Minerals In Thin Section Research Articles

IMAGE-BASED YOLOV4 ARCHITECTURE FOR DETECTING MINERAL IN SEDIMENTARY ROCKS THIN SECTION

Thin section analysis of sedimentary rocks is the basis for identifying minerals and textures. In general, quantitative analysis of thin sections of rock often requires many hours of work when done manually. In today's era, mineralogical interpretation and percentage calculations must be carried out automatically using more practical applications. The research method begins with the identification of 44 thin section samples in parallel plane polarized (PPL) and crossed polarized (XPL) conditions with thin section analysis then mineralogy detection is carried out using a computational approach, namely the use of image-based Deep Learning YoloV4 architecture with 2D RGB image objects from the thin section of sedimentary rock. The results of this study show the best values of Average Precision in Quartz, Feldspar, and lithic are 39.21% in the XPL model, 26.53% in the XPL model, and 15.75% in combined mode, according to the training and testing of YoloV4 Models for the identification of rock minerals in thin sections. Based on the complexity of the mineral types, the granularity of the detection, and the specific geological objectives, establishing a meaningful benchmark or baseline for comparison is always challenging. Additionally, consider discussing the trade-offs between precision and recall, as a higher precision may be more critical in some geological applications. It is expected that the application of this research can produce practical, fast and accurate interpretation of the determination of minerals in sedimentary rocks from all thin-section images of rocks and thus provide a complete understanding of geological views automatically.

Building YoloV4 models for identification of rock minerals in thin section

Rock mineral identification is a costly and time-consuming task using conventional methods of testing physical and chemical properties, especially in the petrographic laboratory. A comprehensive identification model for three rock minerals in sedimentary rocks based on the YoloV4 model is available as a solution. The models predict rock minerals by calculating the pixels and the weights that have been trained previously. First, the YoloV4 models and framework were built. Then, a total of 44 manually labelled thin section images (sedimentary rocks thin section) were used to create the model to detect minerals accurately. The MAP and loss results showed that the parameters of the minerals detection model in PPL are 11% and 1.19, respectively. Meanwhile, The MAP and loss results of XPL are 19% and 1.18, respectively. Finally, Identification of rock minerals using deep learning algorithms is a very promising idea especially the YoloV4 model can build a comprehensive detection of rock samples in thin sections effectively.

Superpixel segmentations for thin sections: Evaluation of methods to enable the generation of machine learning training data sets

Training data is the backbone of developing either Machine Learning (ML) models or specific Deep Learning (DL) algorithms. The lack of well-labeled training image data has significantly impeded the development of novel DL methods like Convolutional Neural Networks (CNNs) in mineral thin section images identification. However, image annotation, especially pixel-wise annotation is always a costly process. Manually creating dense semantic labels for rock thin section images has been long considered as an unprecedented challenge in view of the ubiquitous variety and complexity of minerals in thin sections. To speed up the annotation, we propose a human–computer collaborative pipeline in which superpixel segmentation is used as a boundary extractor to avoid hand delineation of instances boundaries. The pipeline consists of two steps: superpixel segmentation using MultiSLIC, and superpixel labeling through a specific-designed tool. We use a cutting-edge methodology Virtual Petroscopy (ViP) for automatic image acquisition. Bentheimer sandstone sample is used to conduct performance testing of the pipeline. Three standard error metrics are used to evaluate the performance of MultiSLIC. The result indicates that MultiSLIC is able to extract compact superpixels with satisfying boundary adherence given multiple input images. According to our test results, large and complex thin section images with pixel-wise accurate labels can be annotated with the labeling tool more efficiently than in a conventional, purely manual work, and generate data of high quality.

