Natural Geological Samples Research Articles

Calibration of buried NaI(Tl) scintillator detectors for natural radionuclide measurement based on Monte Carlo modelling

Measurements of naturally occurring concentrations of 40K, and of the decay series of 238U and 232Th, are of interest in the earth sciences in general, and in particular, scintillator-based gamma spectrometers can be used for the low cost determination of burial dose rates in natural geological samples. We are currently developing a robust, portable, wireless detector specifically intended for field measurement of natural radionuclide concentrations and hence, the calculation of dose rates. One of the challenges in developing and applying such an instrument is reliable calibration. Most calibrations of field instruments depend on access to non-finite matrices of known K, U, Th activity concentrations, in either a 4π or 2π geometry; these are only available at a few facilities around the world. Here we investigate an alternative approach, based on the measurement of small samples containing well-known activity concentrations of only K or U or Th, and Monte-Carlo radiation transport modelling to convert the observed spectra into those expected from specific activity concentrations in a non-finite 4π geometry. We first validate our modelling procedure by simulating these observed spectra. The non-finite matrix calibration spectra are then predicted, and least-squares fitted to the spectrum observed at the centre of a 1 m3 of granite chips; the resulting predicted U, Th and K activity concentrations are compared with independently known values.

Simultaneous Measurement of 147Sm/144Nd and 143Nd/144Nd Ratios in Natural Geological Samples by MC-ICP-MS After One Stage Chemical Purification Using TODGA Resin

Simultaneous Measurement of 147Sm/144Nd and 143Nd/144Nd Ratios in Natural Geological Samples by MC-ICP-MS After One Stage Chemical Purification Using TODGA Resin

Development of three chalcopyrites and one copper metal as potential reference materials for copper isotopic analysis by LA-MC-ICP-MS.

Laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) has become a powerful technique for in situ Cu isotopic analysis in natural geological samples. Cu isotopic compositions in natural chalcopyrites have been used to reveal aspects of the mineralization processes directly. However, internationally or commercially available matrix-matched chalcopyrite reference materials for mass fractionation correction or quality control purposes are still lacking for in situ Cu isotopic analysis using LA-MC-ICP-MS. Three natural chalcopyrites 14ZJ12-1, JGZ-29 and JGZ-78, and one copper metal GBW02141 with different Cu isotopic compositions were investigated as potential microanalytical reference materials by LA-MC-ICP-MS. Ga element was used as an internal standard to correct the mass fractionation of Cu isotopes during LA-MC-ICP-MS analysis. A large number of Cu isotope ratio measurements using femtosecond LA-MC-ICP-MS were conducted and produced good intermediate precision of δ65 CuNIST976 (0.07-0.08‰, 2 standard deviations), demonstrating the homogeneous Cu isotopic distribution in the recommended samples. The mean δ65 CuNIST976 values of -0.21 ± 0.04‰, 0.46 ± 0.04‰, -0.06 ± 0.04‰ and 0.11 ± 0.05‰ (2 standard deviations) in 14ZJ12-1, JGZ-29, JGZ-78 and GBW02141, respectively, were obtained using solution-MC-ICP-MS in the four recommended samples. Here, we describe three natural chalcopyrites 14ZJ12-1, JGZ-29, JGZ-78, and one copper metal GBW02141 as the potential Cu isotopic reference materials for LA-MC-ICP-MS analysis. Our analyses demonstrate that these recommended materials have a high degree of elemental and isotopic homogeneity, indicating that they are suitable for microanalysis techniques for data quality assurance or interlaboratory calibration.

