Emilien Oliot

Thermodynamic Modeling and Thermobarometry of Metasomatized Rocks

Determining the P-T conditions at which metasomatism occurs provides insight into the physical conditions at which fluid-rock interaction occurs in the crust. However, application of thermodynamic modeling to metasomatized rocks is not without pitfalls. As with “normal” metamorphic rocks, the main difficulty is to select mineral compositions that were in equilibrium during their crystallization. This essential task is particularly difficult in metasomatized rocks because it is often difficult to distinguish textures produced by changes in P-T conditions from those caused by fluid-rock interactions and associated changes in bulk composition. Furthermore, the selection of minerals in equilibrium in metasomatized rocks is made difficult by the great variability of scale of mass transfer (see Chaps. 4 and 5), and therefore equilibrium, which varies from micrometer- to hand-sample or larger scale, depending on the amount of fluid involved and the fluid transport mechanisms (e.g. pervasive or focused). Finally, another major limitation that is discussed in detail in Chap. 5, is that fluid composition coming in or out of the rock is unknown. Since fluid is a major phase component of the system, neglecting its impact on the phase relations might be problematic for thermobarometry. Despite these pitfalls, we describe in this contribution examples where thermobarometry has been apparently successfully applied. We emphasize that pseudosection thermobarometry is particularly suitable for metasomatized rocks because the effects of mass transfer can be explored through P-T-X phase diagrams. Application of thermodynamic modeling to metasomatized rocks requires (1) detailed mineralogical and textural investigation to select appropriate mineral compositions, (2) essential geochemical analyses to define the relative and absolute mass changes involved during the metasomatic event(s), and (3) forward modeling of the effects of mass transfer on phase relations.

Early weakening processes inside thrust fault

Observations from deep boreholes at several locations worldwide, laboratory measurements of frictional strength on quartzo-feldspathic materials, and earthquake focal mechanisms indicate that crustal faults are strong (apparent friction μ ≥ 0.6). However, friction experiments on phyllosilicate-rich rocks and some geophysical data have demonstrated that some major faults are considerably weaker. This weakness is commonly considered to be characteristic of mature faults in which rocks are altered by prolonged deformation and fluid-rock interaction (i.e., San Andreas, Zuccale, and Nankai Faults). In contrast, in this study we document fault weakening occurring along a marly shear zone in its infancy (<30 m displacement). Geochemical mass balance calculation and microstructural data show that a massive calcite departure (up to 50 vol %) from the fault rocks facilitated the concentration and reorganization of weak phyllosilicate minerals along the shear surfaces. Friction experiments carried out on intact foliated samples of host marls and fault rocks demonstrated that this structural reorganization lead to a significant fault weakening and that the incipient structure has strength and slip behavior comparable to that of the major weak faults previously documented. These results indicate that some faults, especially those nucleating in lithologies rich of both clays and high-solubility minerals (such as calcite), might experience rapid mineralogical and structural alteration and become weak even in the early stages of their activity.