Jeffery C. Seitz

Practical aspects of quantitative laser Raman microprobe spectroscopy for the study of fluid inclusions

Abstract This paper is addressed to both geologists who use laser Raman microprobe (LRM) spectroscopy to analyze fluid inclusions and to those who want to evaluate analyses done by this technique. Emphasis is on how to obtain quantitative analyses of fluid inclusions. We discuss the basic method of fluid inclusion analysis by LRM spectroscopy and the levels of accuracy and precision attainable with this technique. We evaluate which kinds of fluid inclusions and host mineral matrices will yield the most reliable compositional data. Necessary sample preparations, detection limits, problems with fluorescence, dependence of Raman scattering efficiencies on density, and many other questions asked at the workshop on Raman spectroscopy during the 1987 ACROFI meeting also are addressed. The complementary nature, advantages, and disadvantages of both LRM spectroscopy and microthermometry, the two techniques most frequently used for the analysis of individual fluid inclusions, are emphasized. Some discussions are intended to help LRM users calibrate, and evaluate the optical characteristics of, their particular instruments. It is hoped that this paper will further LRM users in finding a common ground on which to discuss the differences and similarities among different LRM instruments, and that it will encourage a future consensus on efficient means of calibration and on interlaboratory standards.

Characterization of CO2CH4H2O fluid inclusions by microthermometry and laser Raman microprobe spectroscopy: Inferences for clathrate and fluid equilibria

Microthermometry (MT) and laser Raman microprobe (LRM) spectroscopy (at room temperature and at about 0°C) were done on 33 synthetically produced CO2CH4H2O fluid inclusions in quartz (from R. Bodnar and M. Sterner). At room temperature, the inclusions consist of an aqueous liquid, a CO2CH4 supercritical carbonic fluid and (in most cases) graphite. In all these inclusions, the melting temperature for solid CO2 is less than that for the homogenization of the vapor bubble in the carbonic fluid. A method is described whereby MT data for CO2CH4H2O inclusions can be projected within the CO2CH4 binary phase diagram to infer CO2:CH4 ratios in the carbonic fluid ( 35 mole% CH4 in the inclusions under study). This method takes into account the formation of CO2CH4 clathrate hydrate during MT analysis. Unless clathrate formation is properly considered, errors arise in the determination of the bulk CO2CH4 ratio. For the inclusions in our study, these errors are on the order of 5 to 8 mole% CH4. Our interpretation of the MT data indicates that CH4 is preferentially partitioned into the clathrate over the coexisting carbonic fluid, in contradiction to the prediction from Parrish and Prausnitzs (1972) model for clathrate equilibria. Comparison of LRM analyses on the bulk carbonic fluid (in the absence of clathrate) and the residual carbonic fluid (in the presence of clathrate) confirm the preferential partitioning of CH4 into the clathrate. LRM analyses of the clathrate itself indicate that CH4 occupies both types of cage sites in the clathrate structure, whereas CO2 may only occupy one site. Two by-products of the combined LRM and MT analyses of the same inclusions are derivation of empirical ratios of Raman quantification factors for high-density CO2CH4 fluids and the ability to determine CO2:CH4 ratios of inclusions whose MT data lie near the critical region for CO2CH4. Thus, the joint use of LRM and MT techniques provides information that could not be obtained by either technique alone.

High-density volatiles in the system C-O-H-N for the calibration of a laser Raman microprobe☆