2H-2H clumping in molecular hydrogen method and preliminary results

We present a method for measuring the clumped isotope composition of molecular hydrogen (H2) using a high-resolution mass spectrometer, the Thermo 253 Ultra, improved to address subtle artifacts arising from instrument baselines and non-linear responses. We also present methods for purification and concentration of H2 from natural and experimental samples, tailored to this measurement. We document the accuracy of the method through comparison to established methods for the determination of δD values, and through measurements of H2 gases of widely varying D content that were driven to isotopic equilibrium with respect to their distributions of isotopologues by heating in the presence of a catalyst. Experimental reproducibility of δD and ΔDD values over months averages ±0.5 and ± 6.9 ‰, respectively (1σ) — both small fractions of common natural variations. We explore methods of gas purification and handling, and show that preferred methods result in low (0–4 ‰) changes in δD and undetectable changes in ΔDD. Our methods and data processing procedures were further tested by comparing measurements of mixtures of H2 gases that varied widely in δD and ∆DD with a model describing proportions of isotopologues in such mixtures. Application of these methods to H2 that is residual to laboratory consumption by cultured methanogens shows that metabolic ‘back reaction’ (metabolic production of H2 from water-derived protons during net H2 consumption) is responsible for driving the ΔDD value of residual H2 toward equilibrium at environmental temperatures. Finally, we report the first measurements of the clumped isotope composition of molecular hydrogen in natural geological samples collected from high and low temperature submarine hydrothermal vents (Lost city, Rainbow, Ashadze) and an intracontinental natural reservoir in Mali; initial findings suggest that ∆DD of H2 generally records temperatures of fluid venting or long-term storage, even in cases where the δD of H2 has not equilibrated with water at those temperatures. This study establishes the first clumped isotope systematics of molecular hydrogen based on both experimental and natural samples, including key processes in the biogeochemical cycle of H2.

The Application of Full Spectrum Analysis to NaI(Tl) Gamma Spectrometry for the Determination of Burial Dose Rates

In this study, we explored the potential of a NaI(Tl) scintillator-based gamma spectrometer for the accurate determination of burial dose rates in natural geological samples using a full spectrum analysis (FSA) approach. In this method, an iterative reweighted least-square regression is used to fit calibration standard spectra (<sup>40</sup>K, and <sup>238</sup>U and <sup>232</sup>Th series in equilibrium) to the sample spectrum, after subtraction of an appropriate background. The resulting minimum detection limits for <sup>40</sup>K, <sup>238</sup>U, and <sup>232</sup>Th are 4.8, 0.4 and 0.3 Bq·kg<sup>−1</sup>, respectively (for a 0.23 kg sample); this is one order of magnitude lower than those obtained with the three-window approach previously reported by us, and well below the concentrations found in most natural sediments. These improved values are also comparable to those from high-resolution HPGe gamma spectrometry. Almost all activity concentrations of <sup>40</sup>K, <sup>238</sup>U, and <sup>232</sup>Th from 20 measured natural samples differ by ≤5% from the high resolution spectrometry values; the average ratio of dose rates derived from our NaI(Tl) spectrometer to those from HPGe spectrometry is 0.993 ± 0.004 (n=20). We conclude that our scintillation spectrometry system employing FSA is a useful alternative laboratory method for accurate and precise determination of burial dose rates at a significantly lower cost than high resolution gamma spectrometry.

Accurate Determination of Barium Isotopic Compositions in Sequentially Leached Phases from Carbonates by Double Spike-Thermal Ionization Mass Spectrometry (DS-TIMS).

Recent studies have proposed barium isotopes as a novel proxy for studying primary productivity in paleo-oceangraphical studies and elements cycling through the critical zone. Pristine marine carbonates are generally assumed to preserve Ba isotope compositions of ancient seawater. However, Ba incorporated in or adsorbed on detrital minerals such as clays in impure carbonates may limit the accurate application of the Ba isotope proxy for paleo-ocean environmental reconstruction purposes. We present here a sequential extraction procedure and show that a considerable range of Ba concentrations can be associated with the four operationally defined sequential leaching fractions (water-soluble, exchangeable, carbonate, and oxidizable fractions). Chemical separation of Ba from these leachates is achieved with a recovery of >98.6% by our modified ion exchange procedure. Potential instrumental mass bias effects and barium isotope fractionation during the chemical separation are corrected using a carefully optimized 130Ba-135Ba double-spike method. A long-term reproducibility better than ±0.03‰ (2SD) for δ137/134Ba has been achieved by using the double spike-thermal ionization mass spectrometry (DS-TIMS) in this study. We demonstrate that significant variations of δ137/134Ba in the analyzed leachates suggest a considerable Ba isotope fractionation between carbonate mineral phase and noncarbonate phases of marine carbonate rocks. The barium isotope distribution in a set of standard reference materials and natural geological samples under various geological settings has been presented. When utilizing Ba isotopes as a proxy for primary productivity and the biogeochemical cycling of Ba, our new findings from sequential Ba extraction as well as our modified precise DS-TIMS analytical setup should be taken into account.