Automatic identification of minerals in thin sections using image processing

Automatic identification of minerals in thin sections using image processing

Formation of Diagenetic Minerals in the Carboniferous Rock Complex from the Fore-Sudetic Monocline (SW Poland): Fluid Inclusion, Isotopic and Raman Constraints

The paper presents the latest state of knowledge on clastic sedimentary rocks from the Carboniferous complex in the SW part of the Polish Lowlands, studied to help determine their potential prospectivity for the occurrence of oil and/or gas deposits. Rocks were analyzed with respect to the petrographic-mineralogical characteristics of the Carboniferous deposits, their diagenesis, determinations of pressure-temperature conditions of mineral formation and the hydrocarbon occurrence. Analyses were carried out on samples from four selected boreholes in the Fore-Sudetic Monocline. After microscopic analysis of rocks and minerals in thin sections, the following techniques were used: luminescence analysis (UV, blue light), microthermometric analysis of fluid inclusions in double-sided polished wafers, cathodoluminescence analysis, electron scanning microscope studies, XRD analyses, stable isotopic analyses (carbon, oxygen) on calcite and dolomite-ankerite and Raman spectra of fluid inclusions. Orthochemical components, such as carbonates and authigenic quartz, that form cements or fill the veins cutting the sample material have been studied. Fluid inclusion data in quartz and carbonates result in homogenization temperatures of 74–233 °C. The Raman analysis gives temperature estimations for the organic matter of about 164 °C and 197 °C, depending on the borehole, which points to a low coalification degree. The post-sedimentary processes of compaction, cementation and diagenetic dissolution under eo- and meso-diagenetic conditions to temperatures of over 160 °C influenced the present character of the deposits. P-T conditions of brines and methane trapping have been estimated to be ~850–920 bars and 185–210 °C (vein calcite) and ~1140 bars and 220 °C (Fe-dolomite/ankerite). Therefore, locally, temperatures might have been higher (>200 °C), which may be a symptom of local regional metamorphism of a very low degree.

Altered mineral segmentation in thin sections using an incremental-dynamic clustering algorithm

Intelligent mineral segmentation in thin section images of rocks still remains a challenging task in modern computational mineralogy. The objective of the paper is segmenting minerals in geological thin section’s images with special attention on altered mineral segmentation. In this paper, an efficient incremental-dynamic clustering algorithm is developed for segmentation of minerals in thin sections containing altered and non-altered minerals. In the clustering algorithm, there is no need for determining the number of clusters (minerals) existed in thin section images, and also it is able to deal with color changing and non-evident boundaries in altered minerals. We have solved two main existing limitations: segmentation of mineral pixels that are frequently labeled as background pixels, and segmentation of thin sections containing altered minerals. Moreover, we created an open database (Alborz Mineralogical Database), as a benchmark database in computational geosciences regarding image studies of mineral. The proposed method is validated based on the results provided by the segmentation maps, and experimental results indicate that the proposed method is very efficient and outperforms previous segmentation methods for altered minerals in thin section images. The proposed method can be applied in mining engineering, rock mechanics engineering, geotechnique engineering, mineralogy, petrography, and applications such as NASA’s Mars Rover Explorations (MRE).

Characterizing Complex Mineral Structures in Thin Sections of Geological Samples with a Scanning Hall Effect Microscope.

We improved a magnetic scanning microscope for measuring the magnetic properties of minerals in thin sections of geological samples at submillimeter scales. The microscope is comprised of a 200 µm diameter Hall sensor that is located at a distance of 142 µm from the sample; an electromagnet capable of applying up to 500 mT DC magnetic fields to the sample over a 40 mm diameter region; a second Hall sensor arranged in a gradiometric configuration to cancel the background signal applied by the electromagnet and reduce the overall noise in the system; a custom-designed electronics system to bias the sensors and allow adjustments to the background signal cancelation; and a scanning XY stage with micrometer resolution. Our system achieves a spatial resolution of 200 µm with a noise at 6.0 Hz of 300 nTrms/(Hz)1/2 in an unshielded environment. The magnetic moment sensitivity is 1.3 × 10−11 Am2. We successfully measured the representative magnetization of a geological sample using an alternative model that takes the sample geometry into account and identified different micrometric characteristics in the sample slice.

