WATERMELON TOURMALINE: MULTIVARIATE ANALYSIS OF LASER-INDUCED BREAKDOWN SPECTROSCOPY AND ELECTRON MICROPROBE ANALYSES

Watermelon tourmaline describes a variety of tourmaline with the distinctive zoning pattern of a pink core and a green rim. This project examines compositional variations in these zoning patterns, which provide a detailed record of changes in the environment of crystallization. Previous studies (Gibson and McMillan, 2019, 2020 GSA Abstracts with Prog.) using laser-induced breakdown spectroscopy (LIBS) and electron microprobe analyses (EMPA) on samples from the Otjua Mine in Namibia and Brown Derby Mine in Colorado indicated that the green zones are enriched in Mn, which is commonly purported to be the chromophore in the pink zones, which have lower Mn concentrations. Similar results were observed by Tollefson (2018 GSA Abstracts with Prog.) with the possible explanation that the oxidation state of Mn, rather than concentration, plays a large role in producing color. To better understand these zoning patterns, nine zoned elbaite-fluorelbaite tourmalines from various localities were investigated using EMPA and LIBS.

Similar locations on cores and rims of tourmaline samples were analyzed by both methods. Substitutions that control chemical zoning in tourmaline during crystallization were evaluated by plotting binary composition diagrams of the EMPA data and establishing chemical exchange vectors. Principal component analysis (PCA) of LIBS spectra was used to evaluate compositional differences between cores and rims without determination of actual elemental concentrations. Preliminary results indicate that LIBS PCA score and loading plots produce similar results as EMPA exchange vectors diagrams. For example, PCA of LIBS spectra show that cores are consistently higher in La, Al, and Si and lower in Mn, Mg, Fe, and Na relative to rims. This is consistent with the exchange vector LiAlR_2- (Henry and Dutrow, 1996) observed to describe core and rim compositions in the EMPA data. Further analysis is required to extract a more detailed record of compositional trends as they relate to changes in the environment of crystallization. Specifically, variations of trace element concentrations that are below detection by EMPA are recorded in the LIBS spectra.