Rheology of host-inclusion mineral systems by in situ Raman spectroscopy

Earthquakes and stress distribution inside the earth depend on the rheology of rocks and their constituent crystals at variable conditions of temperature (T) and pressure (P). The contrast in the thermoelastic properties between a mineral inclusion and its surrounding host often leads to a positive inclusion residual pressure (P_inc) at ambient conditions, which can be used to retrieve the P and T of entrapment (e.g. Angel et al. 2015). In addition, the evolution of the inclusion residual strain and P_inc as function of P and T also provides the opportunity to explore the rheology of the crystals involved.

In this study, we explored the rheology of zircon inclusions within pyrope-rich garnet by in situ Raman spectroscopy at high T.

Because garnet has a larger thermal expansion than zircon, inclusions showing a positive P_inc at ambient conditions experience continuous relaxation upon heating until the P gradient disappears. Available equations of state (EoS) can predict the experimental results within uncertainties, implying that the system behaves purely elastically up to a certain temperature. At higher T, zircon inclusions have a negative P_inc, which increases in magnitude according to the EoS predictions. However, at T corresponding to P_inc of about -0.2(5) GPa, the residual strains deviate from those predicted by EoS and the inclusion approaches the strain of a free zircon crystal at the same T. We interpret such a deviation as the result of plastic relaxation of the system.  On cooling, a new stress gradient in the host and a positive P_inc in the inclusion developed within the same T range where a negative P_inc was observed upon heating. Importantly, the new residual strains can be predicted by the EoS only if the entrapment conditions correspond to the first T where P_inc = 0 after plastic deformation occurred. Thus, the system underwent resetting within the time-scale of laboratory experiments. In addition, multiple heating-cooling cycles carried out on the same inclusions show that the maximum negative P_inc attainable does not change within a T range of about 200 K. These results suggest that the resistance to plastic deformation (i.e. yield strength) of garnet decreases under tensile stress. Therefore, we conclude that the sign of the stress field affects the yield strength of crystals and may have important consequences on the overall rock rheology and related processes.

Financial support by the Alexander von Humboldt Foundation and the ERC grant agreement 714936 (ERC-STG TRUE DEPTHS) to M. Alvaro

Angel, R. J., Nimis, P., Mazzucchelli, M. L., Alvaro, M. & Nestola, F. (2015). Journal of Metamorphic Geology, 33(8), 801-813.