The Fractal Nature of Structural Controls on Ore Formation: The Case of the Iron Oxide Copper-Gold Deposits in the Carajás Mineral Province, Brazilian Amazon

The formation of ore minerals in hydrothermal deposits is the result of a complex interplay between physical and chemical processes that are conditioned by the geologic environment in which they occur. In the last decades, research has increasingly indicated that many of these processes display different forms of scale invariance, i.e., they show fractal geometry. This suggests that behind the apparent disorder and irregularity of the geometry of mineral deposits at different scales, an underlying regular pattern is present. If properly understood, this regular geometrical pattern could be useful in a variety of theoretical and applied fields. A great portion of this scale invariance is given by the structural framework during mineralization because structures are a dominant factor controlling fluid flow. Here, we assess the geometry of iron oxide copper-gold (IOCG) mineralization in the world-renowned Carajas mineral province, focusing on one of the largest and most economically important orebodies, the Sossego deposit. The geometry of mineralization is evaluated at the microscale (ore minerals in thin sections), local scale (orebodies in mine maps), and regional scale (deposit distribution on regional maps). We show that the spatial distribution and shape of ore minerals at the microscale is largely nonrandom, presents fractal geometry, and displays defined trends in spatial distribution and anisotropy. Additionally, the geometric trends observed at the microscale mimic those of the local-scale geometry of orebodies, as well as the regional-scale distribution of mineralization. The main property controlling the observed scale invariance is permeability, which is intrinsically associated with structures at multiple scales. These results contribute to further understanding the fractal nature of processes controlling mineral deposit formation and revealing new multiscale approaches to investigate structural controls on ore deposition.

Halogen (F, Cl, Br, I) behaviour in subducting slabs: A study of lawsonite blueschists in western Turkey

Abstract We examined the F, Cl, Br and I abundance of minimally retrogressed lawsonite blueschists from the Tavsanli Zone in northwest Turkey to evaluate the behaviour of halogens in subduction zones, and to determine the role coexisting high pressure minerals may play in transporting the halogens to the Earth's mantle. The blueschists contain sodic amphibole and lawsonite, with variable amounts of phengite and chlorite, and minor apatite. A positive correlation between Cl, Br and I contents in bulk rocks suggests their overall coherent behaviour in subduction zones, although high ratios of I/Cl and Br/Cl compared to altered oceanic crust indicate that Cl is preferentially lost relative to Br and I before or during blueschist metamorphism. Iodine and F are enriched relative to altered oceanic crust, suggesting incorporation from marine sediments. In situ analyses of minerals in thin sections reveal F preferentially concentrates in apatite (avg. 3.13 wt%), over phengite (482 ppm), lawsonite (avg. 413 ppm) and Na-amphibole (257 ppm). Chlorine also preferentially resides in apatite (138 ppm), followed by equal partitioning between phengite (59 ppm) and Na-amphibole (56 ppm), and lower concentrations in lawsonite (27 ppm). Upon apatite decomposition at a depth of ∼200 km, F may redistribute into lawsonite and phengite in slabs, whilst Cl is likely expelled to the overlying mantle wedge. Given the stability of lawsonite and phengite to a depth of 280–300 km in cold subduction zones, they may transport F beyond subarc depths, contributing to the high F in magmas derived from the deep mantle.

Different effects of temperature and salinity on permeability reduction by fines migration in Berea sandstone

Hot water injection into geothermal aquifers is considered in order to store energy seasonally. Berea sandstone is often used as a reference formation to study mechanisms that affect permeability in reservoir sandstones. Both heating of the pore fluid and reduction of the pore fluid salinity can reduce permeability in Berea sandstone. These effects could be caused by mobilisation of fines by increasing the repulsive electrical double layer forces among sandstone grains and the fines. We investigated the reversibility and the dependence on flow velocity and flow direction of the permeability change by means of flow through experiments and examined thin sections of samples prior to and after tests. A permeability reduction at 20°C with decreasing salinity was not reversed by restoring the salinity, whereas a permeability reduction due to heating to 80°C was reversible by restoring the temperature to 20°C. A reversible permeability increase with increasing flow rate was observed at 80°C but not at 20°C. We observed no difference in the distribution of kaolinite clay minerals in thin sections of untested and tested samples. Dissolution of iron bearing carbonates and precipitation of iron hydroxides was observed but no effect on permeability was found. The experimental results suggest that different mechanisms are responsible for permeability reduction depending on temperature and salinity.

