Speleothems are mineral precipitates found primarily in karstic caves which are formed by infiltration of water from surficial precipitation. The most common paleoclimate proxies use isotopic ratios of U-Th and δ18O to measure deposition timing as well as precipitation variations, respectively. By measuring flux in precipitation patterns such as temperature and frequency it’s possible to reconstruct historic climate conditions through environmental models. Measurements of isotopic ratios require mass spectrometry where samples need to be micro drilled, micro milled, or ablated by laser. This destructive technique of measurement is necessary but can be plagued by complications such as secondary mineral alterations where unstable aragonite converts to calcite creating erroneous isotopic ages.In this study we investigate the use of correlative chemical imaging techniques to nondestructively characterize areas most suitable for accurate isotopic ratio measurements. We demonstrate that non-destructive full sample elemental and mineralogical imaging can improve selection of suitable locations line profiling as well as spot analysis.Polished speleothem slabs from karstic caves in Morocco and China were imaged using quantum cascade laser direct infrared spectroscopy creating maps based on mineralogy contrast. Micro x-ray fluorescence (µXRF) is then used to create elemental distribution maps. Additionally, Bragg diffraction peaks were mapped to visualize phase changes and delineate crystal grains. Scanning electron microscopy was used to capture backscatter imagery as well as produce high resolution energy dispersive x-ray maps in regions of interest where smaller spot size allowed for better resolving power of fine details. Using each imaging method correlatively provides new and helpful insight when selecting line profiles for isotopic measurements.

The characterization of rocks and minerals often benefit from multiple techniques used in concert. Unfortunately, many of the most common techniques used are destructive and labor intensive such as powder x-ray diffraction and petrographic microscopy of thin sections. Nondestructive and rapid methods of analysis therefore can be a beneficial tool to use in conjunction with more traditional methods of identification in order to expediate and simplify the process. Raman spectroscopy has the advantage of limited sample preparation with the drawback of limited signal and therefore slow acquisition times. FTIR spectroscopy is usually utilized in reflectance mode by attenuated total reflectance and is limited by the relatively large crystal posing limits on spatial resolution.Recently quantum cascade lasers (QCL) have become commercially available and are offered in an increasing number of infrared spectroscopy instruments by multiple manufacturers. QCLs offer a tunable quasi-monochromatic excitation source enabling greater signal at each discrete wavelength of interest over traditional thermal sources. Increased signal to noise enables relatively short acquisition times creating a promising new way to acquire infrared spectrum in the fingerprint mid-IR region. An additional benefit is that while samples require smooth and parallel faces, they don’t need to be brought to thin section. Currently applications are limited and often narrow in scope, in part due to the relatively short commercial lifespan of the technology.For this study we performed chemical imaging on a variety of samples up to 27 x 72 mm using QCL technology creating hyperspectral and multispectral datasets. Results were correlated to conventional imaging techniques such as polarized light microscopy, fluorescence microscopy, micro x-ray fluorescence and scanning electron microscopy. By utilizing multidimensional analysis such as principal component analysis we aim to produce a framework by which other analysts can utilize QCL instruments in novel ways, specifically within the field of mineralogy and geochemistry. Results show promising potential for mineral phase mapping, grain mapping, and grain morphology with limited sample preparation compared to traditional techniques.

U is a naturally occurring heavy metal that exists primarily in granites, granodiorites, and their sedimentary derivatives such as sandstones. In reducing environments U is relatively immobile and can be deposited as its tetravalent form U(IV) in minerals such as uranite (UO2). Conversely, oxidizing environments may result in oxidation and subsequent dissolution of U(VI), which is ≥ 10,000 times more soluble. Once dissolved into groundwater, the now mobile U can enter drinking wells and result in elevated concentrations in residential drinking water.Jacobsville Sandstone is a red and mottled sandstone which lines the northern shore of Michigan’s Upper Peninsula deposited as basin fill during the 1.1 Ga Midcontinent Rift under reducing conditions which aid in the deposition of U(IV) in minerals. Previous studies have shown an elevated level of naturally occurring U and As in drinking water as high as 415 µgL-1 and 13 µgL-1 respectively, harvested from aquifers where Jacobsville Sandstone constitutes the underlying bedrock. The same studies indicate correlation between well bottom height and concentration of U suggesting a heterogenous distribution. Rock samples were obtained from three different locations in the region where Jacobsville Sandstone is exposed. By performing large area scans with µXRF we were able to determine points of interest which were further investigated with SEM-EDS where smaller spot sizes allow for better resolving power of fine details in matrix particles and cement between sandstone grains. Utilizing both methods of analysis allowed us to investigate mineralogic and elemental composition of samples revealing V, Ce, Nd, and La co-existing with U in matrix particles bound to cement between silicate grains along with As bearing minerals.