Abstract:

Apatite (U-Th)/He dating, a widely used low-temperature thermochronometer sensitive to shallow crustal processes within the uppermost ~3km of the crust, is critical for constraining tectonic, landscape, and basin thermal evolution. This study aims to validate the accuracy and reliability of the(U-Th)/He dating workflow at Zhejiang University’s thermochronology Laboratory through a systematic analysis of the international standard sample, Durango apatite. The core objective is to ensure the laboratory’s capability to generate precise age and Th/U ratio data consistent with global benchmarks, thereby providing a robust methodological foundation for low-temperature thermochronological studies focusing on near-surface cooling processes(closure temperature <90℃).

To achieve this goal, the laboratory conducted rigorous experimental analyses on a total of 60 Durango apatite grains across three batches. The specific procedures were as follows: First, Durango apatite particles with a 20-micron surface removed were crushed, and apatite fragments were screened under a polarizing microscope. These grains were encapsulated in platinum(Pt)capsules—chosen for their high thermal conductivity and chemical inertness—to facilitate efficient laser heating at 900℃ for 5 minutes and subsequent acid digestion. This single-grain laser heating approach enables complete and rapid degassing of helium from individual apatite crystals, minimizes the risk of cross-contamination between samples, and ensures a higher helium extraction efficiency compared to conventional bulk furnace heating methods. Helium(⁴He)extraction and analysis were performed using an Alphachron mass spectrometer equipped with a diode laser system, a high-vacuum maintenance system(including mechanical, turbo, and ion pumps), and a quadrupole mass spectrometer(QMS). The ⁴He concentration was quantified using the ³He isotope dilution method, with precise corrections for thermal blanks(empty Pt capsules) and line blanks(system background), which were found to be extremely low(average background value of 1.66×10^-12mL, only 1/1 248 to 1/29 280 of the sample signal value). For uranium(U) and thorium(Th) analysis, digested samples were measured by inductively coupled plasma mass spectrometry(ICP-MS). Using a spike solution with known U/Th isotopic ratios(²³⁵U/²³⁸U=838±7,²³⁰Th/²³²Th=10.45±0.5), elemental abundances and Th/U ratios were accurately calculated, with rigorous subtraction of reagent and Pt capsule background values.

The test data showed high consistency with international standards. The measured ages of the 60 Durango grains ranged from 28.48 to 33.20Ma, with an average age of(31.30±2.19)Ma(2σ), closely matching previously reported values such as(31.71±1.1)Ma(Wang et al., 2017)and(31.61±2.7)Ma(Wu et al., 2016). A normal probability plot of the age data revealed a tight distribution(R²=0.980 6) with a slope close to 1, indicating data concentration and high reliability. The Th/U ratios ranged from 17.10 to 26.13, overlapping with global datasets(e.g., 17.23-23.60, 19.2-21.8, 18.0-22.5. Although two grains exhibited slightly higher Th/U ratios(25.76 and 26.13), these values remained within the natural variation range reported in the literature(up to 34.5±3.9), confirming their geological plausibility. This minor Th/U dispersion aligns with the well-documented intra-grain trace element heterogeneity of Durango apatite, confirming our workflow preserves primary sample geochemical signatures without artificial fractionation.

The study concludes that Zhejiang University’s(U-Th)/He dating workflow for single apatite grains is accurate and reliable, with systematic uncertainties effectively controlled through strict background corrections and standardized operations. The extremely low ⁴He background values and precise Th/U measurements validate the laboratory’s capability for high-precision testing. The successful validation of Durango apatite as a reference sample not only demonstrates the laboratory’s contribution to global calibration standard data but also provides an important benchmark for quality control in similar laboratories.

Through testing the Durango apatite standard sample, this study validates the feasibility of the analytical workflow, ensures consistency with international datasets, and promotes the development of low-temperature thermochronology. The capability of Zhejiang University’s (U-Th)/He Chronology Laboratory to generate reliable (U-Th)/He age data positions it as a key participant in global thermochronological research, facilitating the resolution of fundamental questions about thermal and tectonic processes in the field of geosciences. Additionally, this validated standardized workflow supports domestic inter-laboratory comparison, promotes the standardization of (U-Th)/He dating in China, and provides reliable analytical support for the geoscience community.

Key words: (U-Th)/He dating, Durango apatite, international standard sample

摘要：

(U-Th)/He同位素年代学方法在地质体的热史研究中独具优势, 诸多基于不同矿物的(U-Th)/He年代学体系应运而生。其中, 磷灰石(U-Th)/He年代学体系应用极为广泛, 这得益于磷灰石在地壳岩石中分布广泛, 单颗粒有效铀含量较高, 且封闭温度低于90℃, 能够反映岩体在近地表的热史过程。Durango磷灰石作为磷灰石(U-Th)/He定年的国际标准样品, 对其年龄进行测定, 对于验证实验流程的精准性与可靠性意义重大。浙江大学(U-Th)/He年代学实验室于2024年建成, 实验室精心构建了完备且严谨的磷灰石(U-Th)/He单颗粒测试流程, 并对3个批次共计60个Durango磷灰石颗粒展开测试。结果显示, 颗粒年龄介于28.48~33.20Ma, 平均年龄为(31.30±2.19)Ma(2σ), 与国际标定值高度吻合; Th/U介于17.10~26.1, 同样与国际报道相符。充分证实实验室测试结果精确可靠, 测试流程科学合理, 测试方法切实可行。

关键词: (U-Th)/He年代学, Durango磷灰石, 国际标准样品