The memory of crystals: microstructures in UHP garnets from Dora Maira

Microstructures within rocks and crystals describe their past deformation conditions, which may vary in time and space even within a single rock sample. This variation depends on pressure, temperature and strain rate, and determines whether minerals deform in a brittle or ductile manner. Investigating deformation mechanisms under extreme conditions, such as ultra-high-pressure (UHP), is particularly useful. Indeed, the analysis of well-preserved UHP rocks provides insights into crystal’s behavior over a wider range of pressures. For these reasons, we studied Dora Maira whiteschists, and specifically the pyrope crystals forming these rocks.

Dora Maira is one of the internal crystalline massifs of the European Western Alps, formed by HP and UHP units. The latter is famous for the presence of coesite-bearing whiteschists. These rocks are foliated, with a spatially variable foliation defined by the shape preferred orientation of phengite and garnet crystals. Garnet grains can be either rounded or elongated and show different sets of fractures. Moreover, garnet crystals are locally recrystallized.

The first set of garnet fractures is represented by parallel fractures oriented at high angle with respect to the main rock schistosity and affecting garnet crystals in the entire outcrop. These fractures are locally associated with another set developed at ca. 45°, formed together with small (µm-scale) rotating volumes of garnet. The parallel fractures are dislocated by the local recrystallization of some garnet grains and by radial fractures developed around coesite/palisade quartz inclusions. These radial fractures formed due to the large volumetric change happening at the coesite-quartz transition.

We analyzed the described microstructures using optical microscope and SEM in (HR)-EBSD mode. Additionally, we investigated garnet crystals’ composition with SEM-EDS and microprobe. In this contribution, we show the results of this combined analysis.

Our results provide new microstructural evidence that garnet can record alternating brittle and ductile deformation under UHP conditions. Besides, we document a correlation between deformation-related microstructures and major-element redistribution within garnet, highlighting the deep connection between these two aspects which was previously underestimated.