Tracking mechanical Dauphiné twin evolution with applied stress in axial compression experiments on a low-grade metamorphic quartzite

Abstract

The stress-dependent evolution of mechanical Dauphiné twinning has been investigated in axial compression experiments on a low-grade metamorphic quartzite, applying both time-of-flight neutron diffraction and electron backscatter diffraction. The data of the experimentally stressed quartzite samples were compared with those of the naturally deformed starting material to monitor Dauphiné twinning in relation to different experimental stress states. This comparison shows that in the experimental conditions of 500°C temperature and 300 MPa confining pressure, Dauphiné twinning initiates below 145 MPa differential stress and saturates between 250 MPa and 460 MPa differential stress. A single grain orientation analysis (SGOA) has been developed based on the distinction of quartz grains free of Dauphiné twin boundaries (DTBs) and containing Dauphiné twin boundaries. Comparing pole figures and inverse pole figures of DTB-free grains of the starting material with those of the experimentally stressed samples shows a significantly different orientation distribution of the positive {1011} (r) and the negative {0111} (z) rhombs. In DTB-containing grains, the SGOA allows to distinguish between host and twin domains. Using DTBfree grains, the SGOA furthermore reveals a particular pattern, with one of the r rhomb maxima parallel to the axial compressive stress direction and a girdle with two r rhomb submaxima perpendicular to it. We believe that this relationship between the axial compressive stress direction and the rhomb orientation distribution shows the potential of the SGOA in the reconstruction of the paleostress state in naturally stressed quartz-bearing rocks.

Preferred Orientation of Quartz in Metamorphic Rocks from the Bergell Alps

“…All of these samples are Triassic quartzites. Similar high ratios were observed for experimentally stressed quartzites [143] that we show for comparison as sample Br in Table 2C.…”

“…After discussing the geological significance of these observations of quartz preferred orientation, let us look at some methodological aspects. As was mentioned earlier, a range of methods were used to investigate quartz textures, ranging from optical methods [148,149], conventional X-ray diffraction [19,65], synchrotron X-ray diffraction [150,151], neutron diffraction (ranging from monochromatic [149,152] to time-of-flight [49]), and electron backscatter diffraction (EBSD) [79,143,144,146]. Results from different methods were compared [25,49,63,149,153,154].…”

“…Mechanical twinning causes a microscopic distortion of the lattice without significant macroscopic strain and occurs during applied stress to reduce the strain energy to switch the stiff (0111) pole parallel to compression to the soft (1011) pole, without changing the orientation of the c-axis, {1010} and {1120} orientations. In polycrystalline aggregates twins form rather easily at moderate stresses [20,142,143] and twin boundaries propagate through crystals [144].…”

The Anatomy and Origin of a Synconvergent Grenvillian-Age Metamorphic Core Complex, Chottanagpur Gneiss Complex, Eastern India

“…For instance, the z orientation lacks orientations parallel to the vorticity axis (center of pole figure) as well as along the upper left and lower right margins of the pole figure. Both experimental [85] and observational [86][87][88] studies have shown that rhomb-dominated fabrics may develop even in the absence of strong c-axis fabrics. This pattern reflects Dauphiné twinning (a 60°rotation around the quartz crystallographic c-axis) triggered by a response to stress.…”

Residual lattice strain in quartzites as a potential palaeo-piezometer

“…These twins are common in quartz, both as growth twins and as deformation twins (e.g. Minor et al 2018).…”