扩散年代学：进展与展望

地质年代学为地球与行星科学研究提供时间坐标，以定量解析地质过程先后关系和时间尺度。历经百余年发展，定年技术在研究对象、测试效率、空间分辨率和时间分辨率等维度均得到大幅度提高，地质年代学研究已从仅提供时代约束过渡到更加强调对地质过程时间尺度与节律的研究，进而约束地质事件的驱动机制和互馈机理。然而，基于放射性同位素衰变的绝对定年技术精度存在物理极限，不能无限提高，且其时间分辨率一般随年龄增加而变差，难以满足深时地质研究的高时间分辨率需求，发展时间分辨率不受绝对年龄约束的相对定年技术是地质年代学的重要发展方向。本文围绕扩散年代学这一具有重要发展前景的相对定年技术，在系统回顾其理论模型和测量技术的基础上，重点对制约扩散年代学准确性和精确性的问题，如扩散系数的不确定性、扩散初始条件假设、浓度曲线的测试质量以及误差评估等进行了探讨。本文还对扩散年代学近年来在岩浆储存与运移、成矿时间尺度与节律和变质过程等领域取得的部分重要进展予以评述。精确的扩散系数是开展扩散年代学研究的前提，以石英中的Ti为例，不同实验给出的扩散系数差异超过3个数量级，据此计算的花岗岩岩浆在固相线上的储存时间从几十年变化到百万年尺度，显著影响我们对岩浆储存状态的理解。高质量的元素浓度剖面测量是扩散年代学的关键，因石英Ti含量的高空间分辨率精准测量较为困难，CL灰度常被作为Ti含量的替代指标，但这需要考虑Al等元素对CL灰度的影响，并严格评估Ti含量校正曲线和扩散剖面空间尺度不匹配对定年结果的影响。高温变质过程U Pb定年通常给出较为离散的表观年龄，并被解释为变质过程具有较长的持续时间，这一定程度上可以通过高温下同位素体系因扩散引起的不封闭予以解释。展望未来，进一步完善扩散年代学在高温体系中的研究，拓展在中低温条件下的应用，并与绝对定年深度结合，是扩散年代学的重要发展方向，也是实现深时地质研究高时间分辨率解析的必由之路。

Progress and prospects of diffusion chronometry

Geochronology provides temporal coordinates for Earth and planetary sciences, enabling the quantitative analysis of the sequence and timescales of geological processes. After over a century of development, dating techniques have greatly improved in terms of research targets,analytical efficiency, spatial and temporal resolution. Geochronology research has transitioned from merely providing chronological constraints to emphasizing the timescales and rhythms of geological processes, thereby constraining the driving mechanisms of, and feedbacks/interplay between geological events. However, absolute dating techniques based on the decay of radioactive isotopes have a physical limitation to their precision and cannot be infinitely improved. Additionally, their temporal resolution generally deteriorates with increasing age, making it difficult to meet the high temporal resolution requirements of deep time research. Developing relative dating techniques whose temporal resolutions are not constrained by absolute age is an important direction for the development of geochronology. This paper focuses on diffusion chronology, a relative dating technique with significant potential which remains to be fully explored. Building upon a systematic review of its theoretical foundations and analytical techniques, the paper discusses key issues that limit the accuracy and precision of diffusion chronology, such as the uncertainty of diffusion coefficients, assumptions about initial boundary conditions, and the quality of measuring concentration profiles. The paper also reviews some recent important advances in diffusion chronology in areas such as magma storage and migration, timescales and rhythms of ore forming, and metamorphic processes. Accurate diffusion coefficients are essential for conducting diffusion chronology research. For example, the differences in diffusion coefficients of Ti in quartz obtained from three experiments can exceed three orders of magnitude. Consequently, the calculated storage time of felsic magma above the solidus can vary from decades to millions of years, significantly affecting our understanding of magma storage conditions. High quality measurements of elemental concentration profiles are critical for diffusion chronology. Due to the challenges in accurate measurement of titanium content in quartz with high spatial resolution, CL grayscale is often used as a proxy for titanium content. However, this requires consideration of the effects of elements such as aluminum on CL grayscale, and a rigorous evaluation is warranted for the impact of spatial mismatch between titanium content calibration curves (measured by EPMA or LA ICPMS) and diffusion profiles (derived from CL grayscale). U Pb dating of high temperature metamorphic processes typically yields relatively dispersed apparent ages, which are interpreted as a long duration of metamorphic processes. This can be partly explained by the open isotopic system behaviour due to diffusion at high temperatures. Looking into the future, to meet the high demand of high temporal chronological data in deep time research, further advancing diffusion chronology in high temperature systems, and expanding its application under medium to low temperature conditions, and integrating it with absolute dating are important avenues.