High-Resolution, All-Element Imaging of a Garnet Grain with the icpTOF

The animation below shows a series of trace element maps for a 5×7 mm garnet grain in a micaschist from the South Carpathians, Romania.   The images were recorded with an icpTOF  coupled to a CETAC Analyte G2 193 nm excimer laser (3 J/cm2 fluence, 20 µm spot size, and 100 Hz laser repetition rate) equipped with a HelEx II cell and an ARIS device for fast washout.   With this setup the washout time could be optimized to ~10 ms. Combining this fast wash out with the speed of the icpTOF enabled all-element imaging of garnet grain in just 55 minutes.



Each pixel in these image represents the integrated signal from a single laser shot.  The icpTOF inherently acquires a complete mass spectrum (all elements) for each pixel.  Here, only the maps for Ti and the rare earth elements (REE) are shown.

The images are intensity maps, but they have also been quantified against the NIST SRM 612 reference material using the Iolite software. The Ti map reveals abundant micro-inclusions of rutile within the garnet crystal.  This spectacular “rutile dust” shows syn-kinematic growth of the garnet core, followed by slow static growth of an inclusion-free mantle and extensive growth of post-foliation rims. The REE show remarkable zoning patterns. The intensities increase from rim to core, and from light to heavy REE, while also recording spectacular ring-shaped compositional oscillations, especially for gadolinian lanthanides. This zoning pattern originates from an interplay between fractionation in garnet according to lattice compatibility, and elemental supply ensuing breakdown of lanthanide-bearing phases in the rock matrix. The elemental correlations enable highlighting of substitutional mechanisms in garnet.

These experiments were conducted in collaboration with Ciprian Stremtan of Teledyne CETAC Technologies and Gavril Săbău of the Geological Institute of Romania.  The data were first presented as an entry in the NAWLA 2017 Laser Ablation Image Contest.