Systematic variations in shape preferred orientation and crystallographic orientation relationships of rutile inclusions in garnet upon fractional crystallization of pegmatoid melt

Almandine-spessartine garnet in a Moldanubian peraluminous pegmatoid  (Bohemian Massif, Austria) shows asymmetric morphology, compositional zoning, and microstructural zoning, indicating directed crystal growth. Sector-specific variations in inclusion abundance and microstructures in {112} and {110} garnet sectors indicate facet-specific crystallization processes, associated with individual garnet surface configurations and a compositional boundary layer (CBL) present in the melt adjacent to growing garnet (Kohn et al., 2024). Rutile inclusions show distinct changes in abundance, aspect ratio, shape preferred orientations (SPOs) and crystallographic orientation relationships (CORs) between garnet growth zones. We quantified the SPOs of > 2400 rutile needles in two crystallographically equivalent {112}_Grt rim sectors, and recorded the COR, location, habit and SPO of > 350 rutile inclusions in a transect across core and rim zones within one {112} _Grt sector.

Rutile inclusions are elongated parallel to the four ⟨111⟩ _Grt directions, the three ⟨100⟩ _Grt directions and one ⟨112⟩ _Grt direction. The most frequent SPO for a given {112} _Grt sector is the one closest to the garnet growth direction (i.e. the facet normal), whereas the SPO lying in the facet is exceedingly rare. Sectioning effects cannot explain these frequency variations. Based on the facet-specific SPO and COR statistics, we infer rutile inclusions formed by nucleation at the advancing garnet surface and subsequent co-growth with the host.

Based on directly correlated SORs and CORs between elongate rutile inclusions and garnet host, specific CORs were pooled into three COR groups: 103R/111G (“one <103>Rutile direction one <111>Garnet direction”), 001R/111G and 001R/100G. Within one {112} _Grt sector, core domains exhibit lower aspect ratios and higher abundance of rutile inclusions, with COR group 103R/111G being predominant. Contrastingly, the rim domain exhibits highly elongate rutile needles with lower abundance. The dominant COR group changes to 001R/111G and COR group 001R/100G appears. We suggest the decrease in inclusion abundance signals a decrease in the ratio of rutile nucleation rate to rutile growth rate, while the increase in aspect ratio signals an increase in the growth rate of rutile compared to the garnet growth rate (normal to the facet). The needle-bearing rim supposedly crystallized from a melt with higher Na, Si and OH^– content compared to the core (Kohn et al. 2024). A corresponding increase in diffusion rates of components in the melt is hypothesized to have decreased supersaturation with respect to rutile in a CBL, decreasing rutile nucleation rates and affecting relative growth rates. The preference for particular SPO-COR combinations should be influenced by the garnet surface configuration upon heterogeneous nucleation of rutile. Radial and lateral variations of rutile CORs and SPOs are thus attributed to changes in the nature of the garnet/melt interface, and/or the garnet growth mechanism.

Based on comparison with previous studies, changes in rutile COR group frequencies associated with increasing Si- (and likely OH^–) content of the melt are a systematic feature of magmatic fractional crystallization in peraluminous pegmatitic systems containing rutile-bearing garnet.

Funded by Austrian Science Fund (FWF): I4285-N37 and Slovenian Research Agency (ARRS): N1-0115

Kohn et al. (2024), Lithos, DOI: 10.1016/j.lithos.2023.107461