Heart of Glass: Volatile Release and Petrogenesis of Columbia River Flood Basalt Lavas Through Study of Melt Inclusions and Glassy Tephra

历史经验告诉我们，火山爆发会向大气中释放气体，从而直接造成短期（几年）的全球气候扰动。不太清楚的是，大规模火山爆发是否有能力造成灾难性和/或长期的气候变化。这样的火山爆发并不存在于人类经验的记录之中，但它们的产物在地质记录中却很明显。在这类事件中，有一大堆冰冻的熔岩流，它们构成了大陆泛滥玄武岩地区。个别的洪水熔岩喷发可能比历史上最大的熔岩喷发大数百倍，对气候的影响可能相当严重。最年轻和研究最好的大陆洪水玄武岩省是哥伦比亚河玄武岩（CRB），它形成于大约1600万年前，现在覆盖了美国太平洋西北地区约20万平方公里。以前的研究气体释放与这些熔岩的喷发集中在后期流动的日期后的玄武岩的主要涌出。该项目采用了一种新的方法，目标是在火山口周围地区发现的玻璃状火山灰，而不是同等的结晶“石质”熔岩本身。玻璃和它们所持有的晶体，比熔岩保存了更高保真的喷发放气记录，允许估计CRB活动最强烈阶段造成的大气污染。这些估计将构成“地面实况”，有助于限制喷发时的区域和全球气候扰动模型。在此之前，CRB脱气的研究主要集中在斑状Wanapum熔岩（占整个CRB的6%）上，它晚于Grande Ronde熔岩（占整个CRB的70%）所代表的洪水火山活动的峰值。研究大回旋区的主要障碍是认为它们是无定形的，即它们不包含含有熔融包裹体的“斑晶”（在喷发前在岩浆中生长的晶体）。晶体中的熔体包裹体保留了岩浆的原始气体含量（主要是H2O，CO2，S物种），然后可以与完全脱气的基块玄武岩进行比较，以估计气体损失。然而，对Grande Ronde熔岩流喷口地区的自然水淬火玄武岩玻璃的研究表明，它们确实含有含有熔融包裹体的斑晶，这些斑晶很小，但可以通过微束方法进行分析。此外，一些玻璃质灰碎片在脱气之前已经淬火，并且也可以进行分析以获得气体损失的估计。这些数据将被用来估计从CRB活动的S，H2O和CO2到大气中的通量。这反过来又会限制CRB对中新世气候的冷却（由于大气中的S污染）与变暖（由于CO2释放到大气中）的潜力，这两种假设都是由以前的工作者提出的，但在这两种情况下都没有什么数据支持。另一个研究领域是，玻璃保存了比同等熔岩更广泛的整体化学成分，可能为洪水玄武岩岩浆的最终起源提供新的见解。

Historical experience tells us that volcanic eruptions can directly cause short-term (few years) global climate perturbations through the release of gases to the atmosphere. What is less clear is the capacity for very large eruptions to produce catastrophic and/or long-lasting climate change. Such eruptions lie outside recorded human experience but their products are evident in the geologic record. Among this class of events are the gigantic piles of frozen lava flows that make up continental flood basalt regions. Individual flood lava eruptions may be hundreds of times bigger than the largest historically-erupted lavas, with, potentially, proportionately severe effects on climate. The youngest and best-studied continental flood basalt province is the Columbia River Basalts (CRB), which formed around 16 million years ago and now cover about 200,000 square kilometers of the Pacific Northwest region of the United States. Previous studies of gas release associated with the eruption of these lavas have focused on late flows that post-date the main outpouring of basalt. This project employs a new approach that targets glassy volcanic ash found in the areas around the volcanic vents, rather than the equivalent crystallized, 'stony' lavas themselves. The glasses, and the crystals that they hold, preserve a higher fidelity record of eruption outgassing than do the lavas, allowing estimates of atmospheric pollution caused by the most intense phase of CRB activity. These estimates will constitute 'ground truth' that can help constrain, for instance, models for regional and global climate disturbance at the time of eruption.Before now, studies of degassing from CRB have focused on the porphyritic Wanapum lavas (6% of the whole CRB), which post-date the peak activity of flood volcanism represented by the Grande Ronde lavas (70% of the whole CRB). The principle barrier to study of the Grande Ronde has been a perception that they are aphyric, i.e. they do not contain melt inclusion-bearing 'phenocrysts' (crystals that grew in the magma prior to eruption). Melt inclusions in crystals preserve the original gas content of the magma (chiefly H2O, CO2, S species) that can then be compared with fully degassed groundmass basalt to gain an estimate of gas loss. However, study of naturally water-quenched basaltic glasses from the vent areas of Grande Ronde lava flows shows that they do indeed contain melt inclusion-bearing phenocrysts, small, but nonetheless amenable to analysis by microbeam methods. Additionally, some of the glassy ash fragments have been quenched prior to degassing and can also be analyzed to gain an estimate of gas loss. The data will be used to estimate fluxes of S, H2O and CO2 to the atmosphere from CRB activity. This in turn will provide constraints on the potential of the CRB for cooling (due to S pollution of the atmosphere) vs. warming (due to CO2 release to the atmosphere) the Miocene climate, both of which hypotheses have been advanced by previous workers, but in both cases supported by little data. A further area of study is afforded by the fact that the glasses preserve a broader range of overall chemical compositions than do the equivalent lavas, potentially providing new insights into the ultimate origins of the flood basalt magmas.