Melting and storage conditions of rhyolites in the Snake River Plain Province , USA

蛇河平原-黄石省保存了玄武岩岩浆底侵作用和随后的地壳再循环导致的双峰式岩浆作用(玄武岩-流纹岩)的独特记录。拟议的项目侧重于蛇河平原流纹岩的生成和储存，旨在阐明玄武岩岩浆底侵作用导致的深部“地壳循环”过程。目前提出了各种生成模型(分馏、同化、部分熔化)，但它们没有经过实验测试，因此往往保持定性。从一个端元假说出发，假设流纹岩是由地壳物质部分熔融形成的，将进行高压实验，以确定典型原岩产生的部分熔体的地球化学特征(主量元素和微量元素)。将测试两种正在争论的主要物源类型：热液蚀变流纹岩和镁铁质玄武岩。实验结果和实验熔体的成分将与来自ICDP岩芯和覆盖SRP 16 Ma活动的选定喷发单元的天然样品中的玻璃成分进行比较。天然玻璃的地球化学成分也将被用来应用最近由支持者校准的新气压计。从几个喷发单元(如金伯利和糖城核心)中挑选的样本将用于检查大型喷发是否是在不同的P-T条件下储存的不同岩浆批次造成的。这些结果将有助于了解流纹岩的源岩性质、喷发前的熔融和岩浆储存条件(深度、温度、挥发分含量)以及岩浆储存条件在空间和时间上的演化。

The Snake River Plain-Yellowstone province preserves a unique record of bimodal magmatism (basalt-rhyolite) resulting from basaltic magma underplating and subsequent crustal recycling. The proposed project focuses on the generation and storage of rhyolites in the Snake River Plain (SRP) and is designed to clarify the “crustal recycling” processes occurring at depth as a result of basaltic magma underplating. A variety of generation models are currently proposed (fractionation, assimilation, partial melting) but they are not tested experimentally and, as a consequence tend to remain qualitative. Starting from one end-member hypothesis and assuming that rhyolites are formed from partial melting of crustal material, high pressure experiments will be conducted to determine the geochemical signature (major and trace elements) of partial melts produced from typical protoliths. Two main source types which are under debate will be tested: hydrothermally altered rhyolites and mafic basaltic rocks. The experimental results and the composition of the experimental melts will be compared with the composition of glasses in natural samples from the ICDP cores and from selected eruptive units covering the 16 Ma activity of the SRP. The geochemical compositions of the natural glasses will also be used to apply a new barometer recently calibrated by the proponents. Selected samples from a few eruptive units (e.g., Kimberley and Sugar City core) will be used to check if large eruptions result from different magma batches stored at various P-T conditions. The results will contribute to understand the nature of the source rocks of the rhyolites, the conditions of melting and of magma storage prior to eruption (depth, temperature, volatile contents), and the evolution of magma storage conditions in space and time.