Quantitative micro‐X‐ray fluorescence scanning spectroscopy of wet sediment based on the X‐ray absorption and emission theories: Its application to freshwater lake sedimentary sequences

Abstract

Micro‐X‐ray fluorescence scanning spectroscopy of marine and lake sedimentary sequences can provide detailed palaeoenvironmental records through element intensity proxy data. However, problems with the effects of interstitial pore water on the micro‐X‐ray fluorescence intensities have been pointed out. This is because the X‐ray fluorescence intensities are measured directly at the surfaces of split wet sediment core samples. This study developed a new method for correcting X‐ray fluorescence data to compensate for the effects of pore water using a scanning X‐ray analytical microscope. This involved simultaneous use of micro‐X‐ray fluorescence scanning spectroscopy and an X‐ray transmission detector. To evaluate the interstitial pore water content from the X‐ray transmission intensities, a fine‐grained sediment core retrieved from Lake Baikal (VER99‐G12) was used to prepare resin‐embedded samples with smooth surfaces and uniform thickness. Simple linear regression between the linear absorption coefficients of the samples and their porosity, based on the Lambert–Beer law, enabled calculation of the interstitial pore spaces and their resin content with high reproducibility. The X‐ray fluorescence intensities of resin‐embedded samples were reduced compared with those of dry sediment samples because of: (i) the X‐ray fluorescence absorption of resin within sediment; and (ii) the sediment dilution effects by resin. An improved micro‐X‐ray fluorescence correction equation based on X‐ray fluorescence emission theory considers the instrument's sensitivity to each element, which provides a reasonable explanation of these two effects. The resin‐corrected X‐ray fluorescence intensity was then successfully converted to elemental concentrations using simple linear regression between the data from micro‐X‐ray fluorescence scanning spectroscopy and from the conventional analyzer. In particular, the calculated concentration of SiO2 over the depth of the core, reflecting diatom/biogenic silica concentration, was significantly changed by the calibrations, from a progressively decreasing trend to an increasing trend towards the top of the core.

Tools for uranium characterization in carbonate samples: case studies of natural U–Pb geochronology reference materials

“…While optical orientation is preserved and fluid inclusions and aragonite relict mineral inclusions delineate the original acicular growth habit, the carbonate is almost entirely replaced by cloudy brown calcite (Mazzullo, 1980). This neomorphic replacement may facilitate retention of U (Kelly et al, 2003), or perhaps the organic matter (that gives the sample its color and pungent smell when broken) complexed U and retained it in the calcite. Remarkably, Sr is lost well before U in this system, as evidenced by the rather homogeneous U concentration at about 4.5 ppm (which makes this a good U-Pb reference material) and the highly variable Sr (Fig.…”

“…The TES beamline has a monochrometer that is very stable, allowing for hours worth of scanning without drift. Speleothem from a cave in Austria (Kelly et al, 2003) that has U 6+ for reference. Figure 9.…”

“…However, aragonite, which is metastable, does not exclude U. Reeder et al (2001) and Kelly et al (2003) suggested that high U contents in a speleothem calcite (which we use as a U 6+ standard) might have been inherited from high concentrations in primary aragonite and preserved during neomorphic inversion to calcite. No experimental studies for U incorporation have been reported for dolomite, but it is unlikely to have a higher affinity for U than calcite as they have the similar structures and there is more of a size mis-match with Mg than Ca.…”

“…Laser ablation mapping (Woodhead et al, 2010(Woodhead et al, , 2007Piccione et al, 2019;Drost et al, 2018;Roberts et al, 2020), synchrotron XRF mapping (Cole et al, 2004;Piccione et al, 2019;Frisia et al, 2008;Vanghi et al, 2019), PIXE (Ortega et al, 2005(Ortega et al, , 2003, µXRF (de Winter et al, 2017), and EMPA (Suzuki et al, 2016) permit tying petrographic observations of different phases to in situ chemical maps of U and other elements that inform fluid chemistry. For the purpose of U-Pb dating, knowledge of the mineral lattice location of U and its oxidation state and speciation provide a fundamental framework for interpreting ages, as well as revealing details of the fluid chemistry (Kelly et al, 2006(Kelly et al, , 2003Sturchio, 1998;Cole et al, 2004). In some cases, uranium minerals are encapsulated in calcite, which can protect them from alteration (Suzuki et al, 2016;Ludwig, 1978).…”

A Sample Characterization Toolkit for Carbonate U-Pb Geochronology

“…15-25 µm) with laboratory X-ray sources. µXRF has been used for chemical imagining in diverse fields, including materials characterization, archaeology, and earth sciences (de Allwood et al, 2018;Katsuta et al, 2019;Haschke , last;, and related instruments will be deployed on interplanetary spacecraft (Williford et al, 2018). Qualitative and semi-quantitative imaging and standardless analysis is rapid and versatile, but quantitative composition determination of carbonate rocks is challenging due to the X-ray-transparent carbonate matrix (de .…”