TiO₂nanoparticle trails in garnet: implications of inclusion pressure‐induced microcracks and spontaneous metamorphic‐reaction healing during exhumation

Abstract

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 ¼ 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, TiO 2 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.

Origin of rutile needles in star garnet and implications for interpretation of inclusion textures in ultrahigh‐pressure metamorphic rocks

“…d) were also observed in samples G1 and G2‐P. In addition, submicrometre‐sized facetted multiple‐phase‐inclusion of zircon ± kyanite ± quartz ± brookite ± aluminium hydroxide (Ti‐akdalaite) (not shown), as well as trails of titania nanocrystals lying parallel to the {110} grt plane (not shown), similar to those reported in the garnet of Tananao Metamorphic Complex, Taiwan (Hwang et al ., , ), are also omnipresent.…”

Rutile inclusions in garnet from a dissolution‐reprecipitation mechanism

“…Focusing observations throughout the thin section thickness show that rutile crystals in such domains in fact lie in the diffused bands close to the ~{110} grt vicinal cleavages/cracks (Figure b,c,e), which could represent the healed decompression cracking due to volume mismatch between garnet and quartz, or coesite–quartz transformation. (These oriented rutile domains are closely similar to the nanosize rutile trails decorating the healed {110} grt radial cracks surrounding submicron polyphase inclusions in garnet from the Tananao metamorphic complex; Hwang, Shen, Yui, & Chu, . )…”

Thermodynamic equilibrium at heterogeneous pressure

“…Ruling out a magmatic origin of the myrmekite, they conclude that it formed during local stress differences on K-feldspar grain boundaries. Hwang et al (2007) showed the interplay of crack formation due to stress concentrators around inclusions and the GRAIL reaction (garnet + rutile = kyanite + ilmenite + qtz), known for its barometric purposes (e.g., Bohlen et al 1983). These studies suggest that pressure variations have not dissipated during metamorphic reactions and therefore do not occur independently from reactions.…”