Accurate Determination of Lithium Isotopic Compositions in Geological Samples by Multi-collector Inductively Coupled Plasma-Mass Spectrometry

Abstract Accurate determination of lithium (Li)isotopic composition in natural geological samples is the basis for Li isotope geochemical studies. In this study, a method contained preparation of geological materials (water and rock) and accurate determination of Li isotopic composition was set up. The separation of Li from water and rock samples was implemented by a single column containing 1.5 mL of Bio-Rad AG 50W-X12 (200–400 mesh) resin, with 0.40 M HCl and 1.0 M HCl as eluents. Only 8.5 and 14 mL of eluents were used to separate Li from water and rock samples with this method, respectively. Blank signal of the operation procedure was (2.4 ± 0.1) mV, which was almost same as the 2.3 mV of the 2% HNO3 signal used in this study. Experimental results showed that Li isotopic fractionation during leaching process was significant and deviation of δ7Li values in these samples with incompletely recovered Li reached up to 50‰. Lithium isotopic ratios were determined by multi-collector ICP-MS (Nu Plasma II) using the sample standard bracketing (SSB) method. L-SVEC standard with similar Li concentration to samples (about 80 ng mL−1) was used in this study. The external precision (2σ) of this technique, determined by repeated measurement of pure Li standard solutions and seawater was

Quantitative compositional mapping of mineral phases by electron probe micro-analyser

Abstract Compositional mapping has greatly impacted mineralogical and petrological studies over the past half-century with increasing use of the electron probe micro-analyser. Many technical and analytical developments have benefited from the synergies of physicists and geologists and they have greatly contributed to the success of this analytical technique. Large-area compositional mapping has become routine practice in many laboratories worldwide, improving our ability to measure the compositional variability of minerals in natural geological samples and reducing the operator bias as to where to locate single spot analyses. This chapter aims to provide an overview of existing quantitative techniques for the evaluation of rock and mineral compositions and to present various examples of applications. A new advanced method for compositional map standardization that relies on internal standards and accurately corrects the X-ray intensities for continuum background is also presented. This technique has been implemented into the computer software XMapTools. The improved workflow defines the appropriate practice of accurate standardization and provides data-reporting standards to help improve petrological interpretations.

Recognition of 25 natural geological samples using a modified correlation analysis method and laser-induced breakdown spectroscopic data

The recognition of natural geological samples is of great significance for mineral exploration and geological research. Because of the powerful element analysis ability, the laser-induced breakdown spectroscopy data was widely used in the classification and recognition of samples in recent two decades. Correlation coefficient, as a classical statistic parameter, reflects the degree of a close correlation between variables, and can be used to compare the LIBS data of test samples with the existing data. But it is difficult to get high accuracy in dealing with the identification problems of natural geological samples, which are usually composed of similar elements and mixed with minor impurity constituents. So, some typical spectral data segments and quantitative features, such as the intensity ratios of elements of LIBS data, are extracted and combined with the correlation coefficient by a specially designed analytical model to identify the geological samples. The modified correlation coefficient method shows a positive improvement in the recognition accuracy, while less complexity than the widely used pattern recognition methods.

Determination of Rare Earth Elements in Geological Samples Using Laser-Induced Breakdown Spectroscopy (LIBS).

Laser-induced breakdown spectroscopy (LIBS) was used to detect rare earth elements (REEs) in natural geological samples. Low and high intensity emission lines of Ce, La, Nd, Y, Pr, Sm, Eu, Gd, and Dy were identified in the spectra recorded from the samples to claim the presence of these REEs. Multivariate analysis was executed by developing partial least squares regression (PLS-R) models for the quantification of Ce, La, and Nd. Analysis of unknown samples indicated that the prediction results of these samples were found comparable to those obtained by inductively coupled plasma mass spectrometry analysis. Data support that LIBS has potential to quantify REEs in geological minerals/ores.