Mineralogy and chemistry of altered Icelandic basalts: Application to clay mineral detection and understanding aqueous environments on Mars

We used a suite of techniques, including those emulating compositional data sets obtained from Mars orbit and obtainable at the Mars surface, to examine aqueous alteration of basaltic rocks from Iceland as a mineralogic and geochemical analog for Noachian environments on Mars. A sample suite was collected for laboratory measurement of (1) whole‐rock visible/near‐infrared (VNIR) reflectance and thermal infrared (TIR) emission spectra; (2) VNIR and TIR reflectance spectra of particle‐size separates derived from the bulk rock and from materials extracted from fractures/vesicles; (3) X‐ray diffraction (XRD) patterns for determination of quantitative modal mineralogy; (4) major element chemistry using flux fusion of whole‐rock powders; and (5) electron microprobe analyses of minerals in thin sections. Conclusions about aqueous alteration can be influenced by technique. For these basalts, whole‐rock chemical data showed scant evidence for chemical fractionation, but TIR, VNIR, and XRD measurements identified distinctive assemblages of hydrous silicate minerals, differing by sample. XRD provided the most complete and accurate quantitative determination of sample mineralogy. However, VNIR spectroscopy was the technique most useful for determining composition of low‐abundance smectite clays, and TIR spectroscopy was the most useful for recognizing hydrated silicates in thin surface coatings. High spatial resolution mineralogical and chemical data sets were useful for understanding the texture and distribution of alteration products and variations in fluid chemistry. No single approach provides a complete assessment of the environment of alteration, demonstrating the importance of employing multiple, synergistic mineralogical and geochemical techniques and instruments in exploration of rock strata from aqueous paleoenvironments on Mars.

Petrography and Petrogenesis of Pre-Mesozoic rocks, Ago-Iwoye NE, SW Nigeria

Detailed geological mapping was carried out in a part of Ago-Iwoye NE with a view to identify the rock types and determine their origin. Petrographic and whole rock analysis of seven rock samples collected during the initial mapping exercise was done through thin sectioning, and X-ray fluorescence. Minerals were studied in cross and plane polarized light; their contact angle and boundary interaction were inferred in thin section. Normative mineralogy of the rocks was also determined using CIPW Norm. The study area is underlain by both igneous and metamorphic rocks; the gneisses are migmatized and intruded by granitic rocks and pegmatite. The dominant minerals in thin section include quartz, biotite, muscovite and hornblende. Estimated mineral contact angle is ~56° and ~26° for hornblende-biotite and hornblende-muscovite, respectively. Mineral grain boundaries are planar and undulating in thin section. The geochemical study revealed major oxides such as SiO2, Al2O3, Fe2O3, CaO, MgO, K2O and Na2O. The highest estimated liquid temperature occurred in the gneisses with a value of ~1014°C, mean of ~862°C, range of ~281°C and water content of 1.6% which shows that the rocks are metamorphic in origin. The volume composition shows that quartz has the highest volume in all the seven samples with a percentage value of ~68%. Discriminant plots show that the granites are quartz-rich granitoid and granodiorite while the gneisses have high liquidus temperature; all the rocks samples record high alteration indices and are derived from passive and continental margins. Rocks of the study area are Pan African granitoids emplaced into the polycyclic Pre-Cambrian migmatized complex host rock. Citation: 2012. Petrography and Petrogenesis of Pre-Mesozoic rocks, Ago-Iwoye NE, SW Nigeria. In: (Ed.) Gordon S. Lister, Journal of the Virtual Explorer, volume 40, paper 1, doi: 10.3809/jvirtex.2012.00313 Journal of the Virtual Explorer, 2012 Volume 40 Paper 1 http://virtualexplorer.com.au/

Methods for the microsampling and high-precision analysis of strontium and rubidium isotopes at single crystal scale for petrological and geochronological applications

