TiO2 nanoparticle trails in garnet: implications of inclusion pressure-induced microcracks and spontaneous metamorphic-reaction healing during exhumation

A new mechanism of cleaving-healing of garnet in metamorphic rocks during exhumation is reported. Almandine garnet with submicron multiple-phase inclusions characteristic of the GRAIL reaction, garnet + rutile = kyanite + ilmenite + low-quartz, was found by analytical electron microscopy to have non-epitaxial rutile nanoparticles distributed along internal {110} microcrack trails at the acute corner of submicron multiple-phase inclusions defined by the intersecting (110) and (011) mould surfaces. Such garnet microcracks were formed during lithostatic decompression due to a relatively high compressibility of quartz in the inclusion pockets and stress concentration at the acute corner. Spontaneous healing with accompanied formation of non-epitaxial rutile nanoparticles was activated predominantly via the GRAIL reaction due to high surface tension at the tip of microcracks and, to a lesser extent, decreasing lithostatic pressure or increasing inclusion pressure upon exhumation. This new mechanism of microcracking and healing involves stress build up, fracture propagation, mineral reaction, transport of elements to the reaction site and lowering of reaction boundaries by capillarity effect, all under the influence of thermodynamic and kinetic factors at the submicron-scale. Thus, TiO₂ nanoparticle trails in garnet provide additional information on the P–T path and may shed light on exhumation rates/mechanisms and metamorphic reactions/processes. Careful scrutiny of host minerals on the submicron scale is required to tell whether there are other metamorphic-reaction facilitated healing processes being overlooked by inappropriate techniques or being obliterated by the predominant healing processes of fluid infiltration and resorption zoning.