Mechanisms of serpentinization and some geochemical effects

Abstract

The thesis is presented as a collection of three scientific papers dealing with the chemical and mechanical feedbacks accompanying hydration processes in ultramafic rocks. The first paper deals with the petrological and chemical effects associated with the hydration of ultramafic rocks in the Leka Ophiolite Complex. Although, major elements present in the ultramafic rocks do not seem to be mobile on the regional scale during hydration, textural evidence shows that Fe, Mn and Ca can be transported along grain-scale distances and play an important role in the formation of metamorphic minerals. Textural observations and equilibrium thermodynamic calculations help constrain the sequence of phase transitions through the evolution of the ultramafic rocks during hydration. An important outcome of the serpentinization reactions is the volume change occurring in the affected rocks which may cause deformation. A key observation is that the major volume changes taking place in the dunites occur after the hydration of the orthopyroxenite dykes, i.e. at lower temperatures. The second paper deals with the fracturing of the orthopyroxenite dykes as a response to the stresses set up during the expansion of the dunites. The fracture patterns in the dykes have been statistically characterized as hierarchical patterns which are dominated by four-sided polygons. The fractures in such patterns develop sequentially and later fractures develop orthogonally to earlier fractures. A fragmentation model was also developed to provide insight into the number of generations of fractures in the observed patterns and the initial fracture lengths. An important observation is that the thickness of the orthopyroxenite dykes controls the density of fractures in the dyke. A 2-D FEM model was developed to study the effect of layer thickness on the fracture spacing within a layer with perfectly welded interfaces. The model shows that there exists a critical fracture spacing to layer thickness ratio (0.8-1.0) below which the horizontal stress component turns compressive, i.e. no tensile fractures can form. However, field data shows that the fracture spacing to layer thickness ratios in the orthopyroxenite dykes are below the critical value with the mean value of 0.45±0.2. The third paper investigates the effect of friction along the dyke-dunite interface on the fracture spacing to layer thickness ratios. Results from a 1-D FDM model of a layer with frictional interfaces shows that the minimum fracture spacing to layer thickness ratio is directly dependent on the ratio of the tensile and shear strength of the material. The ratios obtained for a layer with frictional interfaces can be as low as 0.10 and are in agreement with field observations.