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首页> 外文期刊>Contributions to Mineralogy and Petrology >U-Pb isotopic dating of titanite microstructures: potential implications for the chronology and identification of large impact structures
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U-Pb isotopic dating of titanite microstructures: potential implications for the chronology and identification of large impact structures

机译：钛铁矿微观结构的U-PB同位素约会：对大冲击结构的年表和识别的潜在影响

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摘要

Identifying and dating large impact structures is challenging, as many of the traditional shock indicator phases can be modified by post-impact processes. Refractory accessory phases, such as zircon, while faithful recorders of shock wave passage, commonly respond with partial U-Pb age resetting during impact events. Titanite is an accessory phase with lower Pb closure temperature than many other robust chronometers, but its potential as indicator and chronometer of impact-related processes remains poorly constrained. In this study, we examined titanite grains from the Sudbury (Ontario, Canada) and Vredefort (South Africa) impact structures, combining quantitative microstructural and U-Pb dating techniques. Titanite grains from both craters host planar microstructures and microtwins that show a common twin-host disorientation relationship of 74 degrees about 102. In the Vredefort impact structure, the microtwins deformed internally and developed high- and low-angle grain boundaries that resulted in the growth of neoblastic crystallites. U-Pb isotopic dating of magmatic titanite grains with deformation microtwins from the Sudbury impact structure yielded a Pb-207/Pb-206 age of 1851 +/- 12Ma that records either the shock heating or the crater modification stage of the impact event. The titanite grains from the Vredefort impact structure yielded primarily pre-impact ages recording the cooling of the ultra-high-temperature Ventersdorp event, but domains with microtwins or planar microstructures show evidence of U-Pb isotopic disturbance. Despite that the identified microtwins are not diagnostic of shock-metamorphic processes, our contribution demonstrates that titanite has great potential to inform studies of the terrestrial impact crater record.
机译：识别和约会大的冲击结构具有挑战性，因为可以通过后冲击过程来修改许多传统的休克指示阶段。耐火材料阶段，如锆石，而冲击波通道的忠实记录器，通常在影响事件期间重置部分U-PB年龄。 Titanite是Pb闭合温度低于许多其他强大的计数器的附件阶段，但其潜在的影响与冲击相关过程的计时器仍然受到严重的限制。在这项研究中，我们研究了萨德伯里（安大略省，加拿大）和Vredefort（南非）冲击结构的二氧化钛谷物，相结合了定量的微观结构和U-PB约会技术。来自两个陨石坑的二岩谷物宿主平面微结构和微曲线，显示常见的双宿主迷向关系，其约为74度约102＆在VredFort冲击结构中，微曲线内部变形并开发出高角度和低角度的晶界，导致新细胞微晶的生长。来自Sutbury撞击结构的变形微管的岩浆二岩晶体的U-Pb同位素约会产生了1851 +/- 12MA的PB-207 / PB-206岁，记录了冲击加热或冲击事件的火山口修饰阶段。来自Vredefort冲击结构的二岩石谷物主要产生预碰撞年龄，记录超高温Ventersdorp事件的冷却，但具有微管或平面微观结构的结构域显示U-Pb同位素干扰的证据。尽管所确定的微杂体不是诊断休克变质过程，但我们的贡献证明了钛铁矿有巨大潜力，可通知陆地冲击火山口记录的研究。

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