The role of water on the reaction kinetics of a nominally dry reaction: Experimental study on the albite breakdown to form jadeite and quartz

The albite breakdown reaction to jadeite and quartz is frequently utilized for the calibration of piston-cylinder apparatuses due to its well-established and highly precise equilibrium conditions. In nature, jadeitic pyroxene serves as an indicator for high-pressure conditions during metamorphism. Quite a few granites can be found in association with mafic eclogites, so that jadeite + quartz would be expected to be common. While jadeite has been documented in a number of locations, it is often absent in continental high-pressure rocks. Several possible explanations for the absence of jadeite have been proposed; a) the nominally dry reaction may be kinetically too slow in the absence of free water in the system; b) complete retrogression of jadeite paragenesis by later lower pressure metamorphism, or c) that the pressure gradients between mafic eclogites and granites lead to P-T conditions that prevent jadeite formation.

This experimental study aims to determine whether jadeite can form without free water or if slow reaction kinetics hinder its formation. Isochemical phase diagrams have been calculated within the NCKFMASH system to model the water acitivity. It is shown, that biotite breakdown at elevated P-T conditions may provide free water on the grain boundaries even at nominally dry experimental conditions, thus increasing water activity and passively promote jadeite formation. To investigate this effect, three sets of piston-cylinder experiments are conducted using natural granite (100-400 µm). The experiments were annealed just outside the jadeite stability field to eliminate grain boundary water introduced by the powder, before exposing them to eclogite facies conditions. Kyanite is added in some experiments to induce the formation of white mica, thereby reducing the water activity by adsorbing the water in the crystal lattice.

The results of this study will elucidate whether jadeite formation is kinetically inhibited in the absence of free water. If jadeite forms only when free water is present during the reaction, it can be concluded that indeed sluggish kinetics, driven by the absence of free water, impede jadeite formation in high-pressure metagranites.