Directed garnet growth in a peraluminous pegmatoid from the Moldanubian zone in the Bohemian Massif (AT) formed almandine-spessartine garnet that crystallized at the transition from melt to subsolidus stage. One side of the garnet crystal was in contact with a solidified matrix, the other with melt, resulting in asymmetric microstructure, crystal morphology and major element compositional zoning.

Based on major element composition, three garnet growth stages are inferred: (i) magmatic stage during which garnet grew as part of the coarse-grained assemblage Pl + Grt + Ky + Bt. This growth zone has the highest Mn content and P concentrations of 0.4 – 0.5 wt%, (ii) intermediate stage with decreasing Mn, and increasing Fe, Mg and Ca contents and Mg# (=Mg/(Mg+Fe)), (iii) subsolidus stage forming garnet reaction rims with highest Mg and Ca contents and Mg#.

Growth stage (i) involves the development of sector zoning, which is defined by the colour of garnet and the presence of < 1 micrometer sized inclusions observed in optical light microscope (OM). These are dominated by phosphates in Grt{110} growth sectors and by rutile in Grt{112} sectors. Additionally, nanoinclusions of 20 – 50 nm size were identified by scanning transmission electron microscopy of garnet even for zones that appear inclusion-free in OM.

Chemical compositions obtained from electron microprobe analyses integrate over garnet and the inclusions and thus also reflect the different nanoinclusion-contents of the two garnet growth sectors. Compared to the Grt{112} sector, the Grt{110} sector is c. 0.1 wt% higher in P, 100 – 150 ppm higher in Na, and 100 – 200 ppm lower in Ti, which is in line with the prevalence of Na-containing phosphates and the comparatively lower abundance of rutile in this sector.

The following scenarios are considered for the genesis of the nanoinclusions: As sector zoning has developed during magmatic conditions of growth stage (i), overgrowth of pre-existing accessory phases is implausible, as no mechanism for selective incorporation of different phases at different facets is known. Instead, facet specific minor and trace element partitioning during garnet growth and subsequent exsolution, or alternatively, sector specific nucleation and co-growth of accessory rutile and phosphates are both reasonable explanations for the observed distribution of inclusions in specific garnet sectors. These considerations indicate facet-selective processes likely related to the different crystal structure features exposed at the interfaces in contact with the melt.

We conclude that facet specific formation of nanoinclusions is an important factor controlling the trace element composition of pegmatoid garnet apart from bulk melt composition and pT-conditions. When interpreting P, Na and Ti contents in garnet, the potential presence of nanoinclusions that are invisible in the optical light microscope needs to be accounted for, as they may be more widespread in pegmatoid garnet than expected.

The study was funded by the Austrian Science Fund (FWF): I4285-N37 and the Slovenian Research Agency (ARRS): N1-0115.