Evaluation of the odd-even effect in limits of detection for electron microprobe analysis of natural minerals

Limit of detection (LOD), being a fundamental quality parameter for analytical techniques, has been recently investigated and a systematic behavior has been observed for most odd–even element pairs for many techniques. However, to the best of our knowledge very few LOD data are available in published literature for electron microprobe analysis; these consist of three papers, two being on rare-earth elements and the third covering a large number of elements of atomic number between 21 and 92. These data confirm the systematic behavior of LODs for many odd–even pairs. To initiate to full this gap, we determined LODs for several major rock-forming chemical elements from Na to Fe with atomic numbers between 11 and 26, during the microprobe analysis of common minerals (olivine, plagioclase, pyroxene, amphibole, quartz, and opaques) in volcanic rocks. The odd–even effect of nuclear stability seems to be present in LOD data for most odd–even pairs investigated. Nevertheless, the experimental strategy concerning the reference materials, calibration procedure, and blank measurements, should be substantially modified to better evaluate the systematic behavior of LOD values in microprobe analysis

Statistically coherent calibration of X-Ray Fluorescence Spectrometry for major elements in rocks in minerals.

We applied both the ordinary linear regression (OLR) and the new uncertainty weighted linear regression (UWLR) models for the calibration and comparison of a XRF machine through 59 geochemical reference materials (GRMs) and a procedure blank sample. )e mean concentration and uncertainty data for the GRMs used for the calibrations (Supplementary Materials) (available here) filewere achieved from an up-to-date compilation of chemical data and their processing from well-known discordancy and significance tests. )e drift-corrected XRF intensity and its uncertainty were determined from mostly duplicate pressed powder pellets. )e comparison of the OLR (linear correlation coefficient r∼0.9523–0.9964 and 0.9771–0.9999, respectively, for before and after matrix correction) and UWLR models (r∼0.9772–0.9976 and 0.9970–0.9999, respectively) clearly showed that the latter with generally higher values of r is preferable for routine calibrations of analytical procedures. Both calibrations were successfully applied to rock matrices, and the results were generally consistent with those obtained in other laboratories although the UWLR model showed mostly narrower confidence limits of the mean (slope and intercept) or lower uncertainties than the OLR. Similar sensitivity (∼2.69–46.17 kc·s1·%1 for the OLR and ∼2.78–59.69 kc·s1·%1 for the UWLR) also indicated that the UWLR could advantageously replace the OLR model. Another novel aspect is that the total uncertainty can be reported for individual chemical data. If the analytical instruments were routinely calibrated from the UWLR model, this action would make the science of geochemistry more quantitative than at present

Evaluation of geochemical sedimentary reference materials of the Geological Survey of Japan (GSJ) by an objective outlier rejection statistical method

Nine geochemical sedimentary reference materials (RMs), JCh-1, JDo-1, JLs-1, JSl-1, JSl-2, JLk-1, JSd-1, JSd-2 and JSd-3, of the Geological Survey of Japan (GSJ) were evaluated by an objective outlier rejection statistical method. An extensive chemical database for these reference materials, created from the Internet website of the GSJ Geochemical Reference Samples Database (http://riodb02.ibase.aist. go.jp/geostand/) and from research articles published up to December 2008, is evaluated by a statistical scheme consisting of: (i) detection and elimination of the discordant outlier values with the application of 33 discordancy test variants (instead of routinely used inaccurate �two-standard deviation� method); (ii) calculation of new central tendency and dispersion parameters; and (iii) comparison of confi dence limits (calculated by incorporating the recently available new, precise critical values for Student t-test) and normalized mean difference percentages of the geochemical parameters obtained in the present work with those calculated for the data of the literature. Evaluation of these RMs by application of the more appropriate statistical method resulted in more precise new central tendency and dispersion parameter values, and also facilitated to propose recommended values for the fi rst time for some of the geochemical parameters. The results obtained in this work could be useful for better calibrations models and for evaluation of method precision, accuracy, sensitivity and detection limits.Nueve materiales sedimentarios de referencia geoquímica, JCh-1, JDo-1, JLs-1, JSl-1, JSl-2, JLk- 1, JSd-1, JSd-2 y JSd-3, del Geological Survey of Japan (GSJ) fueron evaluados por una metodología estadística objetiva para la eliminación de valores discordantes. Una extensa base de datos químicos para estos materiales de referencia, creada a partir de la base de datos de Muestras de Referencia Geoquímica de la GSJ (http://riodb02.ibase.aist.go.jp/geostand/) y complementada con artículos de investigación publicados hasta diciembre de 2008, se evaluó con un esquema estadístico que consiste en: (i) la detección y eliminación de los valores discordantes por medio de la aplicación de 33 variantes de pruebas de discordancia (en vez del método inexacto rutinariamente utilizado de �dos-desviaciones estándar�); (ii) el cálculo de nuevos parámetros de tendencia central y de dispersión; y (iii) la comparación de límites de confi anza (calculados con nuevos y precisos valores críticos, recientemente disponibles para la t de Student) y los porcentajes de diferencia de la media obtenidos en el presente trabajo con respecto a la reportada en la literatura. La evaluación de estos materiales de referencia mediante este método estadísticamente más apropiado resultó en nuevos y más precisos valores de tendencia central y dispersión. Así mismo, permitió, por primera vez, proponer valores recomendados para algunos parámetros geoquímicos. Los resultados obtenidos en este trabajo podrían ser útiles para mejores modelos de calibración y para la evaluación de la precisión exactitud, sensibilidad y límites de detección de método