IUPAC-IUGS status report on the half-lives of 238U, 235U and 234U

The current state of knowledge on the half-lives of the long-lived U radionuclides has been reviewed by the IUPAC-IUGS joint Task Group “Isotopes in Geosciences”. 238U is assigned a half-life of (4.4683±0.0096)Ga, i.e. a decay constant λ238=(0.155125±0.000333)Ga−1. The coverage factor is k=2 for this and all other estimates presented here. The 238U half-life can be used as a reference for the half-lives/decay constants of all other isotopic geochronometers.A revision of the half-life of 235U based on intercomparison of natural geological samples is premature. The improved repeatability of mass spectrometric measurements has revealed Type B uncertainties that had been dismissed as subordinate in the past. The combined uncertainty of these as yet incompletely charted and quantified sources of Type B uncertainty may be no smaller than the currently accepted uncertainty of the α counting experiments.A provisional value for the 234U half-life can be calculated with the assumption of secular equilibrium in the analyzed natural samples. This assumption has not yet been verified independently and its metrological traceability appears sub-optimum. A Type B evaluation suggests that the ca. 0.17% offset between the N(234U)/N(238U) number-ratios of the natural samples used to estimate the 235U half-life and those of the four samples used to estimate the 234U half-life should be compounded into the standard measurement uncertainty of the latter. The resulting provisional uncertainty interval (k=2) for the 234U half-life is (244.55–247.77)ka, corresponding to λ234=(2.8203–2.8344)Ma−1.

Atom Probe Tomography Analysis of Bulk Chemistry in Mineral Standards

Atom Probe Tomography (APT) analysis of materials is an established technique for atomic level compositional analysis. Extensive research has been performed on many alloys, compounds, multilayered thin films, integrated circuits, and even polymers [1]. While some geological materials, mainly minerals and biominerals, have been previously studied [2-4], the use of APT for the resolution of bulk chemistry of minerals and its relationship to stoichiometry are insufficiently constrained. Often, the mass spectra from natural geological samples exhibit many complex ionic species, and their interpretation can lead to differing determinations of composition.

Azo resorcin[4]calixpyrrole grafted Amberlite XAD-2 polymer: an efficient solid phase extractant for separation and preconcentration of La(III) and Ce(III) from natural geological samples

The impregnation of new azocalixpyrroles obtained from resorcinol derivative into a polymeric chelating resin, Amberlite XAD-2. A simple and effective method is presented for the separation and preconcentration of U(VI)] and Thorium Th(IV) by solid phase extraction on synthesized modified polymeric chelating resin. The synthesized chelating polymeric sorbent showed superior binding affinity towards U(VI) and Th(IV) under selective pH conditions. The optimum pH range was established and the flow rates for U(VI) and Th(IV) were studied respectively. U(VI) and Th(IV) are quantitatively eluted with suitable eluants like HCl and HNO3 and determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES). Other parameters such as total sorption capacity, exchange kinetics, preconcentration factor, distribution coefficient, breakthrough capacity and resin stability. The uptake and stripping of these metal ions on the resin were fast, indicating a better accessibility of the metal ions towards the chelating sites. Chromatographic separation of Th(IV) and U(VI) was accomplished by adjustment of pH in the range of 3.0–9.0 pH, respectively. Separation of Th(IV) and U(VI) from other metal cations, in synthetic solution was achieved. The synthesized resins were recycled at least 14–16 times without much affecting column sorption capacity.

Raman imaging of extraterrestrial materials

Laser Raman Spectroscopy has been proposed and is under extensive development for surface exploration missions to planetary bodies of our Solar System. It reveals information on molecular structure and chemistry. The spatial distribution of molecular species in natural geological samples and planetary materials has significance for the geological processes by which they formed. Raman imaging is the best way to combine the molecular identification and characterization of geologic materials with their spatial distribution. This paper reports Raman imaging studies of five types of extraterrestrial materials and three terrestrial samples using a state-of-the-art Raman imaging system. The Raman spectral features of major, minor, and trace species in these samples, together with their spatial correlations revealed by these Raman imaging studies indicate the genetic relationships and the geological processes that these materials have been experienced. For robotic planetary surface exploration mission, a simple yet very useful molecular map of a sample can be generated by using line-scan or grid-scan of an in situ Raman system with tightly focused laser beam.

