Frederick J. Wicks

THE ORIGIN OF RODINGITES FROM CASSIAR, BRITISH COLUMBIA, AND THEIR USE TO ESTIMATE T AND P(H2O) DURING SERPENTINIZATION

Abstract The Cassiar serpentinite, located in north-central British Columbia, is a low-grade, alpine-type serpentinite within the Sylvester allochthon. Two stages of rodingitization have occurred at Cassiar and these correlate with two stages of serpentinization that have occurred in the ultramafic rock. The first stage is characterised by an increase in Ca ++ activity and decreases in Mg ++ and SiO 2 activities in the rodingite, culminating in the crystallization of grossular in the rodingite and by the formation of lizardite in the serpentinite. At a later time, a second event occurred, characterised by a decrease in Ca ++ activity and increases in Mg ++ and SiO 2 activities both in the rodingite and the serpentinite. This was marked by the crystallization of diopside, prehnite, and lizardite in the rodingite, and by the recrystallization of lizardite to chrysotile and antigorite in the serpentinite. The serpentine recrystallization and the development of the second-stage rodingite minerals indicate that the changes in rodingite and serpentine mineral assemblages were driven by similar changes in the bulk-rock compositions. This suggests that the second-stage event in both rodingite and serpentinite was caused by an externally derived fluid. Measurements on fluid inclusions in the rodingite minerals clinozoisite, grossular, and diopside of the second event indicate that the externally derived fluid was CO 2 -poor, but relatively saline (8 ± 1.5 equiv. wt% NaCl). By combining the pressure-dependant temperature measurements from fluid inclusions with the pressure independant temperature measurements on δ 18 O in coexisting serpentine-magnetite pairs the pressure of formation can be estimated. The second event of serpentine recrystallization and rodingitization was an isothermal process at 300° C ± 36° C , P ( H 2 O )

Antigorite in deformed serpentinites from the Mid-Atlantic Ridge

Deformed, non-psxeudomorphic serpentinites from fault zones in the Rainbow and Menez Hom areas, in the Mid-Atlantic Ridge, contain antigorite associated with variable amounts of chrysotile, while pseudomorphic or non-pseudomorphic lizardite + chrysotile serpentinites are the rule in this and other oceanic environments. A detailed TEM study of these deformed serpentinites shows that antigorite (polysomes m = 12 to 16) replaces chrysotile through dissolution-recrystallization, rather than through solid-state transition. This dissolution-recrystallization process is probably favoured by intense shear stress, the effects of which are preserved in the textures of these rocks. Oxygen isotope temperature estimates for these serpentinites fall well below 300 °C, confirming that Mid-Atlantic Ridge antigorite does not result from high-temperature prograde metamorphism, as it often does in other geological environments. Antigorite-bearing serpentinites, therefore, may occur locally in low-temperature, high-deformation settings, characterized by intense tectonic activity and major shear zones, as frequently found along the slow-spreading Mid-Atlantic Ridge. Technical difficulties may have limited the access to and sample recovery from important deformation settings, such as shear zones and fault scarps, thus explaining the relative scarcity of antigorite-bearing deformed serpentinites recovered from oceanic environments.

Atomic force microscopy studies of layer silicate minerals

Abstract The surfaces of silicate minerals are important in controlling a number of geochemical phenomena from chemical weathering, to the transport of pollutants in solution. The atomic force microscope (AFM) has proven invaluable for obtaining atomic resolution images of the surface topology and structure of a wide variety of minerals. While the physical basis for the sample—tip interactions responsible for the images remains poorly understood, comparison of AFM images with the calculated bulk mineral structures, provides clear evidence that the AFM is truely capable of resolving structural features on the atomic scale. The phyllosilicate or layered silicate minerals are particularly suited to study by atomic force microscopy because of their perfect {001} cleavage, wide range of structure types and variety of surface properties. Atomic resolution images have been obtained on lizardite, chlorite, apophyllite and muscovite. The AFM resolves the Angstrom-scale surface structure of these minerals and is able to clearly elucidate the different structural layers. These studies have been able to observe surface relaxation effects on the surface of chlorite, as well as the progressive amorphization (through ion beam damage) of the muscovite surface. Further, studies in solution are able to image the equilibrated near surface structure providing fresh insight into the nature of the mineral—water interface.