| Geochemical analysis (Analytical Geochemistry) is an important part of modern geochemistry, is an interdisciplinary subject of earth and analytical chemistry. This subject provide the elements or compounds composition information in geological samples for geologists, using chemical analysis as tool. Geochemical analysis is an important foundation of the contemporary earth science research, mineral resources and geological environment evaluation. The development of geochemical analysis closely around the needs of the development of modern earth science.For many decades Inductively Coupled Plasma Mass Spectrometry (ICP-MS) has occupied an outstanding position among the analytical techniques due to its universality, high sensitivity and wide fields of application in elemental analysis, especially in the determination of trace and ultratrace elements and the precise and accurate isotope ratio measurements. Knowledge of the chemical composition of geological samples is the prerequisite for understanding their characteristics, which provide us with important information regarding crust and mantle evolution and allow for the quantification of other geochemical processes. In this study, we introduce a new NH4HF2-open vessel acid digestion method, using this method to dissolve bauxite samples. We carried out in situ Pb isotopic ratio analysis method for feldspars and sulfides using LA-MC-ICPMS. The studies involved Pb isotope accurate determination for poor-lead samples, the matrix matching correction strategy for sulfides and the development of preparation for sulfides and feldspars standard samples. Fangshan pluton was selected as an example to evaluate the in situ Pb isotope antalysis.1. The development of total rock dissolution technology using ammonium bifluoride (NH4HF2) in screw-top Teflon beakers vials.Complete sample digestion is prerequisite for achieving reproducible and accurate analytical results for geological samples. Open vessel acid digestions successfully dissolve mafic samples, but this method cannot achieve complete dissolution of felsic samples due to the presence of refractory minerals such as zircon. In this study, an efficient and simplified digestion technique using the solid compound ammonium bifluoride (NH4HF2) in a screw-top vial has been developed for multi-element analysis of different types of rock samples. NH4HF2 has a higher boiling point (239.5℃) than conventional acids such as HF,HNO3, and HCl, which allows for an elevated digestion temperature in open vessels, enabling the decomposition of refractory phases. Similar to HF, HNO3 and HCI, ultra-pure NH4HF:can be produced using a conventional PFA sub-boiling (heating and cooling) purification system. A digestion time of 2-3 hours for 200 mg NH4HF2 in a Savillex Teflon vial at 230℃ is sufficient to digest 50 mg of the felsic rock GSP-2, which is about five times faster than using conventional closed vessel acid digestion at 190℃ (high-pressure PTFE digestion bomb). The price of a Savillex Teflon vial is far less than the price of a high-pressure PTFE digestion bomb (consisting of a PTFE inner vessel and an outer stainless steel pressure jacket). Moreover, the NH4HF2-open vessel acid digestion can apparently suppressis not hampered by the formation of insoluble fluorides. We have successfully applied the NH4HF2-open vessel acid digestion to the digestion of a series of international geological reference materials, including mafic to felsic igneous rocks and shales. This method provides an effective, simple, economical and comparatively safe dissolution method that combines the advantages of both the open and closed vessel digestion methods.2. An application of ammonium bifluoride digestion methods in bauxite materials using ICP-MS.Bauxites are the major raw material source of aluminum, and an important resource for Ga and REE. In recent decades, geological, petrographic, mineralogical and geochemical investigations of bauxite deposits have been carried out to research their formation by a number of authors. The composition of trace elements in bauxite samples can provide useful information about bauxite deposits. Sample digestion is a fundamental and critical stage in the process of geochemical analysis, which enables the acquisition of accurate trace element data by ICP-MS. However, the conventional bomb digestion method with HF/HNO3 results in a significant loss of trace elements (Li、Cr、 Ga、 Sr、Y、Mo、Th、REEs) due to the formation of insoluble AIF3 precipitates during the digestion procedure of the bauxite samples. In this study, the bauxites digesting capability of the following methods was investigated:the "Mg-addition" bomb digestion method. the NH4HF2 open vessel digestion method and the NH4F open vessel digestion method. The "Mg-addition" bomb digestion method can effectively suppress the formation of AlF3 and simultaneously ensure the complete decomposition of resistant minerals in the bauxite samples. The addition of MgO to the bauxite samples produces (Mg+Ca)/Al ratios≥1. However, adding a large amount of MgO leads to significant blank contamination for some transitional elements (V, Cr, Ni and Zn). The NH4HF2 or NH4F open vessel digestion method can also completely digest the resistant minerals in the bauxite samples in a short period of time (5 hours). Unlike the conventional bomb digestion method with HF/HNO3, the white precipitates and the semitransparent gels present in the NH4HF2 or NH4F digestion methods can be efficiently dissolved by evaporation with HCIO4. Based on these 3 optimized digestion methods,37 trace elements including REEs in the 10 bauxite reference