Experimental constraints on the kinetics of the albite = jadeite + quartz reaction depending on grain size and reaction overstepping

Densification and deformation during eclogitization govern the strength and buoyancy of orogenic roots and the stability of mountain ranges over geological timespans. The breakdown of albite to jadeite + quartz represents a key end-member reaction that is associated with densification of about 20%; shear stresses induced by such volumetric changes may cause brittle failure and have been linked to intermediate-depth seismicity (Yamato et al., 2022). Eclogitization is a kinetically sluggish process that requires significant reaction overstepping and may proceed far beyond the equilibrium pressure–temperature conditions, and/or remain largely incomplete – particularly in fluid-deficient felsic crust as evidenced by field observations (Palin et al., 2017) and geophysical constraints (Hetényi et al., 2021). However, the kinetics of the albite = jadeite + quartz reaction is poorly constrained, especially regarding the roles of grain size, pressure–temperature overstepping, and reaction duration. To address this gap, we conducted high-pressure experiments using a piston-cylinder apparatus at the Institute of Geosciences, JGU Mainz. Natural albite crystals were crushed and sieved into grain size fractions between 50 and 500 µm, loaded into Au-capsules, and separated by Au-foils. A subset of experiments involved furnace-drying (~500°C) of the starting materials followed by hot-welding of the capsules to minimize atmospheric moisture contamination. In experimental stage I, pressure was initially set just below (~1 kbar) the albite = jadeite + quartz reaction boundary (Holland, 1980), followed by heating to target temperature. In stage II, pressure was increased at constant temperature to variable target pressures above the reaction to systematically explore the effect of reaction overstepping. Samples where quenched by power shutdown, and reaction progress was quantified using scanning electron microscopy (backscattered electron imaging and cathodoluminescence) based on the relative fractions of reactant albite and products jadeite–quartz. Preliminary results reveal highly variable degrees of reaction progress. Where present, jadeite–quartz occur as finely intergrown symplectites, typically decorating albite grains at the rims, as well as forming within larger albite grains. The latter textures indicate complications arising from fluid inclusions in the starting material. By combining constraints on P(T) overstep, grain size, and experimental run duration, we determine effective reaction rates for albite breakdown. These results provide end-member kinetic constraints on high-pressure transformation in fluid-deficient, coarse-grained felsic rocks, which constitute the bulk of many well-known (U)HP terranes such as the Western Gneiss Region (Norway) and the Dabie–Sulu belt (China).

References

Hetényi, G. et al. (2021). Metamorphic transformation rate over large spatial and temporal scales constrained by geophysical data and coupled modelling. Journal of Metamorphic Geology, 39(9), 1131–1143.

Holland, T. J. (1980). The reaction albite = jadeite + quartz determined experimentally in the range 600–1200°C. American Mineralogist, 65(1-2), 129–134.

Palin, R. et al. (2017). Subduction metamorphism in the Himalayan ultrahigh-pressure Tso Morari massif: an integrated geodynamic and petrological modelling approach. Earth and Planetary Science Letters, 467, 108–119.

Yamato, P. et al. (2022). Reaction-induced volume change triggers brittle failure at eclogite facies conditions. Earth and Planetary Science Letters, 584, 117520.