Kinetically driven successive sodic and potassic alteration of feldspar
Gan Duan, Rahul Ram, Yanlu Xing, Barbara Etschmann, Joël Brugger
Abstract
Abstract The dynamic evolutions of fluid-mineral systems driving large-scale geochemical transformations in the Earth’s crust remain poorly understood. We observed experimentally that successive sodic and potassic alterations of feldspar can occur via a single self-evolved, originally Na-only, hydrothermal fluid. At 600 °C, 2 kbar, sanidine ((K , Na)AlSi 3 O 8 ) reacted rapidly with a NaCl fluid to form albite (NaAlSi 3 O 8 ); over time, some of this albite was replaced by K-feldspar (KAlSi 3 O 8 ), in contrast to predictions from equilibrium reaction modelling. Fluorine accelerated the process, resulting in near-complete back-replacement of albite within 1 day. These findings reveal that potassic alteration can be triggered by Na-rich fluids, indicating that pervasive sequential sodic and potassic alterations associated with mineralization in some of the world’s largest ore deposits may not necessarily reflect externally-driven changes in fluid alkali contents. Here, we show that these reactions are promoted at the micro-scale by a self-evolving, kinetically-driven process; such positive feedbacks between equilibrium and kinetic factors may be essential in driving pervasive mineral transformations.