How, when and what do geochronometers record in deformed metamorphic rocks V

地质年代学通过提供埋藏、变质和变形的速率和时间尺度，从根本上巩固了我们关于大陆地壳如何形成和演化的知识。高空间分辨率的原位分析(通过激光消融)能够在薄片内精确和准确地测量单个地质计时矿物的同位素比率。这些同位素比值提供了严格限制的年龄，可以与岩相学观察和矿物化学分析联系起来，所有这些都是现代“岩石年代学”领域的基础。对于在经历了漫长的地质历史的变形和变质岩石中，改变变质条件、块体岩石化学、变形和流体渗透在确定地质时钟何时开始滴答作响方面的重要性和作用，仍然存在相当大的争论。来自变质和变形岩石的原位U-Th-Pb年代学数据通常会产生一系列的时间跨度，而不是单一“年龄”的分析不确定性。因此，这一年龄跨度要么表明：(1)在一定的压力、温度和变形(P-T-d)条件下发生了长期结晶；(2)在冷却和折返过程中，存在不完全的同位素重置；或者(3)存在不同年龄的矿物域的分析混合。最近的研究表明，经历了类似P-T-d条件的单个样品，即来自同一露头的样品，可以产生截然不同的矿物日期，这表明岩石的整体化学成分对允许地质计时矿物结晶或溶解的反应具有很强的控制作用。同样众所周知的是，同一岩石中不同的地质计时矿物对压力、温度和变形的反应也不同。

Geochronology fundamentally underpins our knowledge of how the continental crust forms and evolves by providing the rates and timescales of burial, metamorphism and deformation. High spatial resolution in-situ analyses (via laser ablation) allow for the precise and accurate measurement of isotope ratios from individual geochronometer minerals within thin sections. These isotope ratios provide tightly constrained ages that can be linked to petrographic observations and mineral chemical analyses, all of which underpin the modern field of 'petrochronology'. There is a still considerable debate about the importance and role of changing metamorphic conditions, bulk rock chemistry, deformation and fluid infiltration in determining when the geological clock starts ticking in deformed and metamorphosed rocks that have experienced a lengthy and protracted geological history. In-situ U-Th-Pb geochronology datasets from metamorphosed and deformed rocks commonly yield a range of dates that spans more time than the analytical uncertainty of a single "age" would suggest. This span of ages therefore suggests either that: (1) protracted crystallization took place over a range of pressure, temperature and deformation (P-T-d) conditions, (2) there was incomplete isotopic resetting during cooling and exhumation, or (3) there has been analytical mixing of mineral domains of different age. Recent studies have demonstrated that individual samples that have undergone similar P-T-d conditions, i.e. from the same outcrop, can yield strikingly varied mineral dates, indicating that the rock's bulk chemical composition exhibits a strong control on the reactions that allow the geochronometer minerals to crystallise or dissolve. It is also well known that different geochronometer minerals within the same rock respond differently to pressure, temperature and deformation.