Zircon (ZrSiO4) is the most commonly used by-mineral for dating geological samples, and it is widely present in a variety of medium- acidic igneous, metamorphic and sedimentary rocks. However, the basal-super-bedrock rock is not easy to grow zircon because of Si unsaturated, and foreign zircon may be captured in the magmatic process, in which case the age of zircon is more complicated and easy to cause controversy. So, for basal-supermassive rocks, what minerals should be used to date them? This brings us to the protagonist we are going to talk about today— the slanted zircon.
The chemical formula of oblique zircon is ZrO2, which exists in a variety of silicon unsaturated rocks (such as kimberlite, carbonate rock, alkaline syringite, magnesium ferrous-supermagomaglucite intrusive rock, etc.), with high U content and minimal initial Pb, which is very suitable for U-Pb system dating. It rarely exists in the form of trapped crystals, and is less likely to lose Pb than zircon, so the genesis of oblique zircon is clear, can represent the crystallization age of magma, and is considered to be the most important dating mineral of silicon unsaturated rocks. It is no exaggeration to say that for the dating of basal-superstition rocks, only the age of the slanted zirconite is convincing. However, in basal-superseditive rocks, oblique zircon is generally low in content and small in particle size. Because the particles are small, it is not easy to dissociate with other low-density minerals, resulting in low density density, and the mineral processing method will lose more; at the same time, because the sheet crystal shape of the oblique zircon makes it produce a higher specific surface, the dissociated single crystal is easy to float and washed away by the water flow, so the traditional sand pan / shaker method is used to sort, the recovery rate of the oblique zircon is often very low, and the general selection of dozens of particles by several kilograms of samples is quite successful. The double difficulties of low content and low recovery rate make oblique zircon a rare mineral sought by researchers, and sometimes even can only "look at the stone and sigh". In order to get a sufficient amount of oblique zircon single minerals, every time in the wild to recover as many samples as possible, always afraid of not enough samples, it is not easy to go out of the field, from the implementation of funds to route planning, it takes time and energy to achieve the purpose as much as possible. If it is only one sample, it may not be a big problem to bring more, but it is often studied in a series of samples, so is it necessary to take back hundreds of kilograms of samples? Sampling in the plains is good, if you sample in the plateau above 4,000 meters above sea level, the taste of carrying the weight forward can be imagined. Going abroad to attend conferences often visits the classics, and at that time there was a feeling of "inadequacy". Deep-sea samples or core samples are small and precious, so how to deal with it?
In view of the above problems, Guo Qian, senior experimentalist of the Institute of Geology and Geophysics of the Chinese Academy of Sciences, and researcher Li Qiuli and others have developed a new method for efficient sorting of oblique zircon - acid solution. This method mainly takes advantage of the different solubility of different minerals in HF, HNO3 and HCl to dissolve the non-destination minerals and leave the minerals of interest. In order to demonstrate the feasibility and superiority of the method, several processes were compared. A uniform diabase sample was selected, of which ~1 kg was sent to a professional mineral processing company, and 12 oblique zircons were selected. Another stone weighing 118 grams, after being crushed and dried by high pressure, obtained 113 grams of powder, randomly divided into 6 parts, each part ~ 19 grams. Using the improved shaker method of S derlund and Johansson (2002), under the operation of senior technicians with more than 5 years of beneficiation experience, 2 samples were divided into ~12 oblique zircons. Take 2 samples by acid-soluble method, and each sample is divided into ~160 oblique zircons (10 to 100 μm long and 4 to 50 μm wide), that is, an average of 1 gram of samples are selected. Through the comparison of the same samples, the recovery rate of the acid solution method can be increased by dozens of times, and the technical requirements for the experimenters are not high.
The main sorting process is shown in Figure 1:
(1) Crushing samples: Crushing rock samples to ~200μm (it is recommended to high-pressure pulse crusher, conventional jaw crusher and disc crusher to avoid pollution).
(2) Dissolved silicate: Add samples and 150 mL of 22 M HF to a 300 mL Teflon container and react at room temperature for 36 h. Oscillate the Teflon container every few hours in the middle (tighten the lid) to help the sample react adequately with HF. The upper layer of clear liquid is then poured out, leaving behind a white precipitate, which is an insoluble fluoride produced by the reaction of HF and minerals.