Micromilling of single crystals to yield microgram-sized solid samples for subsequent isotopic analysis can yield important petrogenetic information from crystals (especially feldspars) in magmatic rocks. Exceptional detail can be gained because the textural context of the sample area can be fully evaluated before milling. Such information is otherwise unknown or lost when performing analysis at the bulk rock scale. Here we present a comprehensive methodology for the precise analysis of ng-levels of Rb and Sr purified from microgram-sized solid samples extracted from minerals in thin sections. Physical sampling techniques are based around a computer numerical control (CNC) milling machine (Micromill™), newly designed specifically for the sampling of complex accretionary and growth structures in crystalline materials. Chemical procedures for the isolation of Sr and Rb for TIMS and MC-ICPMS analysis, respectively, are presented, and the performance of these analytical techniques is assessed in the context of the small sample sizes produced by micromilling. Despite being time consuming, mechanical sampling, conventional dissolution and chemical separation followed by analysis by TIMS remain the best methods for both highly accurate and precise determinations of Sr isotopic compositions in most geological materials over a wide range of Sr concentrations, Rb/Sr ratios and matrix types. Using these techniques, it is possible to achieve long-term 2 S.D. external precisions of 50 ppm for load sizes as small as 3 ng Sr. We demonstrate the validity of these techniques using two examples. The first is from a < 50 ka single feldspar crystal from Parinacota volcano (Chile) to show that 87Sr/ 86Sr isotope ranges as small as 0.00006 (2 × 2 S.D. external) can be resolved under conditions where radiogenic-Sr ingrowth can be neglected. The second is from the 28.4 Ma Fish Canyon Tuff (Colorado) to demonstrate the utility of isotope dilution measurements of Rb and Sr contents to calculate 87Rb/ 86Sr and thus perform age corrections to the 87Sr/ 86Sr ratios in order to establish 87Sr/ 86Sr i variations between single crystals or zones. We demonstrate that biotite crystals in the Fish Canyon Tuff show Sr-isotopic variations well beyond analytical errors and hence that the crystals involved were not in isotopic equilibrium and cannot be used to establish an isochron age. On the other hand, the precision of our isotope dilution measurements may be used to obtain Rb–Sr geochronological information at the single crystal scale, provided that homogeneity in 87Sr/ 86Sr at the time of crystallisation can be demonstrated.

In situ U–Pb dating of zircon formed from retrograde garnet breakdown during decompression in Rogaland, SW Norway

AbstractRegionally metamorphosed metapelites from Rogaland, SW Norway, contain zircon formed during the decompression reaction garnet + sillimanite + quartz → cordierite. The zircon, which occurs as inclusions in cordierite coronas around garnet, is texturally, chemically and isotopically distinct from older zircon in other textural settings in the matrix. A SHRIMP U–Pb age of 955 ± 8 Ma based on analyses in thin section on the decompression zircon from the cordierite coronas, therefore dates a point on the retrograde path, estimated from garnet–cordierite equilibria to be 5.6 kbar, 710 °C. This population was under‐represented in conventional SHRIMP analyses of individual zircon in a mono‐mineralic grain mount and, in the absence of a textural context, its significance unknown. The dominant age identified from SHRIMP analyses of the grain mount, in combination with analyses from matrix zircon in thin section, was 1035 ± 9 Ma. Based on the lack of consistent textural relationships with any specific minerals in thin section, as well as rare earth element chemistry, the 1035‐Ma population is interpreted to represent zircon growth during incipient migmatization of the rocks at 6–8 kbar and c. 700 °C. This is consistent with previous estimates for the age of regional M1 metamorphism during the Sveconorwegian Orogeny. The most important outcome of this study is the successful analysis of zircon grains in a specific, well‐constrained reaction texture. Not only does this allow a precise point on the regional P–T path to be dated, but it also emphasizes the possibility of zircon formation during the retrograde component of a typical metamorphic cycle.

Petrological and physico-chemical investigations on an Tunisian glauconitic deposit

The glauconite bearing rocks occurring in lower Eocene deposit of North-East of Tunisia were examined to study the composition and association of glauconite with different minerals. The study was carried out by petrographic investigation, X-ray powder diffraction, Scanning Microscopy with an Energy Depressive System methods. Green grains are a conspicuous component of the studied material. Although resembling glauconitic minerals in thin-section, electron microprobe analyses and acid treatment indicate that the green grains are formed by glauconite core and phosphate rim. The nature of (001), (002) and (003) spacing as found in the XRD patterns obtained for purified sample suggest the presence of glauconite mica mineral. XRD identification is amply supported by results of chemical analyses. Those revel high K and Fe contents in purified sample.