Abstract Three methods have been used to produce high-density volatiles in the system C-O-H-N for the calibration of a laser Raman microprobe (LRM): synthetic fluid-inclusion, sealed fused-quartz-tube, and high-pressure-cell methods. Because quantitative interpretation of a Raman spectrum of mixed-volatile fluid inclusions requires accurate knowledge of pressure- and composition-sensitive Raman scattering efficiencies or quantification factors for each species, calibrations of these parameters for mixtures of volatiles of known composition and pressure are necessary. Two advantages of the synthetic fluid-inclusion method are that the inclusions can be used readily in complementary microthermometry (MT) studies and that they have sizes and optical properties like those in natural samples. Some disadvantages are that producing H2O-free volatile mixtures is difficult, the composition may vary from one inclusion to another, the exact composition and density of the inclusions are difficult to obtain, and the experimental procedures are complicated. The primary advantage of the method using sealed fused-quartz tubes is its simplicity. Some disadvantages are that exact compositions for complex volatile mixtures are difficult to predict, densities can be approximated only, and complementary MT studies on the tubes are difficult to conduct. The advantages of the high-pressure-cell method are that specific, known compositions of volatile mixtures can be produced and that their pressures can be varied easily and are monitored during calibration. Some disadvantages are that complementary MT analysis is impossible, and the setup is bulky. Among the three methods for the calibration of an LRM, the high-pressure-cell method is the most reliable and convenient for control of composition and total pressure. We have used the high-pressure cell to obtain preliminary data on 1. (1) the ratio of the Raman quantification factors for CH4 and N2 in an equimolar CH4N2 mixture and 2. (2) the spectral peak position of ν1 of CH4 in that mixture, as well as in pure CH4, at pressures up to 690 bars. These data were successfully applied to natural inclusions from the Duluth Complex in order to derive their compositions and total pressures.

Theoretical and practical aspects of differential partitioning of gases by clathrate hydrates in fluid inclusions

Abstract Clathrate hydrates may form in aqueous, gas-rich fluid inclusions during low-temperature microthermometric analysis. Gases are differentially partitioned between the clathrate and residual fluid as a function of temperature, pressure, and bulk composition. Of particular concern in multicomponent gas-bearing inclusions is the fact that the selective partitioning of individual gases into clathrate hydrate can strongly affect the interpretation of microthermometric data. The absolute extent of depletion of gases (consumed by clathrate) in the residual fluid is dependent upon both the composition of the gas and the relative proportions of H2O and gas-rich fluid. The equilibrium phase relations in clathrate systems may be modeled in order to correct for the presence of clathrates. Quantitative modeling of CO2CH4H2O, and consideration of CO2N2H2O, N2CH4H2O, and H2SCH4H2O indicates th and extent of differential partitioning of gases and the temperature-pressure-composition stabilities of clathrates. For instance, under a geologically reasonable range of pressure, H2S and CH4 will partition into clathrate more strongly than N2, whereas the direction of partitioning between CO2 and CH4 and between CO2 and N2 changes as a function of temperature, pressure, and the relative proportions of the components. Comparison of experimental data for unary- and binary-gas systems and the model of John et al. (1985) indicates that the model correctly predicts clathrate stabilities and is useful for fluid inclusion studies.

A vibrating-tube densimeter for fluids at high pressures and temperatures

A vibrating U-tube apparatus has been developed for determining the densities of pure fluids and fluid mixtures at 10-200 MPa and 323-773 K. Measured parameters areP,T, andr (period of vibration). Fluids are injected into the U-tube at constantP andT. Three or more reference fluids are used to calibrate the response of the instrument. Fluid mixtures are produced by pumping pure fluids into T-junctions on the upstream side of the U-tube using high accuracy syringe pumps. An automated syringe pump is used to maintainP at setpoint ±0.01 MPa.T is controlled to ±0.01 K using a closed-loop, electronic signal amplification/feedback system. For mixtures, a statistically significant number of measurements of r are obtained to account for the effects of small heterogeneities in fluid composition (generally <0.005X;). Typically, density data for 15 fluids can be obtained in a 6- to 8-h period. Considering all of the potential sources of error in the experimentation, conservative estimates of uncertainty are as follows:P, ±0.02 MPa;T, ±0.05 K;p (pure fluids), ±0.0005g.cm−3; andp (fluid mixtures), ±0.0005-0.0010g-cm−3.

The CO2-H2O system. II. calculated thermodynamic mixing properties for 400°C, 0–400 MPa

