Titanite can be found in rocks of wide compositional range, is reactive, growing or regrowing during metamorphic and hydrothermal events, and is generally amenable to U–Pb geochronology. Experimental evidence suggest that titanite has a closure temperature for Pb ranging from 550 to 650°C, and thus titanite dates are commonly interpreted as cooling ages. However, this view has been challenged in recent years by evidence from natural titanite which suggests the closure temperature may be significantly higher (up to 800°C). Here, we investigate titanite in an enclave of migmatitic gneiss included within a granite intrusion. The titanite crystals exhibit textural features characteristic of fluid‐mediated mass transfer processes on length scales of <100 µm. These textural features are associated with variation in both Pb concentrations and distinct U–Pb isotopic compositions. Zr‐in‐titanite thermometry indicates that modification of the titanite occurred at temperatures in excess of 840°C, in the presence of a high‐T silicate melt. The Pb concentration gradients preserved in these titanite crystals are used to determine the diffusivity of Pb in titanite under high‐T conditions. We estimate diffusivities ranging from 2 × 10⁻²² to 5 × 10⁻²⁵ m²/s. These results are significantly lower than experimental data predict yet are consistent with other empirical data on natural titanites, suggesting that Pb diffusivity is similar to that of Sr. Thus our data challenge the wide‐held assumption that U–Pb titanite dates only reflect cooling ages.

中文翻译：

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钛矿约会结晶：超固相线再平衡过程中铅的缓慢扩散

钛铁矿存在于广泛的组成范围内的岩石中，在变质和热液事件中是反应性的，可生长的或可再生的，通常适用于U–Pb年代学。实验证据表明，钛矿对Pb的封闭温度范围为550至650°C，因此，钛矿日期通常被解释为冷却年龄。但是，近年来，天然钛矿的证据对这一观点提出了挑战，这表明封闭温度可能会更高（高达800°C）。在这里，我们研究了花岗岩侵入体中包含的片麻片岩飞地中的钛矿。钛矿晶体在<100 µm的长度尺度上表现出流体介导的传质过程的特征。这些纹理特征与铅浓度和独特的U–Pb同位素组成的变化有关。钛酸锆中的Zr测温表明，在高840°C的温度下，钛酸盐的改性发生在T硅酸盐熔体。这些钛矿晶体中保留的铅浓度梯度用于确定高T条件下钛在铅矿中的扩散系数。我们估计的扩散率范围为2×10 ^-22到5×10 ^-25  m ² / s。这些结果明显低于实验数据，但与天然钛矿的其他经验数据一致，这表明铅的扩散率与锶相似，因此我们的数据挑战了普遍认为U–Pb钛矿日期仅反映冷却年龄的假设。 。