Abstract:

(U-Th)/He dating is characterized by a low closure temperature(approximately 70℃) and exceptional sensitivity to low-temperature thermal events, allowing for the reconstruction of detailed thermal histories in geological bodies below 300℃. This method has significant potential for constraining the timing of ore deposit formation, documenting uplift and erosion, investigating mountain denudation processes, deciphering deep-time thermal histories, pinpointing the timing of metamorphic deformation, and tracing thermal-tectonic processes in continental rifts and passive margins. Careful consideration of mineral internal structure, closure temperature,⁴He diffusion and retention behavior, and radiation damage mechanisms is critical for accurate data interpretation. In addition to commonly used apatite and zircon, the method has been expanded to include minerals such as titanite, xenotime, rutile, and garnet, which are suitable for investigating ancient metamorphic thermal events(up to 3 950Ma), including the emplacement age of kimberlites and the burial-exhumation histories of cratons. Moreover, (U-Th)/He dating applied to monazite, garnet, and olivine presents promising solutions for dating young volcanic rocks(as recent as 2.0ka), offering new geological insights. Minerals such as hematite, magnetite, and fluorite allow for the direct dating of ore deposits. Studies on goethite offer new methods for determining the timing and rates of precipitation and weathering in extremely young geological events(~0.4 Million Years), which aids in continental surface reconstruction. The inclusion of minerals such as calcite, conodonts, and crinoids enriches the toolkit for studying sedimentary basin evolution, while investigations of meteorites offer unique perspectives on planetary formation and evolution. Accessory minerals such as spinel, perovskite, and epidote highlight their potential applications in future geochronological studies.
In (U-Th)/He geochronology, different minerals demonstrate distinct advantages and limitations. Apatite (U-Th)/He ages are affected by radiation damage and α-particle ejection effects. Age precision can be enhanced through α-particle capture model corrections, multi-elemental analyses, and selection of unaltered grains. The ⁴He diffusion behavior in titanite is strongly influenced by crystal size, with radiation damage as a key factor. Monazite, enriched in cerium and lanthanum, exhibits high resistance to radiation damage, though its ⁴He diffusion is significantly affected by thorium content and lattice defects. Xenotime exhibits anisotropic ⁴He diffusion; high U-Th-induced radiation damage alters diffusion behavior, thereby increasing age uncertainty. Conodonts can constrain the thermal evolution of sedimentary rocks; however, their U-Th content, REE concentrations, and microstructural features influence the reliability of their ages. Zircon, rich in uranium and thorium, displays low ⁴He diffusivity, which is modulated by grain size, morphology, and accumulated radiation damage. An ideal zircon grain is a tetragonal prism with a 2︰1 length-to-width ratio and a size ranging from 75 to 150μm. Selecting transparent grains with minimal internal cracks or inclusions and evaluating them using multiple analytical techniques can improve age precision. The garnet (U-Th)/He method is particularly effective in determining the emplacement ages of kimberlite bodies and constraining the timing of volcanic eruptions. Olivine contains relatively low concentrations of uranium and thorium, yet it exhibits stable ⁴He diffusion properties. However, challenges persist, including reduced age precision resulting from low U-Th content, complexities in correcting for initial ⁴He, and the implantation effects of ⁴He from surrounding basaltic matrices. Rutile's susceptibility to radiation damage and its anisotropic ⁴He diffusion behavior can significantly affect dating accuracy. Nonetheless, rutile remains a promising chronometer for unraveling the thermal histories of metamorphic and igneous terrains. Hematite, characterized by multiple diffusion domains, can effectively retain its initial ⁴He, making it applicable for studying fault slip histories and the timing of hydrothermal fluid circulation. Major limitations include high-temperature ⁴He release, grain size reduction from fault slip, and surface alteration effects. Low U-Th content, intrinsic crystal defects, and hydration behavior further reduce ⁴He retention in hematite, resulting in closure temperatures as low as 25-60℃. Magnetite (U-Th)/He geochronology faces issues including sluggish ⁴He diffusion, variable sample purity, and the risk of uranium loss during high-temperature extraction. Calcite, despite being abundant and chemically stable, is limited in (U-Th)/He dating by its low ⁴He retention, low closure temperatures(40-80℃), and inherently low helium concentrations. Excess ⁴He within inclusions and complex multi-domain diffusion behavior contribute to significant variability in age results. Crinoids are capable of resolving thermal histories within the 60-110℃ range. Fluctuations influence the spatial and temporal distribution of crinoids in sea level and oceanic geochemistry. Despite technical challenges-such as low equivalent uranium content and poorly constrained ⁴He diffusion-crinoid (U-Th)/He geochronology holds considerable promise for paleoenvironmental and paleoclimate studies when optimized analytical protocols, improved diffusion models, and targeted fossil specimens are employed. Fluorite (U-Th)/He dating offers unique advantages, especially in the absence of traditional chronometers, making it an indispensable tool for dating low-to high-temperature hydrothermal systems. Although its closure temperature and ⁴He diffusion behavior remain poorly constrained, fluorite (U-Th)/He dating provides unmatched potential for deciphering ore deposit ages and associated thermal evolution. Phosphate minerals in meteorites enable the determination of formation and evolutionary stages via (U-Th)/He geochronology. Although meteorite (U-Th)/He dating is constrained by sample rarity, acquisition difficulties, and complex thermal evolution, studying He diffusion in extraterrestrial materials expands the method's applicability and accuracy. Additionally, accessory minerals such as spinel, perovskite, and titanite exhibit potential for (U-Th)/He dating of crustal processes, orogenic dynamics, and deep-earth environments. Epidote, widely distributed in sedimentary and metamorphic rocks, is a promising mineral for tracking rapid cooling episodes and reconstructing paleoclimate conditions.
To effectively apply the (U-Th)/He technique and yield robust age determinations, it is crucial to understand the effects of grain size, compositional zoning, internal lattice damage, uranium mobility, inclusions, mineral purity, and closure temperature. This review outlines the characteristics and application scopes of diverse (U-Th)/He minerals, identifies potential sources of data divergence, and proposes corresponding mitigation strategies. The goal is to assist researchers in accurately interpreting (U-Th)/He ages and their geochronological significance, thereby promoting the refinement and broader application of the method.

