Collaborative Research: Crustal Modulation of Flood Basalts--Integrating Field, Geochemical and Computational Data for the Steens Basalts

洪水玄武岩喷发在地质上是短暂的、灾难性的火山事件，它倾泻出大量的熔岩--比1980年的圣海伦斯火山喷发大数万倍？带到地球表面。它们是完全改变地貌的重要地质事件，也有可能影响气候和生态系统。美国唯一的洪水玄武岩是哥伦比亚河洪水玄武岩(CRB)，它主导着华盛顿州和俄勒冈州部分地区的景观。大多数CRB熔岩主要在1600万至1500万年前喷发，并以世界上最大的单个熔岩流而闻名。在CRB中最古老的部分是斯蒂恩斯玄武岩(1680-1600万年前)，它覆盖了现在的俄勒冈州53,000平方公里的陆地表面。斯蒂恩斯玄武岩在成分上比CRB的大部分更具镁质，因此提供了地幔玄武岩与地壳(地表以下延伸约40公里的地球外层岩壳)之间相互作用的独特记录。在俄勒冈州东南部的斯廷斯山，多达200股熔岩流壮观地暴露在一个近千米厚的堆栈中，记录了岩浆(熔岩)在CRB事件的早期阶段如何改变成分。斯蒂恩斯山的野外工作将产生一个临时的(从老到年轻)熔岩样本序列，这些样本将使用一系列工具进行化学分析。这些数据将用一系列计算机模型进行评估，包括名为岩浆室模拟器的最新一代模型。模拟结果将提供从地幔添加到地壳的新岩浆量的估计，地幔是玄武岩的最终来源，以及有多少地壳被合并到岩浆中。顺序采样可以记录这些参数如何随着岩浆脉冲的起伏而随时间变化。野外和化学资料与计算机分析相结合的方法可广泛应用于其他火山地体，本研究结果为认识地球上其他大体积岩浆事件提供了定量依据。我们假设，下斯蒂恩斯玄武岩代表岩浆作用，在岩浆作用的早期，高速率的岩浆输入占主导地位。相反，上斯蒂恩斯玄武岩在一个热衰退系统中经历了更多的与地壳的相互作用，因为CRB活动中心向北迁移，地壳岩浆系统向地壳较浅的深处移动。这些假设将通过野外工作进行验证，其中将收集一套具有代表性的熔岩样品；收集全岩主要和微量元素、O-、Sr-、Nd-、Pb-、Hf-、Os和He同位素、岩石学和矿物化学数据；以及温压计算和其他计算岩石学模型。这项研究的结果将使斯蒂恩斯岩浆的岩石学演化与哥伦比亚河洪水玄武岩事件发生时地壳和地幔之间发生的质量和热交换联系在一起。更广泛的影响包括(1)不同的科学家群体的参与，包括2名PI，1名博士生，2名硕士研究生和几名本科生。至少有5名女科学家(包括一名拉丁裔)；(2)开发/完善和介绍建模教程；(3)以公开讲座和实地考察的形式进行科学宣传；(4)通过出版物和会议传播成果；(5)将成果和岩石样本纳入课堂。

Flood basalt eruptions are geologically short-lived, catastrophic volcanic events that pour enormous volumes of lava - tens to hundreds of thousands of times greater than the 1980 Mount St Helens eruption? onto Earth's surface. They are important geologic events that completely change the landscape and also have the potential to impact climate and ecosystems. The only flood basalt in the United States is the Columbia River Flood Basalt (CRB), which dominates the landscape in parts of Washington and Oregon. Most of the CRB lavas erupted mainly 16 -15 million years ago and are noted for being among the world's largest individual lava flows. Among the oldest part of the CRB is the Steens Basalt (16.8-16 million years old), which overran 53,000 km2 of land surface in what is now Oregon. The Steens Basalts are compositionally more magnesian than the bulk of the CRB and so provide a unique record of the interaction between mantle-derived basalt and the crust (the outer rocky shell of Earth that extends ~40 km below the surface). At Steens Mountain, in southeastern Oregon, as many as 200 lava flows are spectacularly exposed in a nearly kilometer-thick stack that records how magma (molten rock) changed composition during the earliest stages of the CRB event. Fieldwork at Steens Mountain will yield a temporal (older to younger) sequence of lava samples that will be chemically analyzed using an array of tools. These data will be assessed with a range of computer models, including the latest generation of model called the Magma Chamber Simulator. Modeling results will provide estimates of the amount of new magma added to the crust from the mantle, the ultimate source of the basalt, and also how much crust is incorporated into magma. The sequential sampling allows documentation of how these parameters change with time as the magmatic pulse waxes and wanes. The methodology of combining field and chemical data with computer analysis can be broadly applied to other volcanic terrains, and the results of this study provide a quantitative basis for understanding other large volume magmatic events on Earth. We hypothesize that the lower Steens Basalt represents magmatism dominated by high rates of magma input during an earlier, energetically waxing stage of magmatism. In contrast, Upper Steens Basalt experienced more interaction with the crust in a thermally waning system, as the locus of CRB activity migrated north and the crustal magmatic system moved to shallower depths in the crust. These hypotheses will be tested by fieldwork, where a representative suite of lava samples will be collected; collection of whole-rock major and trace elements, O-, Sr-, Nd-, Pb-, Hf-, Os- and He-isotopes, petrography and mineral chemistry data; and thermobarometric calculations and other computational petrologic models. Results of this study will allow the petrologic evolution of Steens magmas to be placed in context with mass and thermal exchange that occurred between crust and mantle during the onset of the Columbia River flood basalt event. The broader impacts include (1) involvement of a diverse group of scientists including 2 PIs, 1 PhD student, 2 MS students and several undergraduates. At least 5 are female scientists (including one Latina); (2) development/refinement and presentation of modeling tutorials; (3) scientific outreach in the form of public lectures and field trips; (4) dissemination of results via publications, and conferences; (5) integration of results and rock samples into classes.