Electrical resistivity imaging over natural (in situ) geological samples using physical model studies

To understand the characteristic responses of natural geological samples, viz., black granite, green marble, and graphite sheet, and to have “an a priori” knowledge of their physical property through electrical resistivity imaging, the physical model laboratory setup has been established to conduct scale model studies over targets of finite dimensions and resistivities. The present experiment involves IRIS make SYSCAL Pro-96 measuring system using 48 electrodes with 2-cm interelectrode separation in the laboratory model tank. In the present communication, we have presented the 2D cross section images using Wenner, Wenner–Schlumberger, and dipole–dipole array configurations over the resistive (granite, marble) and conductive (graphite) sheets. In the case of resistive target (black granite sheet, green marble), the combined usage of dipole–dipole and Wenner–Schlumberger arrays provided more accurate measures on target parameters, i.e., the combined usage of both the arrays is preferable in searching high-resistive targets beneath the low-resistive ones over burden. The shape of the resistive target (green marble sheet) is inappropriate when the thickness of the target is greater than one half of the minimum array separation. As the thickness of the target increases, the signatures of the target become feeble, and hence, the shape of the resistive target is not properly reflected in the corresponding tomogram. The response over graphite sheet indicates that the true parameters of the target are not reflected in the cross section, and the existence of the low-resistive (high-conductive) target in the host medium (water) deviates the resistivity of the medium. The target parameters from the cross section using dipole–dipole array are somewhat correlated with true parameters in the case of thin targets at shallow depths. In the case of the sequence of layers of gravel–marble gravel–sand gravel simulated in a small model tank in the physical model laboratory, the thickness of the high-resistive marble layer beneath the low-resistive gravel layer is enhanced conspicuously because of the significant resistivity contrast between gravel and marble.

Evaluation of Sr chemical purification technique for natural geological samples using common cation-exchange and Sr-specific extraction chromatographic resin prior to MC-ICP-MS or TIMS measurement

In this paper, sample preparation protocols based on common cation-exchange and Sr-specific extraction chromatographic resin were evaluated and investigated for natural geological samples prior to 87Sr/86Sr ratio measurement using MC-ICP-MS or TIMS. Several CRMs and real geological samples were digested using HF, HNO3 and HClO4 in closed vessels prior to sequential chemical purifications and then Sr isotopic ratios were determined by MC-ICP-MS or TIMS. Because HREEs reside in the Sr fraction when common cation-exhange resin is used, this purification technique is unsuitable prior to MC-ICP-MS analysis, as doubly loaded HREE interfere on Sr masses. We observe an obviously positive relationship between the radiogenic 87Sr/86Sr ratio and the content of HREEs in the Sr fractions, as well as a negative relationship between un-radiogenic 84Sr/86Sr or 84Sr/88Sr ratios and the content of HREEs in the Sr fractions during MC-ICP-MS Sr isotopic measurement. Such effects are insignificant for TIMS measurement, because ionization temperatures are generally lower and can be well controlled during the analyses. In contrast to the traditional Sr purification method (cation exchange resin), Sr-specific extraction chromatographic resin produces high purity Sr fractions, making it feasible for both MC-ICP-MS and TIMS.

Precise emissivity of rock samples

To differentiate among geological materials and vegetation using thermal infrared spectra, radiometric measurements require corrections for local temperature variations and reflected radiation. A laboratory technique is described for the determination of spectral emissivity signatures for natural geological samples using a Fourier Transform Infrared Spectrometer and the self-emission of the scene. The approach described does not require control of the sample temperature. A quantitative assessment of uncertainties is presented. Comparison of the emissivity measurements from natural geological specimens with hemispherical reflectance data gave an average difference of about 0.01 in emissivity, except near strong features of the spectra. The technique is applicable to field measurements of high-emissivity objects.