To form or not to form? The epic struggle of jadeite in a water-limited world

The occurrence of jadeite in metagranites is typically interpreted as an indicator of high-pressure conditions during metamorphism. Moreover, the albite breakdown reaction to jadeite and quartz has long been employed for the calibration of piston-cylinder. It is therefore paradoxical that jadeite is frequently absent in metagranitic rocks, even when these are associated with mafic eclogites. Possible explanations for the absence of jadeite include sluggish reaction kinetics under fluid-absent conditions, retrograde overprinting, and mechanical pressure differences between mafic eclogites and adjacent granitic lithologies.

This experimental study investigates the reaction mechanisms and the role of fluid in nominally dry jadeite formation. Piston-cylinder experiments were conducted at 2.2 GPa and 800 °C on two natural granitic rocks. One sample is a fresh granite from Finland, and the other a metagranite from Monte Rosa (Western Alps) with a complex metamorphic history involving two orogenic cycles. The history of the latter led to exsolution of magmatic plagioclase into albite and zoisite and partial replacement of biotite by white mica.

While jadeite formed in the fresh granite at albite–biotite interfaces, no jadeite developed in the Monte Rosa metagranite under identical conditions. Although the albite breakdown reaction is thermodynamically overstepped, jadeite formed not through simple albite breakdown but through localized reactions at albite–biotite interfaces. The results indicate that jadeite formation is controlled by several factors, including biotite stability dependent on aluminium content, slow reaction kinetics in the absent of fluid, and the metamorphic history of the rock, which influences local equilibrium.