Procedimiento estereológico para la cuantificación del tamaño medio de grano de rocas en 2-D: aplicación a la interpretación petrofísica de la velocidad de corte de granitos con útiles diamantados

Grain size is a fundamental rock-forming characteristic of rocks that can be estimated under very different techniques. In this paper a simple and efficient 2-D stereological procedure for estimating the numerical density and the mean area of crystal sections as well as the area percentage of minerals in thin sections of rocks is presented. An example applied to the estimation of the mean section area and numerical density of the biotite crystals in a thin section of a tonalite is described in detail. Some results of the procedure application in a research project about Pink Porrino granite sawability are also presented.

Pertsevite, a new silicatian magnesioborate mineral with an end-member composition Mg2BO3F,in kotoite marble from east of Verkhoyansk, Sakha-Yakutia, Russia

Pertsevite is a new Mg borate from a contact-metasomatic kotoite marble from the region east of Verkhoyansk, Sakha-Yakutia, northeastern Siberia. The end member is defined as Mg 2 BO 3 F, which is the composition of a synthetic phase. The natural material consistently contains from 4 to 12 wt.% SiO 2 as well as some OH. This is due to the substitutions Si + O for B + F and OH for F. A representative composition, which is an intermediate member of a solid solution series Mg 2 BO 3 (F,OH) - Mg 2 SiO 4 , has the composition (EMP data, OH calculated for charge balance) SiO 2 8.25; Al 2 O 3 0.10; B 2 O 3 22.44; FeO 3.71; MnO 0.65; MgO 57.39; CaO 0.24; F 7.29; H 2 O calc 1.67; total 98.97 wt.%, corresponding to (Mg 1.88 Fe 2+ 0.07 Mn 0.01 )(B 0.85 Si 0.18 )O 3.21 F 0.54 (OH) 0.24 or simply Mg 2 (B 0.8 Si 0.2 )O 3.2 (F,OH) 0.8 . A general formula of silicatian pertsevite may be given as Mg 2 (B,Si)O 3 (F,OH,O). Like the synthetic end member Mg 2 BO 3 F, silicatian pertsevite is orthorhombic, space group Pna 2 1 , with a = 20.490(6), b = 4.571(1), c = 11.890(3) A, V=1113.61(5) A 3 , Z = 16 and D calc = 3.12 g/cm 3 for the above composition. Its structure is based on a hexagonal close-packed arrangement of anions similarly as for forsterite, but with cations occupying different sets of voids. BO 3 triangles are randomly replaced by SiO 4 tetrahedra with Si lying outside of the planes defined by the BO 3 triangles. The crystals are colorless in thin section, biaxial (+) with (for the above composition) α = 1.609(1), β = 1.620(1), γ = 1.642(1) for 589 nm, 2V Zmeas = 65(1) ° and no visible dispersion. In the one thin section of the kotoite marble available, pertsevite occurs as anhedral crystals interspersed with calcite, kotoite, forsterite, clinohumite, spinel, ludwigite and a new member of the hulsite group. Based on analytical data, but also on structural considerations, there is a large miscibility gap between silicatian pertsevite and practically boron-free forsterite. Pertsevite is probably more widespread than this single example suggests; its being overlooked may be explained by its close resemblance to forsterite and humite group minerals in thin section.

A Strategy for Teaching an Effective Undergraduate Mineralogy Course

An effective undergraduate mineralogy course provides students with a familiarity and understanding of minerals that is necessary for studying the Earth. This paper describes our strategy for integrating the disparate topics covered in a mineralogy course and for presenting them in a way that facilitates an understanding of mineralogy that enables students to apply it in subsequent courses and research. Our strategy is to organize the course into a well-integrated sequence of lectures, demonstrations and laboratory exercises that unfolds the material logically and at a pace that is responsive to the students' needs. The course begins with six weeks on crystal chemistry, then five weeks covering analytical methods for characterizing minerals and ends with five weeks on the silicates. This order facilitates a progression of learning from the basic concepts to the more advanced and allows us to reinforce the concepts of crystal chemistry during the final section on the silicates. Optical mineralogy is almost entirely taught in the lab and is aided by use of a mineral identification chart developed to help students learn to identify minerals in thin section. Student performance is assessed through one technical paper and presentation as well as homework, essay exams and lab practicals.

