Indicator reactions of K and Na activities in the upper mantle: Natural mineral assemblages, experimental data, and thermodynamic modeling

Abstract

The paper presents a review of data on mineral assemblages and reactions that are potential indicators of K and Na activities in upper mantle fluids and melts modifying upper mantle rocks in the course of mantle metasomatism. Results of experimental modeling of these reactions are discussed. These data are utilized to calculate phase reactions in

\(\log \left( {{a_{{H_2}O}}} \right) - \log \left( {{a_{{K_2}O}}} \right)and\log \left( {{a_{{H_2}O}}} \right) - \log \left( {{a_{N{a_2}O}}} \right)\)

space by minimizing the Gibbs free energy (constructing pseudosections). The calculations of this type make it possible to estimate variations in K and Na activities in processes modifying upper mantle rocks, to predict successions of mineral assemblages that are formed when these parameters vary, and to compare metasomatic processes in rocks of various composition. The approach is illustrated by examples of peridotite and eclogite xenoliths in kimberlite and alkaline basalt.