(3) Remove fluoride: add 200mL of 2.5 M HCl and 6 gH3BO3, heat 4 h at 100 ° to dissolve fluoride. Pour out the solution, leaving the minerals. If there is still a white pellet, repeat this step until the white pellet is completely dissolved.
(4) Dissolving non-silicates: After step (2), the sulfide and magnetite have not yet dissolved. Add 45 ml of 6M HCl and 15ml of 8M HNO3, heat at 120° for 24 hours, cool to room temperature and pour out the solution.
(5) Transfer the sample: Wash the remaining minerals in the Teflon container 4 times with distilled water, then transfer the sample to the surface dish, place it under the binocular scope for examination, and transfer the oblique zircon to the sulfuric acid paper sample bag with a capillary straw.
To improve efficiency and reduce dissolution time, both silicate and non-silicate minerals can be dissolved. ~19 g samples were dissolved with 120 mL 22 M HF and 60 mL 8 M HNO3, heated at 120° for 5 h, cooled to room temperature and then poured out the upper layer of clear liquid. The next steps for fluoride removal and sample transfer are the same as those in steps (3) and (5) above.
It is worth noting that HF, HCl, HNO3 are highly corrosive, acid-soluble operation must be carried out in a fume hood, and the operator wears protective clothing, rubber gloves and gas masks to avoid acid and gas contact with the skin.
Modified from Guo et al. (2022)
Figure 1 Acid-soluble sorting process
Compared with the traditional shaker/sand pan- heavy liquid-magnetic separation method, the main reason why the acid-soluble sorting process can significantly improve the recovery rate of oblique zircon is twofold: (1) The acid-soluble method can expose the oblique zircon in the form of inclusions, so that these oblique zircons in the form of inclusions are sorted out, which is impossible to achieve by traditional methods. (2) The traditional method is to rely on the flushing of water to wash away light minerals and leave heavy minerals, but the special plate-like crystal shape of oblique zircon makes it easy to be washed away like light minerals. The shape of the associated minerals, the velocity of the water flow, the tilt of the shaker and the experience of the sorter will also affect the sorting effect. The acid-soluble sorting result depends only on the solubility of the mineral, so the sorting process is more accurate, which further avoids the loss of oblique zircon.
After watching the sorting process, some people may be worried that the use of concentrated acid will affect the U-Pb system of oblique zircon, and we have also given a comparative analysis. In this study, the U-Pb test of oblique zircon obtained by heating HF+HNO3 to dissolve non-target minerals was carried out by ion probe (SIMS) method, and the results were compared with the results of unaccharged oblique zircon U-Pb determined by isotope dilution thermoionization mass spectrometry (ID-TIMS). Within the margin of error, their results are consistent (Figure 2), indicating that the acid-soluble process does not cause significant Pb loss of the oblique zircon, at least the U-Pb system inside the oblique zircon particles remains closed. Therefore, it is feasible and reliable to sort oblique zircon by acid-soluble method. In the experiment, we found that zircon, rutile and cassiterite are also suitable for the above acid-soluble methods, but further geochemical analysis is required to determine whether the acid-soluble method has caused disturbances to the isotopic system of these minerals.
(a) U-Pb harmonic plot of oblique zircon sorted by SIMS for acid-soluble method determination; Modified from Guo et al. (2022) and Peng et al. (2011)
Fig. 2 Comparison of U-Pb analysis of oblique zircon
The acid-soluble method increases the recovery rate of oblique zircon by at least one to two orders of magnitude, so that the sample usage of sorted oblique zircon can be reduced from kilograms to grams. Once this method was popularized, researchers no longer had to bother to recover so many rocks. This not only reduces the burden of field sampling by researchers and the intensity of work of experimenters sorting oblique zircons, but also provides a solution to the sorting problem of special precious samples.
The research results were published in the international academic journals ACS Omega (Guo Qian, Li Qiuli*, Chu Yin, Ling Xiaoxiao, Guo Shun, Xue Dingshuai, Yin Qingzhu. An Acid-Based Method for Highly Effective Baddeleyite Separation from Gram-Sized Mafic Rocks[J]. ACS Omega,2022, 7, 3634 3638. DOI: 10.1021/acsomega.1c06264)。
Editor: Chen Feifei
Proofreader: Qin Huaqing Jiang Shumin