First high-pressure laboratory experiment on host-inclusion systems

In his new manuscript Nicola Campomenosi describes the results of the first laboratory experiments where quartz in garnet host-inclusion systems have investigated under high pressure conditions using Diamond Anvil Cell (DAC).

Sketch of the real P–T path experienced by a cubic host (blue solid line) subjected to an external pressure Pext, i.e., Phost = Pext, as well as of different P–T paths of an anisotropic inclusion from the entrapment (TtrapPtrap) to the final state (TendPend), while the host experiences an external pressure Pext; the dashed line corresponds to the virtual unrelaxed P–T path of an inclusion forced to fit the cavity of the host during the exhumation. The grey solid line corresponds to the real relaxed P–T path experienced by the inclusion upon exhumation. Pinc (unrel) and Pinc (rel) are derived from the unrelaxed and relaxed residual strain, respectively

The experiment has been carried out to elucidate the behaviour of QuiG at elevated pressures. Three quartz in garnet (QuiG) samples with Pinc close to 0 GPa at room conditions have been investigated.

We provide the first experimental laboratory evidence to demonstrate that upon pressure increase, the garnet host acts as a shield to the softer quartz inclusion. Consequently, the Pinc increases with a smaller rate compared to that of the external pressure.

Up to 2.5 GPa, the evolution of Pinc calculated from the Raman data agrees very well with prediction from the equations of state. The behaviour of a quartz inclusion in a relatively thin host samlpe was explored up to external pressures of 7 GPa.

A Pressure dependence of the FWHM of the 207 mode for a free quartz (red triangles, data from Morana et al. 2020) and for a QuiG (sample S5). The changes in slope are interpreted as indicating the conditions under which quartz (qz) becomes metastable with respect to coesite (coe). The quartz inclusion metastability begins at Pext of ~ 4.4 GPa about 2 GPa above the free crystal. B Pressure dependence of the peak position of the E128 mode for the QuiG sample S5. Above ~ 4.4 GPa, this peak splits in two non-degenerate components (see the blue and green pseudo-Voigt functions in the inset). Red dashed lines represent the pressure of the quartz–coesite thermodynamic phase boundary at ~ 300 K according to Bose and Ganguly (1995). Grey solid lines represent the Pext interpreted as the beginning of quartz metastability with respect to coesite for this sample according to the pressure dependence of the FWHM of the 207 mode.

Our results indicate that the shielding effect of the host can maintain the quartz inclusions thermodynamically stable up to about 2 GPa above the equilibrium quartz–coesite phase boundary.

In addition, the partial shielding leads to the development of anisotropic symmetry-breaking stresses and quartz inclusions undergo a reversible crossover to a lower symmetry state. Given that the presence of non-hydrostatic stress may influence the quartz-to-coesite phase boundary, especially at elevated temperatures relevant for entrapment conditions, our results emphasize the importance of elastic anisotropy of QuiG systems, especially when quartz inclusion entrapment occurs under conditions close to the coesite stability field.