Decoupling of U–Pb dates from chemical and crystallographic domains in granulite facies zircon
Decoupling of U–Pb dates from chemical and crystallographic domains in granulite facies zircon
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DOI：
10.1016/j.chemgeo.2009.11.002
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发表时间：
2010-02
期刊：
影响因子：
3.9
通讯作者：
R. Flowers;A. Schmitt;M. Grove
中科院分区：
文献类型：
--
作者：
R. Flowers;A. Schmitt;M. Grove
获得了加拿大地盾镁铁质麻粒岩中锆石晶体的高空间分辨率（12μm）和高密度（每个晶体高达22个点）U-Pb离子探针数据。锆石颗粒变质的起源，并受到随后的高压麻粒岩相事件。207 Pb/206 Pb年龄跨度≥ 650 Ma，并可用于定义离散的晶内域的日期轮廓的年龄结果。这些域的边界切在高角度的阴极射线发光，背散射，铪和U化学绘图，电子背散射衍射取向差数据显示的组成分区。结果表明，放射成因铅的行为从其他微量元素的锆石晶体去耦。以前发表的U-Pb ID-TIMS数据定义了一个高度线性的不一致阵列，其上交点为2554.5±4.3Ma，与麻粒岩相变质作用期间的初始锆石生长有关，下交点为1896.0±18.0Ma，与导致U-Pb不一致的第二次麻粒岩相变质事件有关。离子探针测年的晶内模式与多个锆石生长事件不相容，并且与锆石重结晶的报道实例不一致，其中U-Pb系统学的重置伴随着锆石化学和CL模式的修改。颗粒缺乏可辨别的核心边缘的变化预计扩散配置文件，和诊断指标的低温辐射损伤增强铅损失是缺席的。相反，这些数据似乎需要非变质变质锆石在高温变质过程中的非均质铅损失，这意味着在某些情况下，其他晶内快速通道促进变质锆石晶体中的铅损失。这一结果与关于锆石中铅的流动性的传统思想是不相容的，并且表明锆石中铅行为的重要方面尚未完全理解。
High spatial resolution (12μm spot size) and high density (up to 22 spots per crystal) U–Pb ion probe data were acquired for zircon crystals from a mafic granulite in the Canadian shield. The zircon grains are metamorphic in origin and were subjected to a subsequent high-pressure granulite facies event.207Pb/206Pb dates span ≥650Ma, and can be used to define discrete intracrystalline domains of dates by contouring the age results. The boundaries of these domains cut at high angles across compositional zonation revealed by cathodoluminescence, backscatter, Hf and U chemical mapping, and electron backscatter diffraction misorientation data. The results indicate decoupling of radiogenic Pb behavior from other trace elements in the zircon crystals. Previously published U–Pb ID-TIMS data define a highly linear discordia array with an upper intercept at 2554.5±4.3Ma associated with initial zircon growth during granulite facies metamorphism, and a lower intercept at 1896.0±18.0Ma associated with a second granulite facies metamorphic event that caused U–Pb discordance. The intracrystalline patterns of ion probe dates are incompatible with multiple zircon growth events, and are inconsistent with reported examples of zircon recrystallization in which resetting of the U–Pb systematics was accompanied by modification of zircon chemistry and CL patterns. Grains lack discernible core–rim variations expected for diffusion profiles, and diagnostic indicators of low temperature radiation damage-enhanced Pb loss are absent. Rather, the data appear to require heterogeneous Pb loss from non-metamict metamorphic zircon during high-temperature metamorphism, implying that under certain circumstances other intracrystalline fast pathways facilitate Pb loss in metamorphic zircon crystals. This result is incompatible with conventional thinking regarding Pb mobility in zircon, and indicates that important aspects of Pb behavior in zircon are not yet fully understood.