The high-pressure, α-PbO2-structured polymorph of titanium dioxide, termed TiO2-II, was recently identified in micrometer-sized grains recovered from four Neoarchean spherule layers. The presence of shock-induced TiO2-II provides physical evidence to support an impact origin of the spherule layers. Detailed characterization of the distribution of TiO2-II in these grains could be used for both correlating the spherule layers and estimating the distances of the spherule layers from their source craters. In this study, grains from four spherule layers were investigated using Raman microspectroscopic mapping with both univariate and multivariate data analysis methods. Raman spectra of the grains provide evidence for geological matrices dominated by rutile and TiO2-II, as shown by Raman bands at 174 cm-1 (TiO2-II), 428 cm-1 (TiO2-II), 443 cm-1 (rutile), and 610 cm-1 (rutile). Initial estimates of the distribution of TiO2-II and rutile were determined by integration of the 174 cm-1 and the 285–357 cm-1 Raman bands of TiO2-II and the 240 cm-1 and 610 cm-1 Raman bands of rutile. Principal component analysis yielded a predominantly three phase system comprised of rutile, TiO2-II, and substrate adhesive epoxy. Scanning electron microscopy suggests heterogeneous grains containing polydispersed micrometer and submicrometer particles. Multivariate curve resolution-alternating least squares (MCR-ALS) applied to the Raman microspectroscopic mapping yielded two additional phases, anatase and quartz, as well as rutile and TiO2-II. Spectral profiles and spatially resolved chemical maps of rutile, TiO2-II, anatase, quartz, and substrate adhesive epoxy were generated using MCR-ALS. The enhancement of the spatial resolution of the MCR-ALS Raman microspectroscopic maps was investigated by optimizing spectral acquisition parameters in comparable analysis times. Using the resolved spectrum of TiO2-II generated from MCR-ALS analysis, a Raman spectrum for pure TiO2-II was estimated in order to further facilitate the identification of TiO2-II within complex materials.

Raman microspectroscopic mapping with multivariate curve resolution-alternating least squares (MCR-ALS) applied to a high-pressure polymorph of titanium dioxide, TiO2-II

251st National Meeting (2016)

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