Rutile inclusions in garnet: Inclusion microstructure as monitor of pegmatoid melt fractionation

The investigation of rutile inclusions in garnet host crystals is a rewarding topic, as this host-inclusion system may form in igneous and metamorphic lithologies in a wide range of rock compositions and pressure - temperature conditions. Different formation mechanisms have been discussed for the genesis of rutile inclusions in garnet, as for example overgrowth of pre-existing rutile by garnet, co-growth of host and inclusion phases, or exsolution of rutile from titanium-bearing garnet host crystals (Kohn et al., 2024b, with references).
The investigated rutile-bearing almandine-spessartine garnet crystal from the Moldanubian Gföhl Unit Bohemian Massif, AT, formed in three growth stages during fractional crystallization of a pegmatoid melt (Kohn et al., 2024a). Here, we focus on the first magmatic growth stage, which formed the core and inner rim domain of garnet, without significant changes in pressure-temperature conditions. This is reflected by the lack of an abrupt change in the major garnet components across the core-rim boundary of garnet. Instead, a gradual change of major element composition continues undisturbed across the boundary. However, the microstructural change (size and habit) of rutile inclusions is significant: while the coloured garnet core bears equant rutile inclusions (80 - 200 nm), the uncoloured rim is dominated by needle-shaped rutile (< 150 µm length, c. 200 nm width) with clear shape preferred orientations (SPOs). The microstructural differences correlate with abrupt changes in trace element composition of garnet (Na₂O and OH^-content relatively higher in the rim than in the core), which are referred to compositional changes of the pegmatoid melt during fractional crystallization. 
The synchronous formation of rutile inclusions and their garnet host crystal can be demonstrated on the basis of a statistical dataset of the SPOs of rutile inclusions with high aspect ratio, showing a selective effect of the local garnet growth direction on the observed SPO frequencies (Kohn et al. 2024b). Changes of the rutile inclusion habit from equant to acicular signals changes in the relative growth rates of garnet and rutile, as well as the nucleation rate of rutile, resulting from a reduction of the melt viscosity. 
Comparable microstructural changes (equant to acicular rutile inclusions, from the core to the rim of garnet) are also documented in a metapegmatite from the Austroalpine Crystalline Basement, Koralpe,  AT (Griffiths et al., 2020), separated in time and space from the studied lithology. Therefore, we conclude that the described microstructure of rutile inclusions in pegmatoid garnet is a potential marker of changes in melt properties upon fractional crystallisation of pegmatitic melts, which are not documented by the major garnet components.

Funded by Austrian Science Fund (FWF): I4285-N37.

References

Griffiths T.A. et al (2020) American Journal of Science 320:753–789

Kohn V. et al (2024a) Lithos 466–467, 107461

Kohn V. et al. (2024b) Contributions to Mineralogy and Petrology 179, 69