Recently, we proposed that diffusion relaxation of major elements in garnet induces pressure gradients within the crystal [1]. This is based on the fact that the molar volumes of the garnet end-members vary roughly by 10%. Consequently, the re-equilibration of a zoned garnet by diffusion will produce pressure gradients due to the elastic (nearly isochoric) response of the crystal. Diffusion experiments [2], which show apparent uphill diffusion, were used to argue that such pressure gradients do exist and that ductile accommodation of volume changes only occured in the experiments at the highest temperature (1250 °C). We examined HP garnets with coesite inclusions in order to investigate the significance of this process for natural environments. Whiteschists from the Dora Maira Massive in the Western Alps underwent eclogite facies metamorphism (3.3-4.3 GPa, 720-780 °C) during the Alpine event at 35Ma [3]. Coesite included in garnet (py0.96gr0.02alm0.02) during the HP stage was partially transformed to quartz during the subsequent, rapid exhumation (from 3.5 to 1 GPa within 2 Ma [4]). Coesite is preserved by maintaining a high pressure on the inclusion wall due to the large volume change of the phase transition. The surface of the host garnet experiences a lower pressure controlled by the exhumation P-T path. This pressure difference should induce a diffusion of major elements in the garnet surrounding the inclusion. Element distribution maps show well-defined Fe-rich, Ca-poor halos surrounding the coesite-inclusions. The observed diffusion profiles are in agreement with predictions, assuming a positive !P around the inclusions. The measured diffusion lengths (40-150 μm) agree with simple diffusion models using published diffusion coefficients. Thus, understanding the effect of pressure gradients on diffusion and, alternatively, the generation of pressure due to relaxation of chemical gradients by diffusion, is crucial for interpreting P-T-t paths of zoned minerals correctly.
[1] Baumgartner et al. (2010), GSA meeting Denver. [2]
Vielzeuf & Saúl (2010), CMP 1-20. [3] Compagnoni & Rolfo
(2003), UHP Metamorphism - EMU notes 5. [4] Rubatto &
Hermann (2001), Geology 29, 3-6.