Garnet is an important rock-forming mineral of various igneous and metamorphic rocks. In the shallow part of the upper mantle, the volume fraction of garnet may reach only ~15%. In the Earth’s mantle transition zone, however, the fractions could be up to ~40 vol% for a pyrolite mantle and ~60 vol% for a piclogite mantle.
The timescales and thermodynamic conditions of a series of mantle processes is largely influenced by the diffusion behaviors of Fe-Mg in the constitutive minerals. Numerous studies have demonstrated that even a small amount of water has a huge effect on the transport properties of mantle minerals such as diffusion, electrical conductivity and rheology. However, the magnitude of the effect of water on diffusivity is still poorly constrained.
To explore the effect of water on Fe-Mg interdiffusion in garnet, a research team led by Prof. ZHANG Baohua from the Institute of Geochemistry of the Chinese Academy of Sciences (IGCAS), systematically measured Fe-Mg interdiffusion rates in garnet aggregates as a function of temperature, composition and water contents at 3 GPa by the diffusion couple method at controlled the oxygen fugacity.
The results demonstrated that the composition of garnet had no influence on the Fe-Mg interdiffusion coefficient (DFe–Mg), however, the rate of interdiffusivity in garnet containing 106–1260 weight ppm H2O was over two orders of magnitude faster than in anhydrous garnet at 1373 K.
Large water exponent (r ≈ 1.38) implied that the incorporation mechanism of water was hydrogen associated with metal and partly silicon vacancies to form neutral point defect complexes.
While at the same time, it also suggested a profound role of water in enhancing the Fe-Mg interdiffusion in garnet. Enhancement of the diffusivity by such a small amount of water was almost identical to that associated with a 300 °C increase in temperature.
The significant water-induced effect on kinetics may have important implications for evaluating the closure temperature of geospeedometry and geothermometry and the time scales of geological processes.
The work titled “Large effect of water on Fe–Mg interdiffusion in garnet” was published in Earth and Planetary Science Letters.
DFe-Mg in garnet as a function of reciprocal temperature and water content (CH2O) at 3 GPa （Image by IGCAS）
Institute of Geochemistry, Chinese Academy of Science
(By Prof. ZHANG Baohua’s group)