Porosity generation via spatially uncoupled dissolution precipitation during plagioclase replacement in quartz undersaturated fluids

Abstract

The replacement of feldspars is commonly characterized by pseudomorphism and reaction-induced pore generation. However, the effects of compositions of feldspars and fluids on porosity generation during alteration are still poorly understood. In this study, we conducted a series of hydrothermal experiments on plagioclase replacement by 2 M KCl or NaCl aqueous solutions at 600 °C and 150 MPa for 1–8 days, using plagioclase with different compositions (anorthite, An96Ab4; labradorite, An66Ab33Or1; albite, An1Ab99) with or without quartz. Albite replacement by K-feldspar was not affected by the presence of quartz, whereas anorthite was unaltered in the quartz-absent fluid. The replacement of labradorite by KCl(aq) showed different results: in the presence of quartz, labradorite was altered by K-feldspar, whereas in the absence of quartz, alteration proceeded significantly with the generation of large pores hosted by secondary anorthite coupled with euhedral K-feldspar overgrowth. Such textural relationship and oxygen isotope-labeled experiments reveal that silica-deficient fluid enhances the uncoupled dissolution reprecipitation process. The Si and Al ions in the reacted aqueous solution diffused outside the labradorite grains and encountered K+-rich solutions to grow K-feldspar. The experiments with polycrystalline rocks composed of amphibole + labradorite using 2 M KCl aqueous solution indicated the replacement of labradorite grains by anorthite and K-feldspar overgrowth, as found in single-crystal experiments. Our results indicate that the silica concentration in the fluids has different influences on the saturation indices of albite, anorthite, and K-feldspars in saline fluids, which significantly affect the replacement textures and porosity generation in crustal rocks.

Fracture evaluation of the plutonic basement in the Upper Magdalena Basin: Implications for the development of naturally fractured reservoirs in the Northern Andes

“…However, the porosity is concentrated in biotite and microfractures (see Figure 9b,c). Porosity formation may result from the replacement of Ca-plagioclase with more stable Na-plagioclase or K-feldspar and the formation of illite aggregates (Nurdiana et al, 2024;Staněk & Géraud, 2019). Dykes of mafic to intermediate composition show less porosity values, associated with filled veins of iron-oxide minerals (Staněk & Géraud, 2019).…”