U-Series Crystal Ages and Implications for Magma Dynamics

了解火山下岩浆的演化方式，以及它们演化的时间尺度，对于获得火山系统如何运作的基本见解，以及提高我们预测火山爆发的能力都很重要。表面喷发的火山岩中的晶体包含了它们与地下岩浆相互作用的化学记录，因此，这些晶体的定年可以提供系统内随时间变化的化学信息。然而，不同类型的晶体（如锆石和斜长石）的年龄通常会产生不同的特征年龄。目前尚不清楚这是由于分析的规模（锆石的单个晶体与斜长石的数千个晶体的集合体相比），还是由于锆石和斜长石通常在不同类型的火山系统中进行分析（锆石的大型火山口形成系统，斜长石的较小系统），还是因为这两种类型的晶体记录了岩浆历史的不同部分。这种不确定性使得比较不同晶体和/或来自不同火山的晶体的年龄变得困难，阻碍了我们对岩浆过程的时间尺度的总体理解。在这个特殊的项目中，研究人员将通过测量两个不同系统中相同样品中的锆石和斜长石的年龄来解决这个问题：新西兰的塔拉韦拉火山（Okataina火山口复合体的一部分）和俄勒冈州南姐妹火山的魔鬼山圆顶（熔岩从复合火山流出）。研究计划将包括原位测量锆石中的238U-230Th年龄，以及从两个岩浆系统中提取的斜长石和锆石样品的散装分离物中的226Ra-230Th和238U-230Th年龄。这些数据将提供有关同一系统中主要相和微量相结晶之间的时间关系的信息，并有助于描绘每种矿物记录的岩浆历史的哪些部分。第二个目标是了解Okataina和South Sister岩浆储存的动力学，它们分别代表了大体积的硅火山口系统和小体积的硅圆顶。总的来说，这些数据将为现有和未来的u系列晶体年龄提供解释背景，并将为两个具有不同喷发历史的系统中的岩浆动力学提供见解。该项目将为解释u系列矿物年龄提供基础数据，这将对利用这些年龄来理解岩浆系统产生广泛的科学影响。Kaharoa项目将促进三个机构（美国加州大学戴维斯分校、加拿大不列颠哥伦比亚大学和新西兰坎特伯雷大学）之间的国际合作，因此，拟议工作的结果将与更大的数据体相结合，否则是不可能的。

Understanding the ways in which magmas beneath volcanoes evolve, and the time scales over which they evolve, is important both in terms of gaining fundamental insights into how volcanic systems work and in terms of improving our ability to forecast volcanic eruptions. Crystals in volcanic rocks erupted at the surface contain chemical records of their interactions with magmas beneath the surface, and dating of these crystals can therefore provide information about chemical changes within the system over time. However, ages of different types of crystals (for example, zircon and plagioclase) generally yield different characteristic ages. It is unclear whether this is due to the scale of analysis (individual crystals for zircon compared to aggregates of thousands of crystals for plagioclase), due to the fact that zircon and plagioclase have typically been analyzed in different types of volcanic systems (large, caldera-forming systems for zircon, smaller systems for plagioclase), or because the two types of crystals record different parts of a magma's history. This uncertainty makes it difficult to compare ages of different crystals and/or crystals from different volcanoes, hampering our ability to develop a general understanding of the timescales of magmatic processes. In this particular project, the investigators will address this problem by measuring ages of zircon and plagioclase in the same samples for two different systems: Tarawera volcano, New Zealand (part of a the Okataina caldera complex) and Devils Hills domes, South Sister volcano, Oregon (lava flows from a composite volcano).The research plan will include measuring in situ 238U-230Th ages in zircon and 226Ra-230Th and 238U-230Th ages in bulk separates of both plagioclase and zircon from samples derived from the two magmatic systems. These data will provide information about the temporal relationship between crystallization of major and trace phases in the same system and help delineate which parts of the magmatic history are recorded by each mineral. A secondary goal is to understand the dynamics of magma storage beneath Okataina and South Sister, as representatives of a large-volume silicic caldera system and small-volume silicic domes, respectively. Collectively, these data will provide an interpretive context for existing and future U-series crystal ages, and will also provide insights into the dynamics of magmas in two systems with disparate eruptive histories. This project will provide data that are fundamental to interpretation of U-series mineral ages, which will have broad scientific impact in use of these ages to understand magmatic systems. The Kaharoa project will foster international collaborations spanning three institutions (UC Davis, US; Univ. British Columbia, Canada; and Univ. Canterbury, New Zealand) and the results of the proposed work will therefore be integrated with a much larger body of data than would otherwise be possible.