Experimental calibration of oxygen diffusion in garnet and implications for retention of primary oxygen isotopic signatures

Knowledge of oxygen diffusion in garnet is crucial for a correct interpretation of oxygen isotopic sig­natures in natural samples. Scicchitano et al. (2022) reported a series of experiments with pyrope and YAG at P-T of 1-atm to 2.5 GPa and 900 °C to 1600 °C, either under nominally-dry or water-saturated conditions, to better constrain the diffusivity of oxygen in garnet. Analysis of ¹⁸O/(¹⁸O+¹⁶O) profiles by Secondary Ion Mass Spectrometry (SIMS) shows that: (i) diffusivities in pyrope and YAG crystals annealed under similar conditions (P = 1 GPa and T = 900 °C) are comparable, suggesting a limited effect of chemical composition on oxygen diffusivity; (ii) diffusivity values calculated for water-saturated experiments at 900 °C fall on the Arrhenius curve described by nominally dry experiments performed at T = 1050-1600 °C; and (iii) several profiles deviate from the Gaussian error function, suggesting complex diffusion behaviour related to diffusion via interstitial (fast) and vacancy (slow) mechanisms. Modelling this process yields oxygen diffusion coefficients, D, that differ by approximately two orders of magnitude between the fast and slow diffusion mechanisms. The new experimental data suggest, however, that the slow mechanism is prevalent in natural garnet compositions and probably controls the retentivity of oxygen isotopic signatures in natural samples. Even though oxygen diffusivity in garnet is comparable to Fe-Mn and Ca diffusivity at high temperature (> 850 °C), oxygen diffusivity is slower than cation diffusivity at P-T conditions typical of crustal metamorphism due to its larger activation energy. Original oxygen isotopic signatures therefore can be retained in garnet showing zoning partially re-equilibrated by the diffusion of other major elements.                 

 

References

Scicchitano et al. (2022), American Mineralogist, 107, 1425-1441.