New method for simultaneous determination of zircon water content and oxygen isotope developed by Guangzhou Geochemical Institute

[ Instrument Network Instrument R & D ] Water is an important medium for the evolution of the earth. The water content of nominally anhydrous minerals has a profound effect on the physical and chemical properties of rock minerals and is an important geochemical indicator. The oxygen isotope also has important indications on the mineral itself and the source of water. Simultaneous testing of these two indicators can not only expand the function of the instrument, double the use efficiency of large-scale secondary ion mass spectrometry (SIMS) instruments, but also avoid data decoupling caused by the inhomogeneities widely present in geological samples To ensure correct scientific interpretation of experimental data.
Although SIMS is widely used in the analysis of water content of nominally anhydrous minerals, it was previously limited to small instruments such as CAMECA IMS 3-7f series and NanoSIMS. Because water is extremely difficult to remove in a vacuum, only a very high vacuum can remove water. The background value was reduced below 10 ppm. This type of instrument has a small cavity and is easy to obtain high vacuum, but it cannot obtain high-precision oxygen isotope information. Large SIMS has the ability to measure mineral water content and oxygen isotopes at the same time, but the cavity is large, and the degree of vacuum is difficult to compare with small SIMS. In the past, the background value of water content testing using large-scale high-precision SIMS was as high as 40 ppm, which is difficult to meet the test requirements of low water content samples.
Zircon is one of the most widely used anhydrous minerals in earth science research. It can be used for U-Pb dating, Li-O-Hf isotope systems, and Ti thermometers, and has formed a special discipline-zircon . Previous studies have shown that crystalline zircon contains a certain amount of water, and the amount of water content is of great significance for the study of geodynamic processes. The SIMS test of zircon water content is facing difficulties such as high background value of the instrument and lack of reference materials for zircon water content. To successfully test the water content and oxygen isotopes in zircon simultaneously, the development of standard samples and the reduction of the background value of the instrument are indispensable.
In response to the above problems, Xia Xiaoping, a researcher at the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, cooperated with the University of Science and Technology of China and Nanjing University to conduct research using CAMECA IMS 1280-HR secondary ion mass spectrometer. After a large number of comparative studies and conditional experiments, this study found a non-gassing tin-bismuth alloy with a melting point of 90-110 ° C and a Brinell hardness of about 20 to replace the epoxy resin that would outgas and destroy the vacuum in a high vacuum environment. Making target. The obtained alloy target can be ground and polished like a resin target, which overcomes the defect that other metal targets such as indium targets need to be ground and polished in advance, making small particle sample preparation possible. The flatness of the target surface of the sample obtained by this method is comparable to that of traditional epoxy resin targets, and the analysis accuracy of oxygen isotopes is guaranteed. This research also independently developed a set of liquid nitrogen automatic filling device suitable for the cooling system of the SIMS sample chamber, which overcomes the shortage of the instrument itself that needs to be frequently filled with liquid nitrogen (about once every 4 hours), making the instrument sample analysis room stable for a long time. It maintains a high vacuum below 2 × 10-9 torr, which reduces the background value of large-scale high-precision SIMS water content measurement to below 10 ppm for the first time in the world. On this basis, the study included 20 gem-grade zircon particles, including some commonly used U-Pb dating and oxygen isotope test standards, and conducted FTIR tests at the School of Earth and Space Sciences, University of Science and Technology of China to verify their water content. Uniformity. Finally, 8 uniform zircon fragments with uniform water content were successfully selected for the next SIMS test.
Infrared spectroscopy test results show that the analyzed zircon fragments have suffered only a low degree of radioactive damage, all of which are non-degraded zircons with relatively uniform water content. After repeated SIMS tests, most of the internal accuracy of the 16O1H / 16O ratio of zircon particles is better than 0.3 ‰, and the external accuracy is better than 5% (2SD). It has high accuracy and reproducibility. The analysis points show outlier characteristics, which may be caused by the small water-rich inclusions inside the sample. The oxygen isotope data of the obtained zircon samples are basically equivalent to that of conventional secondary ion mass spectrometry oxygen isotope analysis, and the internal and external accuracy is better than 0.4 ‰ (2SD). A linear fit between the water content obtained by the FTIR test and the 16O1H / 16O ratio of the SIMS test established a calibration curve for the zircon water content of the secondary ion mass spectrometer. Linear relationship.
Related results were published as a "cover article" in the Journal of Analytical atomic spectrometry, which reviewers considered to be the best data on the water content of nominally anhydrous minerals. The method proposed in this research has also been granted an international invention patent "a method for simultaneous analysis of water content and oxygen isotopes in zircon based on large-scale secondary ion mass spectrometry" (patent number 201711103811.4).

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