Laser ablation tandem mass spectrometry is a burgeoning field for in situ Rb-Sr geochronology. Here, we determined simultaneous isotope ratios of ⁸⁷Sr/⁸⁶Sr and ⁸⁷Rb/⁸⁶Sr in metamorphic biotite from western Maine, using an ESL™ imageGEO™193 excimer laser ablation system coupled to a Thermo Scientific™ Neoma™ MC-ICP-MS/MS. Measurements were made on Faraday cups with Rb⁺ at mass 87; Sr isotopes were reacted with SF₆ gas and measured as SrF⁺ at masses 103–107. Twenty-two laser spots in biotite from a single sample yield a "traditional" Rb-Sr isochron date of 289 ± 6 Ma. Time-resolved signals reveal significant zoning in ⁸⁷Sr/⁸⁶Sr and ⁸⁷Rb/⁸⁶Sr within single spot analyses, which were used to construct single spot isochrons. Individual laser spots contain multiple isochronous subpopulations; some spots contain up to three distinct Rb-Sr isochrons that are decoupled from variations in Rb/Sr. Thirty-five isochron dates were determined using this "sub-spot" approach, with ⁸⁷Sr/⁸⁶Sr intercepts that systematically vary with Rb-Sr date; two-point isochrons were calculated for individual integrations (n = 780) based on these variable intercepts. Both methods yield age peaks at 303, 270 and 240 Ma. These data suggest that the Rb-Sr system has the potential to record multiple heating, cooling or fluid-alteration events spanning ~ 100 My within small domains in single biotite crystals.

中文翻译：

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LA-MC-ICP-MS/MS 黑云母单点 Rb-Sr 等时线测年

激光烧蚀串联质谱是原位铷-锶地质年代学的一个新兴领域。^{在此，我们使用 ESL™ imageGEO™193 准分子激光烧蚀系统与 Thermo Scientific™ Neoma™ MC-ICP-MS 联用，同时测定了缅因州西部变质黑云母中87} Sr/ ⁸⁶ Sr 和⁸⁷ Rb/ ⁸⁶ Sr的同位素比/多发性硬化症。测量是在质量为 87的 Rb ^{+法拉第杯上进行的；}_{Sr 同位素与 SF 6}气体反应并在质量 103–107 处测量为 SrF ⁺。单个样品中黑云母中的 22 个激光点产生的“传统”Rb-Sr 等时线日期为 289 ± 6 Ma。时间分辨信号揭示了单点分析中⁸⁷ Sr/ ⁸⁶ Sr 和⁸⁷ Rb/ ⁸⁶ Sr 的显着分区，这些分析用于构建单点等时线。单个激光点包含多个同步子群；有些点包含多达三个不同的 Rb-Sr 等时线，它们与 Rb/Sr 的变化无关。使用这种“子点”方法确定了 35 个等时线日期，其中⁸⁷ Sr/ ⁸⁶ Sr 截距随 Rb-Sr 日期系统地变化；基于这些可变截距计算各个积分（ n = 780）的两点等时线。两种方法均得出 303、270 和 240 Ma 处的年龄峰值。这些数据表明，Rb-Sr 系统有可能在单个黑云母晶体的小区域内记录跨越约 100 My 的多个加热、冷却或流体改变事件。