A new study by Freya George and Fred Gaidies sheds light on the distribution of trace elements in medium-grade metamorphic rocks from the famous inverted Barrovian sequence of the Sikkim Himalaya. The study details results of high-resolution micro-computed tomography scanning of the whole-rock sample, and laser-ablation inductively coupled mass-spectrometry (LA-ICP-MS) raster mapping of numerous garnets representative of the whole population. Results document unexpected and decoupled zoning patterns of trace elements compared to major elements in all analysed crystals. The heavy rare earth elements in garnet document early rotation in overprinted early-grown cores, with subsequent rim-ward annular structures potentially documenting a change in the diffusivity of these elements along the pressure-temperature path and fluctuating garnet growth rates. The chromium distribution in the same garnets exhibits continuous spiral patterns, a testament to its immobility at conditions ≤560˚C and the continued rotation of garnet during growth. Trace elements in the study sample prove largely to be both spatially and temporally heterogeneous.
The dataset is the most comprehensive collected of its kind, and highlights the importance of thorough investigation of population-wide garnet chemistry prior to interpretive approaches such as garnet geochronology and trace element geothermobarometery, so as to prevent spurious results.