Mineralogical and geochemical effects due to hydrothermal alteration in the Los Azufres geothermal field, Mexico

To investigate the effects of hydrothermal alteration on the chemistry of volcanic rocks, we analyzed the whole rock chemical composition (major and trace elements, including rare-earth elements - REE) of two distinct portions of a drill well core sample from the Los Azufres geothermal field, Mexico. This highly hydrothermally-altered sample allowed us to study, for the first time, the mineralogical and chemical effects imposed by hydrothermal alteration on the mm scale in this important geothermal system. Mineralogically, hydrothermal alteration in the sample is mainly represented by chloritization of primary olivine and pyroxene, argillitization of primary plagioclase, as well as banded chlorite and quartz, with significant amounts of epidote and hematite. The "altered" part of the sample contains intensely altered remnants of the fresh rock, which show intense silicification, hematization, and dissolution boundaries. Most major and trace elements were mobilized from the original rock. Major element composition reflects the silicification, chloritization, and epidotization processes taking place in the geothermal system. The rare-earth elements La and Ce, as well as Yb and Lu were probably partially re-deposited during alteration. The positive anomaly of Eu may suggest that Eu is being concentrated in hydrothermal epidote after its release from plagioclase to the geothermal fluid. The high-field strength elements such as Zr, Ti, and P, show as well significant hydrothermal alteration-related decrease in the highly-altered rock. The geothermal fluid responsible for this hydrothermal alteration was probably oxidizing, of high temperature (>250º C), and enriched in REE and other trace elementsLos efectos de los procesos de alteración hidrotermal en la composición química de rocas volcánicas fueron investigados por medio del análisis químico de roca total (elementos mayores y traza, incluyendo los elementos de las tierras raras) de dos partes diferentes de un mismo fragmento de núcleo de pozo proveniente del campo geotérmico de Los Azufres, México. Esta muestra, con intesa alteración hidrotermal, nos permitió estudiar por primera vez los efectos mineralógicos y químicos impuestos por la alteración en una escala de milímetros para este importante sistema hidroterma

Solute and gas geothermometry of geothermal wells: a geochemometrics study for evaluating the effectiveness of geothermometers to predict deep reservoir temperatures

<div><p>Deep reservoir temperatures of 10 important geothermal systems of the world were estimated by applying 13 solute (Na/K) and 21 gas geothermometers. The predicted temperatures were comprehensively evaluated and compared with measured bottom-hole temperatures using geochemometric techniques. The present study reveals (1) high prediction performances in most of the Na/K geothermometers for the majority of the geothermal fields with liquid-dominated reservoirs, whereas low prediction performances were indicated for the geothermal fields with vapour-dominated and high-temperature reservoirs; (2) the gas geothermometers, in comparison to Na/K, are more successful in predicting the subsurface temperatures in high-temperature geothermal systems; (3) the geothermal systems for which Na/K geothermometers have indicated a high prediction performance, the gas geothermometers have specified a low prediction performances, and vice versa; (4) both Na/K and gas geothermometers, generally, overestimated the reservoir temperatures for the majority of the low-enthalpy geothermal fields and underestimated for the majority of the high-enthalpy geothermal fields; (5) the reservoir temperature predictions of gas geothermometers have more scatter than those temperatures inferred from Na/K geothermometers; and (6) in general, Na/K geothermometers seem to be a more successful geochemical tool in predicting reliable reservoir temperatures than gas geothermometers.</p></div

