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

成分分区爆炸性喷发的触发机制：来自尼加拉瓜科西吉纳火山的见解

• 批准号: 1650379
• 负责人: Marc-Antoine Longpre
• 金额: $ 19.72万
• 依托单位: CUNY Queens College
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-08-15 至 2023-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1650379&HistoricalAwards=false
• 关键词: Trigger; Mechanisms; Compositionally; Zoned; Explosive

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.

了解火山喷发的触发机制是现代火山学的一个重要目标。喷发可能是外部触发的，例如地震，也可能是内部触发的，即火山下方岩浆系统内发生的过程。通常被称为内部触发因素的是岩浆库用新一批岩浆进行补给，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逐渐形成的压力可能最终导致岩浆库顶部破裂，从而导致喷发。这种基于理论基础提出的触发机制最近得到了实验和数值模拟的支持，但很少在喷发沉积物中事后进行指纹识别。一些火山，例如尼加拉瓜的科西吉纳火山，产生成分分层的喷发沉积物，这些沉积物通常被解释为反映了层状岩浆库的逐渐疏散，其中较冷的岩浆与较热的岩浆长期毗邻并源自较热的岩浆。这些分层系统通常在喷发前具有蒸汽饱和的证据，并且通常缺乏岩浆补给触发的诊断线索，尽管通常不能排除神秘补给的可能性。在这里，我们建议使用一种基于火山岩高分辨率采样以及地球化学和结构分析的系统方法，这将有助于确定科西吉纳火山喷发的触发机制。这项研究将增进我们对层状岩浆库的起源和火山喷发的直接原因的理解。岩浆库中溶解在熔体中并溶解成气泡的挥发性元素的数量和分布是对岩浆流变行为以及最终喷发触发机制和喷发类型的关键控制。我们计划测量在科西吉纳成分分区喷发沉积物的战略地层层位收集的浮石和火山渣样品中熔体包裹体中的主要元素、痕量元素和挥发性元素浓度，从而重建喷发前岩浆系统内溶解和溶出的挥发性元素梯度。这项地球化学和结构实验的目的是区分挥发物堆积触发因素留下的预期指纹与镁铁质补给触发因素留下的指纹，而在许多成分分层的喷发矿床中，这方面的证据很少。科西吉纳火山在最近的地质历史中发生了多次分区爆炸性喷发，其中包括公元 1835 年的 VEI 5 级喷发，影响了全球气候。科西吉纳矿床与经典的分区喷发有一些相似之处，例如马扎马山的顶峰和 1912 年卡特迈火山喷发，我们预计我们的结果将对成分分区岩浆系统的挥发梯度和喷发触发机制产生广泛的影响。