Publication Date:
2020-05-20
Description:
We report experimental data for Y, La, Lu and Hf diffusion in garnet, in which diffusant concentrations and silica activity have been systematically varied. Experiments were conducted at 950 and 1050 °C, at 1 atm. pressure and oxygen fugacity corresponding to the quartz-fayalite-magnetite buffer. At Y and REE concentrations below several hundred ppm we observe both slow and fast diffusion mechanisms, which operate simultaneously and correspond to relatively high and low concentrations, respectively. Diffusivity of Y and REEs is independent of silica activity over the studied range. General formulae for REE diffusion in garnet, incorporating data from this and previous studies, are: log10DREEfm2/s=-10.24±0.21-221057(±4284)2.303RT(K) for the ‘fast’ REE diffusion mechanism at 1 atm. pressure, and log10DREEsm2/s=-9.28±0.65-265200±38540+10800(±2600)·P(GPa)2.303RT(K) for the ‘slow’ REE diffusion mechanism. These slow and fast diffusion mechanisms are in agreement with previous, apparently conflicting, datasets for REE diffusion in garnet. Comparison with high-pressure experiments suggests that at high pressures (〉 ∼1 GPa minimum) the fast diffusion mechanism no longer operates to a significant degree. When Y and/or REE surface concentrations are greater than several hundred ppm, complex concentration profiles develop. These profiles are consistent with a multi-site diffusion-reaction model, whereby Y and REE cations diffuse through, and exchange between, different crystallographic sites. Diffusion profiles of Hf do not exhibit any of the complexities observed for Y and REE profiles, and can be modeled using a standard (i.e. single mechanism) solution to the diffusion equation. Hafnium diffusion in garnet shows a negative dependence on silica activity, and is described by log10DHfm2/s=-8.85±0.38-299344±15136+12500±900·PGPa2.303RTK-0.52(±0.09)·log10aSiO2 In many natural garnets, diffusion of both Lu and Hf would be sufficiently slow that the Lu-Hf system can be reliably used to date garnet growth. In cases in which significant Lu diffusion does occur, preferential retention of 176Hf/177Hf relative to 176Lu/177Hf will skew isochron relationships such that their apparent ages may not correspond to anything meaningful (e.g., garnet growth, peak temperature or the closure temperature of Lu or Hf). Late-stage reheating events are capable of causing larger degrees of preferential retention of 176Hf/177Hf relative to 176Lu/177Hf and partial to full resetting of the Sm-Nd system within garnet, thus increasing the separation between garnet Lu-Hf and Sm-Nd isochron dates, due to the fact that these systems are more significantly disturbed through diffusion as more radiogenic 176Hf and 143Nd have accumulated.
Print ISSN:
0022-3530
Electronic ISSN:
1460-2415
Topics:
Geosciences
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