Deformation-enhanced recrystallization of titanite drives decoupling between U-Pb and trace elements
Deformation-enhanced recrystallization of titanite drives decoupling between U-Pb and trace elements
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钛矿的形变强化再结晶驱动 U-Pb 与微量元素之间的解耦
DOI：
10.1016/j.epsl2021.116810
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发表时间：
2021
影响因子：
5.3
通讯作者：
McDonald, B.J.
中科院分区：
文献类型：
--
作者：
Gordon, S.M.;Kirkland, C.L.;Reddy, S.M.;Blatchford, H.J.;Whitney, D.L.;Teyssier, C.;Evans, N.J.;McDonald, B.J.
钛铁矿是一种常见的辅助矿物，可用于确定年龄（U-Pb同位素）和压力-温度（P-T）条件（微量元素组成：Zr，稀土元素（REE））。然而，在变质作用、流体流动和变形过程中，钛铁矿有重结晶的倾向，这可能导致其同位素和微量元素组成的改变。这一修改的解释的钛酸盐日期和评估的压力-温度-时间路径的影响。变形和重结晶对微量元素流动性的影响，在钛铁矿研究通过微观结构和成分映射的剪切正片麻岩内的超高压域的西部片麻岩区（WGR），挪威的钛铁矿晶体。结果表明，光学共格的钛酸盐单晶在位错蠕变机制下变形，并通过晶界迁移过程再结晶，形成钛酸盐晶粒的聚集体。一些聚集的颗粒记录了喀里多尼亚的发掘日期，而另一些则具有继承的同位素组成。单个颗粒内的集合体，无论其U-Pb同位素组成，包含斑片状分区，形成于同-后重结晶流体蚀变，其特征是一般减少钙和钛和增加铝和铁从核心边缘。然而，Zr和Sr浓度相对于主晶体的长轴被广泛分区，而不考虑聚集体晶界。稀土元素与显微结构和年龄没有明显的相关性。这些结果表明，许多微量元素的钛酸盐是不受多阶段，变形驱动的重结晶，相反，铅是无移动的在这些变形的钛酸盐晶体。微结构和高空间分辨率的地球化学和同位素数据的组合揭示了潜在的程度之间的脱钩的U-Pb同位素系统和微量元素的行为的压力-温度条件随时间的变化。
Titanite is a common accessory mineral that is useful in determining both age (U-Pb isotopes) and pressure-temperature (P–T) conditions (trace-element composition: Zr, rare earth elements (REE)). However, titanite has a propensity to recrystallize during metamorphism, fluid flow, and deformation, which can result in modifications to its isotopic and trace-element compositions. This modification has implications for the interpretation of titanite dates and the evaluation of pressure–temperature–time paths. The impact of deformation and recrystallization on trace-element mobility in titanite is investigated through microstructural and compositional mapping of titanite crystals from a sheared orthogneiss within an ultrahigh-pressure domain of the Western Gneiss Region (WGR), Norway. Results show that optically coherent titanite single crystals deformed in the dislocation creep regime and recrystallized by the process of grain-boundary migration, forming aggregates of titanite grains. Some of the aggregate grains record Caledonian-exhumation dates, whereas others have an inherited isotopic composition. Individual grains within the aggregate, regardless of their U-Pb isotopic composition, contain patchy zoning that formed during syn- to post-recrystallization fluid alteration and that is characterized by generally decreasing Ca and Ti and increasing Al and Fe from cores to rims. However, Zr and Sr concentrations are broadly zoned with respect to the long axis of the host crystal, without regard for the aggregate grain boundaries. REE do not show any obvious correlation with microstructure or age. These results indicate that many trace elements in titanite are unaffected by multi-stage, deformation-driven recrystallization; in contrast, Pb is variably mobile in these deformed titanite crystals. The combination of microstructural and high-spatial resolution geochemical and isotopic data reveals the potential extent of decoupling between the U-Pb isotopic system and the behavior of trace elements as pressure–temperature conditions change through time.
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