materials were determined by ICP-MS. The data obtained from different digestion methods showed excellent inter-method reproducibility (agree within 5%for REEs). The precisions for most elements were lower than 6%. The concentrations of trace elements in the 10 bauxite reference materials show good agreement with the provided limited certified values and information values. New trace elements data for the 10 reference materials are provided, some of them are first published, and will be useful to future geochemical studies of bauxite deposit.3. In situ lead isotope measurements for low-Pb materials using LA-MC-ICP-MS:application to the study of the origin of Fangshan pluton, North China.The lead isotope system has been widely used as a highly effective tracer and a valuable geochronological tool in geoscience. tracking magmatic source components and geological processes. Highly precise and accurate measurements of the common Pb isotopic compositions can be obtained using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) and the Tl-normalized method or the Pb double spike method. In comparison with solution introduction MC-ICP-MS, in situ Pb isotopic measurements using laser ablation MC-ICP-MS offer the spatial resolution to identify and distinguish the inter-and intra-crystalline isotopic variations of mineral grains on the scale of tens of microns; it has other advantages as well, such as high throughput and good precision and accuracy for Pb "rich" materials. As a result LA-MC-ICPMS for Pb isotopic measurements has received increasing interest in applications of tracking sources and geological processes, sediment provenance investigations and petrogenetic interpretations of melt inclusions.In this study, a laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) method is developed to obtain precise and accurate Pb isotopic ratios in low-Pb materials (< 10μg g-1) using a combination of Faraday cups and ion counters. The low abundance 204Pb (～1.4%) was collected using an ion counter. An NBS 981 standard solution was used to cross-calculate the Faraday cup and ion counter (FC-IC) gain and to investigate the signal response characteristics of the ion counter. In addition, a non-linear response of the ion counter used in this study was observed and corrected by a non-linear correction algorithm, which was established by measuring a series of gravimetrically prepared NBS 981 standard solutions (NIST SRM 981). Compared to the standard arrangement, the use of the newly designed X skimmer cone and Jet sample cone improved the signal intensities from the Pb isotopes by a factor of 1.9. Eight natural glasses and the NIST 612 reference material glass were measured to evaluate the new protocol for Pb isotope analysis, and NIST 612 was using as external standard. The analytical results were in agreement with the reference values within 2SD measurement uncertainties. For MPI-DING ATHO-G (5.67 μg g-1 total Pb), KL2-G (2.07 μg g-1 total Pb) and ML3B-G (1.38 μg g-1 total Pb), the typical accuracies of 20xPb/204Pb were 0.09% of preferred values with precisions of<0.33%(2RSD).The Pb isotope ratios in feldspars from granodiorite and within mafic microgranular enclaves (MMEs) from the Fangshan pluton, North China, were measured using the present method. The Pb isotopic compositions of feldspars from the whole host granodiorite show that that are radiogenic in the margin zone and gradually become less radiogenic. For the MMEs, the Pb isotopic compositions of feldspars are highly variable and overlap with that of the whole host granodiorite. For single-grain feldspar, the strong rim-core-rim variations of the Pb isotopic compositions and trace elements are interpreted to have been generated via magma mixing. These results suggest that the Fangshan pluton underwent magma mixing of mantle-derived mafic magmas with felsic magmas, and the proportion of the mafic magma influx decreased over time.4. In situ lead isotope measurements for sulfides using LA-MC-ICP-MSIsotope geochemistry is established as a valuable tool in studies of the origin and evolution of ore deposits. In addition to age information, isotope ratios provide sensitive chemical fingerprinting of a wide range of processes related to ore formation:including the sources of metals, magma evolution and fluid sources. Ongoing analytical advances is this field are opening up new possibilities, in particular the ability to analyse ore-forming minerals directly, with increasing precision and on smaller sample volumes. LA-MC-ICPMS have great potential to provide new insights into ore-forming processes. In this study, a 193nm laser ablation systrm combining with MC-ICPMS was used to establish in situ lead isotope ratio analytical method for sulfides. Our research shown that using the silicate glass NIST SRM 610 as an external standard can not correct the instrumental mass fractionation because of the matix effect. The intenference correction of 204Hg in 204Pb can be abtained even 204Hg/204Pb< 2, when the relationship between the mass fractionation factor of Tl and that of Hg is accuracy. Lead isotope ratio do not change when sulfides are ablsted under vatious spot size and pulse frequency. This is benefit to expend the signal range for Pb isotope. We were tried to produce the standard sample by press method and fuse method. Using the established LA-MC-ICPMS method to analyze the synthetic sulfide standard, laser ablation and dissolution based measurements of sulphide powders are in agreement, indicating this mthod allow the Pb isotopic ratio determination for sulfides. |
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