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，火山爆发可以通过向大气释放气体直接引起短期（几年）的全球气候扰动。不太清楚的是，非常大的火山喷发是否有能力造成灾难性和/或长期的气候变化。这样的喷发没有人类的记录，但它们的产物在地质记录中是显而易见的。在这类事件中，巨大的冻结熔岩流堆构成了大陆洪水玄武岩区。个别的洪水熔岩喷发可能比历史上最大的喷发熔岩大数百倍，对气候的影响可能会按比例严重。最年轻、研究最透彻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，现在覆盖了美国西北太平洋地区约20万平方公里。先前对与这些熔岩喷发有关的气体释放的研究集中在玄武岩主要喷发之后的晚期流动上。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，而不是等效的结晶，“石质”熔岩本身。玻璃和它们所含的晶体比熔岩保存了更高保真度的喷发气体记录，从而可以估计出CRB活动最激烈阶段造成的大气污染。这些估计将构成“基本事实”，有助于约束喷发时区域和全球气候扰动的模型。在此之前，对CRB脱气的研究主要集中在斑岩型Wanapum熔岩（占整个CRB的6%）上，它晚于以Grande Ronde熔岩为代表的洪水火山活动高峰（占整个CRB的70%）。研究Grande Ronde的主要障碍是人们认为它们是干燥的，即它们不含含熔体包裹体的“斑晶”（喷发前在岩浆中生长的晶体）。晶体中的熔体包裹体保存了岩浆的原始气体含量（主要是H2O， CO2， S），然后可以将其与完全脱气的地面玄武岩进行比较，以获得气体损失的估计。然而，对来自大朗德熔岩流喷口地区的自然水淬玄武岩玻璃的研究表明，它们确实含有含熔体包裹体的斑晶，虽然很小，但仍然可以用微束方法分析。此外，一些玻璃灰碎片在脱气之前已经被淬火，也可以分析以获得气体损失的估计。这些数据将用于估计CRB活动产生的S、H2O和CO2到大气的通量。这反过来又将限制CRB对中新世气候的降温（由于大气的S污染）和变暖（由于二氧化碳释放到大气中）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数据支持。玻璃比同等的熔岩保存了更广泛的整体化学成分，这一事实提供了一个进一步的研究领域，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.