The Role of Randomly Mixed-Layered Chlorite/Smectite in the Transformation of Smectite to Chlorite

Abstract

Abstract--Vesicular and groundmass phyllosilicates in a hydrothermally altered basalt from the Point Sal ophiolite, California, have been studied using transmission electron microscopy (TEM). Pore-filling phyllosilicates are texturally characterized as having coherent, relatively thick and defect-free crystals of chlorite (14 _A) with occasional 24-.~ periodicities. Groundmass phyllosilicates are texturally characterized as 1) randomly oriented crystals up to 200 A in width and 2) larger, more coherent crystals up to 1000 in width. Small crystallites contain predominantly 14-.~ layers with some 24-.~ units. Large crystals show randomly interlayered chlorite/smectite (C/S), with approximately 50% chlorite on average. Adjacent smectite-like layers are not uncommon in the groundmass phyllosilicates. Electron microprobe analyses show that Fe/Mg ratios of both groundmass and vesicular phyllosilicates are fairly constant.Termination of brucite-like interlayers has been identified in some of the TEM images. The transformation mechanisms represented by these layer terminations are 1) growth of a brucite-like interlayer within smectite interlayer regions and 2) the dissolution and reprecipitation of elements to form chlorite layers. Both mechanisms require an increase in volume as smectite transforms to chlorite.The data, combined with that from previously published reports, suggest that randomly interlayered C/S is a metastable phase formed in microenvironments with low water/rock ratios. Chlorite forms in microenvironments in the same sample dominated by higher water/rock ratios. The relatively constant number of Mg's in the structure (Mg#) of both structures indicates that in both microenvironments the bulk rock composition has influence over the composition of phyllosilicates.

Alteration and dehydration of subducting oceanic crust within subduction zones: implications for décollement step-down and plate-boundary seismogenesis

“…%S value for each decomposed peak from peak position and the working curve of Eq. (2) are also shown a processes proposed by Bettison-Varga and Mackinnon (1997) that leads to the release of fluids, thus contributing to the bulk dehydration of oceanic basalts. Here, we use the Nankai subduction zone as a case study to examine the possible dehydration of underthrusting basalts.…”

Chlorite, Corrensite, and Chlorite-Mica in Late Jurassic Fluvio-Lacustrine Sediments of the Cameros Basin of Northeastern Spain

“…Higher substitutions of octahedral Fe 2+ for Mg 2+ in the chlorites may compensate for the distortions introduced by the substitution of the larger A13+ cation for Si 4+ in the tetrahedral sheet (Shau et al, 1990). However, Hillier (1993), Bettison-Varga et al (1991), andMackinnon (1997) found no changes in the Fe/Mg ratio of coexisting chlorites and corrensites. Our results indicate a clear difference between chlorite and corrensite compositions, with well-defined ranges for each phase (Figures 4 and 5).…”

Hydrothermal Synthesis of Corrensite: A Study of the Transformation of Saponite to Corrensite

“…Bettison-Varga and Mackinnon (1997), in a study of trioctahedral phyllosilicates from the Point Sal ophiolite, showed lattice fringe images of very thick (50 unit cells?) C-S (R0) with a composition -50% chlorite.…”

“…Finegrained phyllosilicates are perhaps the most abundant alteration product. As a consequence, they have been studied extensively to document changes in mineral structure and composition that could aid in reconstructing the geologic history of the alteration process (Kristmannsdottir, 1979;Bettison and Schiffman, 1988;Schiffman and Fridleiffson, 1991;Robinson et al, 1993;Shau et aL, 1990;Shau and Peacor, 1992;Schiffman and Staudigel, 1995;Bettison-Varga and Mackinnon, 1997).…”

“…We did not attempt to study the effect of fluid/solid ratios in our experiments even though several investigators (Shau and Peacor, 1992;Schiffman and Staudigel, 1995;Bettison-Varga and Mackinnon, 1997) have proposed that this factor might be critical in determining whether or not randomly interstratified C-S will form as an intermediate. We have not attempted this because we are not aware of any theoretical framework to guide us in determining what ratios might be critical at the temperatures and pressures that were chosen for our study.…”