Key words: (U-Th)/He, ⁴He diffusion, minerals

摘要：

磷灰石和锆石的(U-Th)/He定年方法得益于成熟的⁴He扩散模型及对低温事件的高度敏感性, 成为揭示成矿时代、 隆升剥露、 地貌演化、 沉积盆地热历史及深时热年代学等关键地质事件的关键技术。该方法已扩展到榍石、 磷钇矿、 金红石及石榴石等多种矿物, 适用于较老变质热事件(3 950Ma)的研究, 例如金伯利岩侵入时代的厘定和克拉通埋藏与剥露历史的重建。此外, 独居石、 石榴石和橄榄石的(U-Th)/He定年方法有望解决年轻火山岩(2.0ka)的定年问题, 并提供新的认识; 将其利用于赤铁矿、 磁铁矿及萤石等矿物, 使得矿床直接定年成为可能。针对针铁矿的研究为了解极年轻地质事件(0.4Ma)的降水和风化时间与速率提供了新的方法, 从而为大陆地表重建贡献了重要信息。方解石、 牙形石和海百合等矿物的应用丰富了沉积盆地演化研究的工具箱, 而对陨石的研究为理解行星形成和演化提供了独特视角。尖晶石、 钙钛矿、 绿帘石等矿物凸显出其在未来地质年代学研究中的潜力。研究者若想成功应用(U-Th)/He方法并获取高质量的年龄数据, 需要深入理解不同矿物的晶粒尺寸、 环带成分效应、 内部晶格损伤、 铀丢失效应、 包裹体、 矿物纯度及封闭温度等因素。文中概述了不同(U-Th)/He定年矿物的特征及应用领域, 汇总归纳了数据偏离的可能因素, 并指出相应的解决措施, 旨在帮助研究者正确理解(U-Th)/He年龄及其时间标尺的内涵, 从而进一步促进(U-Th)/He方法的发展。

关键词: (U-Th)/He, ⁴He扩散, 矿物