Evaluating the Processes and Timescales of Rhyolitic Differentiation

知识价值：u系列同位素彻底改变了我们评估岩浆时间尺度的能力，并将与Rb/Sr系统一起用于限制板内流纹岩生成的时间。硅质岩浆生成的时间和过程与潜在爆炸性火山的模拟活动有关，具有重要的气候和火山危害意义。白头山火山（中国东北/朝鲜）非常适合进行此类研究，因为它产生了多个年轻的高硅流纹岩，其中包括最近在公元1000年的大体积（~100 km3）火山碎屑喷发。所有的喷发单元都非常年轻，因此可以使用寿命较短的238U-230Th-226Ra和235U-231Pa同位素系统以及寿命较长的放射性成因同位素组合来评估岩浆的时间尺度。此外，采样足以进行拟议的工作，从而节省了拟议研究的时间和成本。通过对火山伴生的镁质和硅质岩浆的分析，对岩浆分馏事件的时间进行了评价。单晶Pb和Sr同位素分析也将用作示踪剂来量化开放系统过程的影响，如地壳污染、岩浆补给和喷发后蚀变。综合这些数据，就可以详细评估火山整体演化过程，以及它们对各种同位素测年方法准确性的影响。更广泛的影响包括：(1)本科生参与地质科学的前沿研究，(2)国际合作，(3)火山风险评估的相关性。关于后一点，在过去的十年里，白头山的“地震危机”引起了人们对火山可能在不久的将来再次爆发的担忧。更好地了解岩浆演化的时间尺度可能有助于预测未来的活动。最后，白头山位于中国和朝鲜（DPRK）边境上，具有潜在的政治意义，对了解火山行为的贡献可以促进国际“善意”。

Intellectual merit: U-series isotopes have revolutionized our ability to evaluate magmatic timescales and will be used in conjunction with Rb/Sr systematics to constrain the time in which large volumes of intraplate rhyolite are generated. The timing and processes involved in the generation of silicic magmas are relevant to modeling activity at potentially explosive volcanoes, with significant climate and volcanic hazard implications. Baitoushan volcano (NE China/North Korea) is ideally suited for such studies because it has produced multiple young high silica rhyolites including a large volume (~100 km3) pyroclastic eruption as recently as ~1000 AD. All eruptive units are sufficiently young that both short-lived 238U-230Th-226Ra and 235U-231Pa isotopic systems as well as long-lived radiogenic isotopes can be used in combination to assess magmatic timescales. Moreover, sampling is sufficient to carry out the proposed work, thus saving on the time and cost of the proposed research. Timing of magmatic fractionation events involved in the generation of associated mafic and silicic magmas in this volcano will be evaluated through analysis of accessory phenocryst minerals. Single crystal Pb and Sr isotope analyses will also be used as tracers to quantify the effects of open-system processes such as crustal contamination, magma recharge, and post-eruption alteration. In combination these data will allow detailed assessment of such processes on the overall evolution of the volcano as well as their effects on the fidelity of the various isotopic dating methods.Broader Impacts include (1) involvement of an undergraduate into cutting-edge research in the geological sciences, (2) international collaborations, and (3) relevance to volcanic risk assessment. Regarding the latter point, a "seismic crisis" at Baitoushan within the last decade raised concerns that the volcano could erupt again in the near future. Better understaning of the timescale of magma evolution could be useful in predicting future activity. Finally, Baitoushan's location on the border between China and North Korea (DPRK) has potential political significance, and contributions to understanding volcanic behavior could promote international 'good will'.