Micro-tomography, LA-ICP-MS and electron microprobe investigation of garnet from Val Codera granitic pegmatite (central Alps, Italy)

Selected crystals of garnets from a granitic pegmatite dike located in the Codera Valley (Novate Mezzola, Sondrio Province, Italy) have been analysed by synchrotron X-ray computed micro-tomography, laser ablation ICP-MS and electron microprobe with the aim to describe and quantify their texture and chemistry, including trace elements and REE. The analyses were carried out on a suite of nine trapezohedral garnets, ranging from 0.2 to 1 cm in size and from wine red to reddish brown in colour. The pegmatite dike is reported in literature as "the garnet dyke" (De Michele, 1974) and outcrops for more than 30 m in length in the glacial circle of the upper Val Codera. The dike is discordant with the foliation of both Bergell tonalite and granodiorite and it is classified as "Rare Elements Pegmatite" belonging to the mixed LCT-NYF family. The main minerals, feldspar, quartz and biotite, are associated with beryl (aquamarine) and red garnet. Other subordinate minerals are tourmaline, columbite, euxenite, monazite, xenotime, uraninite, zircon and magnetite (Guastoni, 2014). The garnet crystals may be considered as gem materials but they are commonly broken and contain too many inclusions. The examined samples are mainly a solid solution of almandine (30-54%) and spessartine (36-64%), less pyrope (3-7%) and 1% of grossular with the spessartine content decreasing considerably from the core to the rim together with the decrease of MnO/(MnO+FeO) ratio. All the crystals contain inclusions of niobates, tantalates, chlorite, K-feldspar and mica. The shape, spatial distribution and dimension of the inclusions have been characterized by synchrotron X-ray computed micro-tomography that allows a 3D imaging and identification of microstructural features of the crystals. The rare earth elements show the chondrite-normalized pattern characteristic of magmatic type garnets, presenting considerable enrichment of heavy REE and strong negative Eu anomaly. The substitution of REE and Y in garnet occurs in the eightfold coordinated ?X?site that, in these garnets, is mainly occupied by Mn2+ and Fe2+, so the variation of both these elements controls REE and Y distribution. The ?REE and Y contents increase at the crystal rim (up to 900 and 1200 ppm, respectively) showing a well definite negative correlation with Mn which presents its maximum value at the core of the crystals. This result suggests that the spessartine-rich garnet crystallisation took place from a Mn-rich melt in presence of volatiles which prevented the incorporation of Y and REE in the structure favouring the crystallisation of Y and REE-rich accessory minerals.