When modern petrology meets advanced microstructural analysis

When modern petrology meets advanced microstructural analysis Spiess R.*1 , Festa V.2 & Tursi F.3 1 Dipartimento di Geoscienze, Università di Padova. 2 Dipartimento di Scienze della Terra e Geoambientali, Università di Bari “Aldo Moro”. 3 Dipartimento di Scienze della Terra, Università di Torino. Corresponding author email: richard.spiess@unipd.it Keywords: migmatites, clogged melt channels, garnet coalescence. Petrology and microstructural analysis are inextricably linked together. The recent paper of Festa et al. (2024) on garnet growth to large ellipsoidal crystals by coalescence within residual migmatitic metapelites is a proof for the successful interaction of modern petrological modelling and advanced microstructural analysis. The whole set of data (phase equilibrium thermodynamic modelling, EBSD analysis, EDS and BEX mapping, micro-Raman analysis) obtained by this combined approach fit together like a puzzle. It defines a model showing that melt extraction in a dynamic system stops when melt channels become clogged by coalescence of peritectic phases carried along in anatectic melt migrating towards pull-apart structures, where the melt with entrained garnet pooled as it was progressively squeezed out. The importance of deformation for melt extraction is known since the basic work of Brown et al. (1995). The amount of decompression associated with formation of extensional sites within rocks undergoing ductile deformation has been documented quantitatively by Spiess et al. (2012). Coalescence of garnet aggregates to large single crystals is invariably related to deformational rotation and surface energy reduction of individual grains (Spiess et al., 2001). Whether coalescence finally results in large single crystals or not depends on several factors. Of prime importance is the presence of a grain boundary wetting fluid/melt. Static experiments conducted in a piston cylinder apparatus at 3 GPa confining pressure and 1100°C show that any attempt to epitaxially grow single garnet crystals in platinum capsule, using a fine pyrope powder surrounding a polished pyrope rod acting as seed, fails. The absence of a fluid-saturated system prevents surface energy from becoming a driving force strong enough to crystallographically isorient the garnet crystals growing around the seed. In the migmatite studied by Festa et al. (2024) rotation of coalescing garnet grains is documented by EBSD mapping. EBSD mapping also shows that during coalescence quartz did not activate dislocations that accommodated geometric incompatibility as a consequence of garnet rotation. Instead, quartz became dissolved in contact along melt decorated grain boundaries. Internal deformation within quartz during coalescence is essentially limited by activation of dauphiné twinning. This contrasts with local activation of dislocations in garnet along presumed boundaries of coalesced garnet grains. Activation of dislocations within garnet most reasonably happened once melt channels were clogged, and a rheological change accompanied ongoing deformation within the residual migmatites. Brown M. et al. (1995) - Melt segregation in migmatites. J. Geophys. Res., 100, https://doi.org/10.1029/95jb00517. Festa V. et al. (2024) - Garnet coalescence clogs melt extraction channels in migmatite. Lithos, 472-473, https://doi. org/10.1016/j.lithos.2024.107581. Spiess R. et al. (2001) - Development of garnet porphyroblasts by multiple nucleation, coalescence and boundary misorientation-driven rotations. J. Metamorph. Geol. 19, 269-290, https://doi.org/10.1046/j.0263-4929.2000.00311.x. Spiess R. et al. (2012) - Depressurized Cavities within High-strain Shear Zones: their Role in the Segregation and Flow of SiO2-rich Melt in Feldspar-dominated Rocks. J. Petrol., 53, 1767-1776, https://doi:10.1093/petrology/egs032.