Geochemistry of Rewalsar Lake sediment, Lesser Himalaya, India: implications for source-area weathering, provenance and tectonic setting

Major, trace and rare earth element (REE) chemistry of fresh-water lake sediments, stituated in the middle Siwalik Formation (Pliocene), have been investigated. The log (Fe2O3/K2O) vs log (SiO2/Al2O3) plot of sediments discriminate the rock types of the catchment which are remarkably similar to those observed in thinsections. The Chemical Index of Alteration (CIA) between 75.2 to 78.1 and Chemical Index of Weathering (CIW) between 79.5 to 89.8 are higher than Post-Archean Australian Shale (PAAS), indicating moderate to high weathering at source area. The PAAS normalized REE patterns are similar and differentiate the major rock types on the basis of abundance of REEs in accordance to clay minerals, grain size, with the highest concentration in the finest grain sediments. It also indicates poor mixing of sediments. The consistency of REEs in sediments with those of rock types illustrate almost unaltered, unmodified transport of these from the provenance to the detritus deposited in the basin and also shows that the REEs are enriched in phyllosilicates. The major elements, Fe2O3, TiO2, MgO, K2O show strong positive correlation with Al2O3 and follow the trend of the latter, and REEs also indicate same characteristics, hence phyllosilicates are likely to be the promising host for REEs. The negative correlation of La, Yb and Y with Zr and positive correlation with REEs and Al2O3 do not indicate Zr control on REE distribution in the present case. The petrographic character of minerals in thinsection of rocks as well as of lake sediments and low Na2O/K2O ratio in the latter show short distance transport, textural immaturity and sorting of grains. The higher Zr/Th and Zr/Yb ratios in coarser sediments than in fine grain detritus are supporting evidence of sedimentary sorting. Chondrite normalized REE patterns and negative Eu anomaly of the lake sediments are similar to that of PAAS indicating that they may have been originally derived from differentiated silicic (felsic) and/or recycled sedimentary source and deposited in foreland basin. However, petrographic character, REEs and high field strength elements estimated on sediments, tends to support metamorphic source area for middle Siwalik detritus and is consistent to the observations made on rocks exposed in the catchment area, attesting to the rise of Himalaya in phases, exposing fresh rocks for erosion/weathering, lying northward in Lesser Himalayan domain. Plot of sediment chemistry of lakes on tectonic delineation diagrams show that Rewalsar detritus derived from middle Siwalik rocks are from passive margin setting. The discriminant function plots of sediment samples also indicate that they are derived from craton interior or a quartzose sedimentary orogenic terrain and deposited in a passive margin setting. The chemistry of Rewalsar sediments is thus a proxy for the middle Siwalik rocks deposited in the foreland basin and throw light on provenance, source area weathering, and tectonic setting of the source rock.

Application of a frequency quintupled Nd:YAG source (λ=213 nm) for laser ablation inductively coupled plasma mass spectrometric analysis of minerals

This paper reports the use a frequency quintupled Nd:YAG laser (λ=213 nm) for laser ablation of minerals for ICP-MS analysis. The fifth harmonic was produced by in-house modification to an existing frequency quadrupled Nd:YAG laser (λ=266 nm), using commercially available optical components. The maximum pulse energy of the 213 nm output is 7.5 mJ per pulse, based on an output of 350 mJ per pulse at 1064 nm. The pulse energy is controlled and attenuated by a wave plate and MgF2 polariser combination. The laser sampling system was coupled to an enhanced sensitivity ICP-MS system. Comparison of time resolved signals for extended analyses of several materials using 213 and 266 nm for ablation demonstrates that (1) 213 nm laser ablation greatly reduces the incidence of catastrophic ablation of strongly cleaved minerals in thin section, owing to higher absorption; (2) in each case, 213 nm laser ablation produces a longer, flatter, higher intensity signal than conventional 266 nm laser ablation, suggesting a larger volume of transportable particulate is produced; and (3) inter-element fractionation is reduced during analysis using the 213 nm laser ablation system. This study is the first reported use of frequency quintupled laser ablation microprobe (LAM)–ICP-MS.