Anticipated Materials and Methods
October 28, 2022
For this project, I plan to analyze three ultrahigh-pressure garnet porphyroblasts with diamond inclusions collected from migmatitic pelitic gneiss in the central Rhodope Mountains, Bulgaria. I aim to obtain 10 maps from each garnet of the following elements: 140Ce, 232Th, 90Zr, 89Y, 153Eu, 172Yb, 175Lu, 238U, 47Ti, and 52Cr. I selected 140Ce and 232Th as representatives of the LREEs and to study the relationship between garnet and monazite. I selected 90Zr, 89Y, 172Yb, 175Lu, and 238U as representative HREEs and to study the relationship between garnet and zircon. I selected 47Ti because titanium has been demonstrated to preserve zonation lost at extreme temperature conditions due to intracrystalline diffusional relaxation (Ague and Axler, 2016). Raimondo et al. (2017) achieved maps with pixel sizes of approximately 7 x 16 μm, and I hope to create similar or higher quality maps.
Individual garnet grains have already been mounted in epoxy, cut, and polished for analysis. I plan to follow a modified version of the protocol outlined in Raimondo et al. (2017). Additionally, I plan to use the programs Iolite and XMapTools 2.3.1 for post-acquisition image processing of the element maps (Woodhead et al., 2007; Hellstrom et al., 2008; Paton et al., 2011; Lanari et al., 2014; Raimondo et al., 2017).
A primary concern for trace element mapping is that the selected elements are informative and show zonation across the garnet grain. In order to ensure that the elements I selected are going to be worth mapping across the entire grain, I plan to run test spots on the core and rim of a representative test garnet to ensure that some kind of zonation is present. If testing reveals identical concentrations between the core and rim of the test garnet, I can adjust the targeted elements to a more informative suite.
I am planning on evaluating the optimal resolution for the larger grains by mapping sections along edge-core-edge line scans at different pixel sizes. This will also help evaluate how much, if any additional information gained from 2D mapping over line scans. A downside of producing line scans over 2D maps could be that odd zonation patterns, including spiral zonation, or asymmetric zonation could be missed in a line scan. However, line scans are much cheaper and faster to produce than 2D maps.
Accuracy and precision of both line scans and 2D maps will be assessed using standards. Commonly used standards for trace element garnet mapping include NIST 612 and NIST 610 (e.g., Ulrich et al., 2009; Zhai et al., 2014; Raimando et al., 2017; Zhang et al., 2017; Rubatto et al.,2020).