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对大陆地壳形成和演化的认识。高空间分辨率的原位分析（通过激光烧蚀）可以精确和准确地测量薄片内单个地质时计矿物的同位素比率。这些同位素比率提供了严格限制的年龄，可以与岩石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。对于经历了漫长而漫长的地质历史的变形和变质岩石，在确定地质时钟何时开始滴答作响时，变质条件变化、大块岩石化学、变形和流体渗透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地质年代学数据集通常产生的日期范围比单一“年龄”的分析不确定性所表明的时间范围更长。因此，这个年龄跨度表明：(1)在一系列压力、温度和变形（P-T-d）条件下发生了漫长的结晶，(2)在冷却和挖掘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，经历了相似的P-T-d条件的单个样品，即来自同一露头的样品，可以产生惊人不同的矿物日期，这表明岩石的整体化学成分对使地球时计矿物结晶或溶解的反应具有很强的控制作用。众所周知，同一块岩石中不同的地质时计矿物对压力、温度和变形的反应是不同的。

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.