Mineralogical and geochemical effects due to hydrothermal alteration in the Los Azufres geothermal field, Mexico

To investigate the effects of hydrothermal alteration on the chemistry of volcanic rocks, we analyzed the whole rock chemical composition (major and trace elements, including rare-earth elements - REE) of two distinct portions of a drill well core sample from the Los Azufres geothermal field, Mexico. This highly hydrothermally-altered sample allowed us to study, for the first time, the mineralogical and chemical effects imposed by hydrothermal alteration on the mm scale in this important geothermal system. Mineralogically, hydrothermal alteration in the sample is mainly represented by chloritization of primary olivine and pyroxene, argillitization of primary plagioclase, as well as banded chlorite and quartz, with significant amounts of epidote and hematite. The "altered" part of the sample contains intensely altered remnants of the fresh rock, which show intense silicification, hematization, and dissolution boundaries. Most major and trace elements were mobilized from the original rock. Major element composition reflects the silicification, chloritization, and epidotization processes taking place in the geothermal system. The rare-earth elements La and Ce, as well as Yb and Lu were probably partially re-deposited during alteration. The positive anomaly of Eu may suggest that Eu is being concentrated in hydrothermal epidote after its release from plagioclase to the geothermal fluid. The high-field strength elements such as Zr, Ti, and P, show as well significant hydrothermal alteration-related decrease in the highly-altered rock. The geothermal fluid responsible for this hydrothermal alteration was probably oxidizing, of high temperature (>250º C), and enriched in REE and other trace elementsLos efectos de los procesos de alteración hidrotermal en la composición química de rocas volcánicas fueron investigados por medio del análisis químico de roca total (elementos mayores y traza, incluyendo los elementos de las tierras raras) de dos partes diferentes de un mismo fragmento de núcleo de pozo proveniente del campo geotérmico de Los Azufres, México. Esta muestra, con intesa alteración hidrotermal, nos permitió estudiar por primera vez los efectos mineralógicos y químicos impuestos por la alteración en una escala de milímetros para este importante sistema hidroterma

Statistically coherent calibration of X-ray fluorescence spectrometry for major elements in rocks and minerals

"We applied both the ordinary linear regression (OLR) and the new uncertainty weighted linear regression (UWLR) models for the calibration and comparison of a XRF machine through 59 geochemical reference materials (GRMs) and a procedure blank sample. The mean concentration and uncertainty data for the GRMs used for the calibrations (Supplementary Materials) (available here) filewere achieved from an up-to-date compilation of chemical data and their processing from well-known discordancy and significance tests. The drift-corrected XRF intensity and its uncertainty were determined from mostly duplicate pressed powder pellets. The comparison of the OLR (linear correlation coefficient ?0.9523–0.9964 and 0.9771–0.9999, respectively, for before and after matrix correction) and UWLR models (?0.9772–0.9976 and 0.9970–0.9999, respectively) clearly showed that the latter with generally higher values of is preferable for routine calibrations of analytical procedures. Both calibrations were successfully applied to rock matrices, and the results were generally consistent with those obtained in other laboratories although the UWLR model showed mostly narrower confidence limits of the mean (slope and intercept) or lower uncertainties than the OLR. Similar sensitivity (?2.69–46.17?kc·s?1·%?1 for the OLR and ?2.78–59.69?kc·s?1·%?1 for the UWLR) also indicated that the UWLR could advantageously replace the OLR model. Another novel aspect is that the total uncertainty can be reported for individual chemical data. If the analytical instruments were routinely calibrated from the UWLR model, this action would make the science of geochemistry more quantitative than at present.