural and chemical data suggest that many of the stacks represent altered detrital biotite micas. A four-stage alteration sequence is proposed: (1) Subaerial alteration of biotite, in the source area, to interlayered biotite–hydrobiotite/vermiculite. (2) Post-depositional collapse of vermiculite to form a mica phase under conditions of high K+/H+in the sediment pore waters. (3) Decrease in K+/H+ratio, possibly due to H+ build up in the fermentation zone, causing alteration of biotite layers to chlorite. (4) Kinking of the stacks and pressure solution of chlorite early in the development of cleavage.