Abstract An excess molar volume (Vex)-explicit virial equation, and two empirical Vex expressions developed from experimentally determined densities, were used to calculate excess Gibbs free energies (Gex) and activity-composition (a-X) relations for CO2-H2O fluids at 400°C, 0–400 MPa. Excess Gibbs free energies are continuously positive and asymmetric toward H2O at all pressures up to 400 MPa, rising to peak values of approximately 1300, 1800, 2000 and 2100 J · mol−1 at 50, 100, 200 and 400 MPa, respectively. Calculated activities for H2O and CO2 vary correspondingly, increasing substantially from 0 to 100 MPa, moderately from 100 to 200 MPa, and slightly from 200 to 400 MPa. In addition, because Gex is asymmetric toward H2O, a-X relations for H2O are distinctly different from those for CO2. These results indicate that CO2-H2O fluids are strongly nonideal at 400°C and all pressures above ∼30 MPa, despite the fact that peak values for Vex decrease from ∼50 cm3 · mol−1 at 30 MPa to ∼1 cm3 · mol−1 at 200 MPa, and remain small to pressures at least as high as 500 MPa. Excess Gibbs free energies and a-X relations for CO2-H2O fluids at 400°C and pressures to 400 MPa calculated from modified Redlich-Kwong and Lee-Kesler equations of state generally suggest significantly smaller positive deviations from ideality.

Understanding motivational structures that differentially predict engagement and achievement in middle school science

ABSTRACT Middle school has been documented as the period in which a drop in students’ science interest and achievement occurs. This trend indicates a lack of motivation for learning science; however, little is known about how different aspects of motivation interact with student engagement and science learning outcomes. This study examines the relationships among motivational factors, engagement, and achievement in middle school science (grades 6–8). Data were obtained from middle school students in the United States (N = 2094). The theoretical relationships among motivational constructs, including self-efficacy, and three types of goal orientations (mastery, performance approach, and performance avoid) were tested. The results showed that motivation is best modeled as distinct intrinsic and extrinsic factors; lending evidence that external, performance based goal orientations factor separately from self-efficacy and an internal, mastery based goal orientation. Second, a model was tested to examine how engagement mediated the relationships between intrinsic and extrinsic motivational factors and science achievement. Engagement mediated the relationship between intrinsic motivation and science achievement, whereas extrinsic motivation had no relationship with engagement and science achievement. Implications for how classroom practice and educational policy emphasize different student motivations, and in turn, can support or hinder students’ science learning are discussed.

Quantitative analysis of mixed volatile fluids by Raman microprobe spectroscopy : a cautionary note on spectral resolution and peak shape

Raman analyses of fluid inclusions can yield quantitative information on composition (from peak areas and heights) and density (from peak position and width). In this study, we examine the effect of instrumental spectral resolution on the ratios of these spectral parameters, and the selection of appropriate integration limits for the determination of peak areas in the CO2-CH4-N2 system. Spectral resolution was varied from about 1 to 9 cm−1 by co-varying the widths of all spectrometer slits. Changes in resolution produced a modest effect on peak-area ratios and a significant effect on peak-height ratios. Measured peak-width ratios varied strongly as a function of the spectral resolution. In addition, we observed a moderate shift in the measured peak position of N2, which can be related to the asymmetry of the band. These results indicate that accurate analysis requires careful attention to the selection of quantification factors, especially if the selected values were derived from studies at different spectral resolutions. Another factor that can have a significant effect on the calculated compositions of CH4- and H2-bearing fluid mixtures is the band broadening that occurs with increasing pressure.

Measuring Science Instructional Practice: A Survey Tool for the Age of NGSS

Ambitious efforts are taking place to implement a new vision for science education in the United States, in both Next Generation Science Standards (NGSS)-adopted states and those states creating their own, often related, standards. In-service and pre-service teacher educators are involved in supporting teacher shifts in practice toward the new standards. With these efforts, it will be important to document shifts in science instruction toward the goals of NGSS and broader science education reform. Survey instruments are often used to capture instructional practices; however, existing surveys primarily measure inquiry based on previous definitions and standards and with a few exceptions, disregard key instructional practices considered outside the scope of inquiry. A comprehensive survey and a clearly defined set of items do not exist. Moreover, items specific to the NGSS Science and Engineering practices have not yet been tested. To address this need, we developed and validated a Science Instructional Practices survey instrument that is appropriate for NGSS and other related science standards. Survey construction was based on a literature review establishing key areas of science instruction, followed by a systematic process for identifying and creating items. Instrument validity and reliability were then tested through a procedure that included cognitive interviews, expert review, exploratory and confirmatory factor analysis (using independent samples), and analysis of criterion validity. Based on these analyses, final subscales include: Instigating an Investigation, Data Collection and Analysis, Critique, Explanation and Argumentation, Modeling, Traditional Instruction, Prior Knowledge, Science Communication, and Discourse.