Formation of garnet polycrystals during metamorphic crystallization

Abstract

Garnet polycrystals may form throughout the metamorphic history of a rock, starting at the earliest stages of garnet growth when closely spaced nuclei coalesce. In mica schist from Townshend Dam, VT, electron back-scattered diffraction (EBSD) analysis shows that garnet polycrystals possess two or more distinct lattice orientations separated by high-angle boundaries (28-61°). The minimum rotational displacements required to bring these lattice orientations into concordance with each other are commonly normal to the same low-energy planes that occur as crystal faces of euhedral garnet. There is no evidence for intracrystalline deformation, and the polycrystals therefore probably represent individual garnet crystals that coalesced during growth. The boundaries cross-cut growth zoning and inclusion trails of the polycrystals, indicating that early-formed polycrystals, once coalesced, behave chemically and physically as single crystals. Statistical analysis of a 3D, high-resolution X-ray computed tomographic data set of a large sample (912 cm 3 ) of a Townshend Dam schist, combined with microprobe and EBSD analyses of garnet, are consistent with a high degree of clustering at all stages of garnet growth. The formation and prevalence of polycrystals implies that garnet nuclei impinged on each other and coalesced, and that coalescence was a common feature throughout garnet growth in the rock.

Intergranular diffusion rates from the analysis of garnet surfaces: Implications for metamorphic equilibration

“…There is no evidence from the surface morphology of the porphyroblasts that they might represent multiple crystals (cf. Whitney, Goergen, Ketcham, & Kuze, ). The growth steps often mirror the topography of the matrix on the opposite surface (Figure d).…”

EBSD in the Earth Sciences: Applications, Common Practice, and Challenges

“…Interestingly, the majority of EBSD work on metamorphic problems relates to microstructures of the mineral garnet (e.g., Hawkins et al 2007;Hirsch et al 2003;Mainprice et al 2004;Prior et al 2002;Spiess et al 2001;Storey and Prior 2005;Terry and Heidelbach 2004;Trepmann and Stockhert 2002;Whitney et al 2008). Garnet is an important metamorphic mineral in the earth's crust and mantle, not because it is volumetrically significant, but rather because it is a mineral on which we often depend for estimates of rock pressure and temperature and for dating of metamorphic events, and because we interpret inclusion fabrics in terms of the rocks' thermomechanical history (see introduction in Whitney et al 2008). The focus of EBSD studies on garnet may relate to the fact that garnet is optically isotropic, and until the mid 1990s we were blind to the full range and complexity of garnet internal microstructures.…”

“…The focus of EBSD studies on garnet may relate to the fact that garnet is optically isotropic, and until the mid 1990s we were blind to the full range and complexity of garnet internal microstructures. One extremely important outcome of these studies is the demonstration that we can develop large garnet grains by the coalescence of many smaller grains Whitney et al 2008) grown from distinct nuclei; and that the misorientations between grains may be nonrandom, due to selection processes or rotations that modify the microstructure.…”

Porphyroblast crystallization: linking processes, kinetics, and microstructures

“…Hirsch et al 2003), but electron backscatter diffraction (EBSD) measurements can provide unambiguous confirmation (or contraindication) of multiple proximal nucleation events, even in rare circumstances in which significant reorientation and/or recrystallization has caused partial amalgamation of multiple Downloaded by [Stanford University Libraries] at 17:37 26 September 2012 originally independent crystals by reducing misorientations among them (Spiess et al 2001;Dobbs et al 2003). For garnet porphyroblasts, polycrystalline aggregates resulting from impingement during growth are common and well-documented, for example by Chernoff and Carlson (1997), Hirsch et al (2003), Meth and Carlson (2005), Whitney et al (2008), and others. For QTA, correction techniques have been implemented, based on observability criteria that account for the likelihood of failing to recognize polycrystalline aggregates as separate crystals; these improve assessments of nucleation suppression and growth suppression in such instances (Ketcham et al 2005).…”