Trigger Mechanisms of Compositionally Zoned Explosive Eruptions: Insights From Cosiguina Volcano, Nicaragua

了解火山爆发的触发机制是现代火山学的一个重要目标。喷发可能由外部触发，例如，地震或火山内部的岩浆系统发生的过程。通常被用作内部触发的是一批新的岩浆对岩浆库的补给，其直接证据通常保存在喷发沉积物中。或者，由于岩浆中气泡的形成和膨胀而导致的渐进压力积累可能最终导致岩浆储层顶部的破裂，从而导致喷发。这种触发机制提出的理论基础上，最近得到了实验和数值模拟的支持，但很少被指纹事后在喷发沉积物。几个火山，如Cosiguina火山，尼加拉瓜，产生成分分层喷发沉积物，通常被解释为反映分层岩浆库的逐步撤离，其中较冷的岩浆与较热的岩浆长期相邻并源自较热的岩浆。这些分层系统一般承担的证据蒸汽饱和喷发前，往往缺乏诊断线索的岩浆补给触发，虽然神秘的补给通常不能被排除。在这里，我们建议使用一个系统的方法，基于高分辨率的采样和火山岩的地球化学和纹理分析，这将允许确定在Cosiguina的喷发触发机制。这项研究将促进我们对层状岩浆库的起源和火山爆发的直接原因的理解。岩浆库中溶解于熔体和溶出成气泡的挥发性元素的数量和分布是控制岩浆流变行为的关键因素，并最终控制喷发触发机制和喷发方式。我们计划测量的主要，微量和挥发性元素的熔融包裹体浮石和火山渣样品中收集的战略地层地平线内的成分分区喷发矿床在Cosiguina，允许重建的溶解和exsolved挥发性梯度内的喷发前岩浆系统。这个地球化学和纹理实验的目的是区分预期的指纹留下的挥发物积累触发从镁铁质补给触发，其中有很多成分分层喷发矿床的证据很少。Cosiguina火山在最近的地质历史中产生了多个分区爆炸性喷发，包括公元1835年影响全球气候的VEI 5喷发。Cosiguina存款有几个相似之处，经典的分区喷发，如气候的马扎马山和1912年卡特迈喷发，我们预计我们的研究结果有广泛的影响挥发性梯度和喷发触发机制在成分分区岩浆系统。

Understanding the trigger mechanisms of explosive volcanic eruptions is an important goal of modern volcanology. An eruption may be triggered externally, e.g., by an earthquake, or internally, by processes occurring within the magmatic system underlying the volcano. Commonly invoked as an internal trigger is the recharge of the magma reservoir with a new batch of magma, direct evidence for which is often preserved in eruption deposits. Alternatively, progressive pressure buildup due to the formation and expansion of bubbles in the magma may eventually lead to fracturing of the magma reservoir roof, resulting in an eruption. This trigger mechanism proposed on theoretical grounds has recently received experimental and numerical modeling support, but has rarely been fingerprinted post hoc in eruption deposits. Several volcanoes, such as Cosiguina volcano, Nicaragua, produce compositionally stratified eruption deposits that are typically interpreted to reflect the progressive evacuation of a layered magma reservoir, in which a cooler magma was protractedly contiguous with and derived from a hotter magma. These stratified systems generally bear evidence of vapor saturation prior to eruption and often lack the diagnostic clues for a magma recharge trigger, although cryptic recharge cannot usually be ruled out. Here we propose to use a systematic approach, based on high-resolution sampling and geochemical and textural analysis of volcanic rocks, that will allow identifying the eruption trigger mechanism at Cosiguina. This research will advance our understanding of the origin of layered magma reservoirs and the direct causes of volcanic eruptions. The amount and distribution of volatile elements, dissolved in the melt and exsolved into bubbles, in magma reservoirs are key controls on the rheological behavior of magma and, ultimately, on eruption trigger mechanisms and eruptive styles. We plan to measure major, trace and volatile element concentrations in melt inclusions from pumice and scoria samples collected at strategic stratigraphic horizons within compositionally zoned eruption deposits at Cosiguina, allowing the reconstruction of the dissolved and exsolved volatile gradients within the pre-eruptive magmatic system. This geochemical and textural experiment is designed to differentiate the expected fingerprints left by a volatile-buildup trigger from those of a mafic recharge trigger, for which there is scarce evidence in many compositionally stratified eruption deposits. Cosiguina volcano produced multiple zoned explosive eruptions in the recent geologic past, including a VEI 5 eruption in AD 1835 that impacted global climate. The Cosiguina deposits share several similarities with those of classic zoned eruptions, such as the climactic Mount Mazama and the 1912 Katmai eruptions, and we anticipate our results to have broad implications for volatile gradients and eruption trigger mechanisms at compositionally zoned magmatic systems.