Abstract:

Terrestrial in-situ cosmogenic nuclide dating(TCND)is one of the most important geochronological techniques for the paleoseismic study of bedrock fault scarps, landslides, and rock avalanches. With many target minerals, due to its uncomplicated composition, widespread occurrence, and simple chemical treatment, Quartz has emerged as an ideal dating material for terrestrial in-situ cosmogenic nuclides dating methods, such as ¹⁴C, ¹⁰Be, ²¹Ne, and ²⁶A1. Prior to accelerator mass spectrometry measurement, the separation of pure quartz from field-collected rock samples was a pivotal step in TCND. However, the elevated aluminum content in quartz samples undermines the reliability of TCND results. Generally, most of the aluminum content in samples originates from impurities like feldspar. To ensure accurate dating outcomes, the content of Al in samples should be reduced to less than 200 ppm. Therefore, effective separation of feldspar and quartz in samples and obtaining pure quartz is the first step in TCN dating. The conventional HF/HNO₃ etching method to separate and purify quartz is widely utilized, but it is time-consuming and low-efficiency. Particularly during the HF/HNO₃ etching stage when dealing with granitic samples containing abundant feldspars and mica impurity minerals necessitates repeated treatments to eliminate feldspars completely; this not only increases etching cycles but also leads to sample loss significantly. It has a great impact on the application of in-situ cosmogenic nuclide dating in active tectonics. Consequently, physically separating quartz from samples before chemical purification can effectively shorten the chemical etching duration while the flotation separation method can effectively remove most gangue minerals in quartz and achieve preliminary purification of quartz.

This article presents a laboratory-integrated flotation purification device and proposes enhancements to the conventional quartz etching process in order to improve its purification efficiency. The purification device uses dodecylamine as the collector, hydrofluoric acid as the feldspar activator, nitric acid as the regulator, and eucalyptus oleanol as the foaming agent. The bubbling component within the device provides sufficient carbon dioxide bubbles to float out feldspar and other minerals in the sample reversely. To evaluate its efficacy in flotation separation, enrichment, and purification, this study conducted tests on two commonly encountered bedrock samples of granitic gneiss and quartzite.

Observation results under a stereomicroscope show that the quartz content in the quartz component after floating is more than 90%. The etching results of the whole rock and the floated quartz components show that after etching 2-3 times with HF/HNO₃, the Al concentration can be reduced to less than 200ppm, which fully meets the requirement of cosmogenic nuclide dating. The quartz separated by flotation from cryptocrystalline quartzite samples can also reach the dating requirements after etching 7-8 times.

Compared to direct etching following bulk-rock sample crushing, this approach reduces etching time by over a half, significantly minimizing reagent consumption for HF/HNO₃ etching and thereby enhancing TCND efficiency. The bubbling power section of our flotation device directly introduces carbon dioxide gas into the flotation liquid to increase the bubble content in the slurry. Consequently, there is improved collision and contact between quartz and feldspar particles with bubbles, resulting in enhanced flotation effectiveness. This system can be effectively employed for separating feldspar from other impurity minerals present in gneiss samples. The proposed flotation process in this study is straightforward and user-friendly while allowing flexibility in adjusting sample quantities ranging from tens to hundreds of grams as required. Furthermore, this high-efficiency flotation separation system may offer insights into processing zircon, apatite, and other dating samples.

Key words: TCN dating, Paleoearthquake, Quartz purification, Flotation separation system

摘要：

就地宇宙成因核素测年是基岩断层面和滑坡、 崩塌体古地震研究中至关重要的年代测定方法之一, 其测定的目标矿物较多。石英因其成分简单、 分布广泛且易于进行化学处理而成为就地宇宙成因核素测年中的一种理想测年矿物。从野外采集的岩石中分离出纯石英是就地宇成核素测年实验的首要步骤, 同时也是一个关键环节。常规的HF/HNO₃蚀刻分离提纯石英的方法耗时长、 效率低, 影响就地宇宙成因核素测年方法在活动构造中的应用。文中报道了一种采用实验室集成的石英浮选提纯装置及提纯方法, 选取活动构造研究中2种宇成核素测年常用的长英质岩石样品——花岗质片麻岩和石英岩, 经破碎、 筛分出0.25~0.50mm粒径的颗粒作为试验样, 成功实现了石英与长石等杂质矿物的有效分离, 获得的石英组分中石英占90%以上, 效果显著。结果表明, 花岗岩类样品浮选分离后的石英组分再用HF/HNO₃继续蚀刻2、 3次后, 石英中的Al含量降至200ppm以下, 完全满足宇宙成因核素测年的要求, 隐晶质的石英岩经过浮选分离再蚀刻提纯, 比全岩粉碎后直接提纯减少了一倍以上的时间。文中的浮选提纯装置和方法极大地减少了HF/HNO₃蚀刻时的用酸量, 降低了成本, 同时也节省了蚀刻时间, 提高了宇成核素测年石英分离提纯的效率。同时, 还可为低温热年代学中提取锆石、 磷灰石的处理流程提供借鉴。

关键词: 宇宙成因核素测年, 古地震, 石英提纯, 浮选