Anatomy of a fumarole field: drone remote-sensing and petrological approaches reveal the degassing and alteration structure at La Fossa cone, Vulcano, Italy

IF 3.2 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Daniel Müller, Thomas R. Walter, Valentin R. Troll, Jessica Stammeier, Andreas Karlsson, Erica de Paolo, Antonino Fabio Pisciotta, Martin Zimmer, Benjamin De Jarnatt
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引用次数: 0

Abstract

Abstract. Hydrothermal alteration and mineralization processes can affect the physical and chemical properties of volcanic rocks. Aggressive acidic degassing and fluid flow often also lead to changes in the appearance of a rock, such as changes in surface coloration or intense bleaching. Although hydrothermal alteration can have far-reaching consequences for rock stability and permeability, limited knowledge exists on the detailed structures, extent, and dynamic changes that take place near the surface of hydrothermal venting systems. By integrating drone-based photogrammetry with mineralogical and chemical analyses of rock samples and surface gas flux, we investigate the structure of the evolving volcanic degassing and alteration system at the La Fossa cone on the island of Vulcano, Italy. Our image analysis combines principal component analysis (PCA) with image classification and thermal analysis through which we identify an area of approximately 70 000 m2 that outlines the maximum extent of hydrothermal alteration effects at the surface, represented by a shift in rock color from reddish to gray. Within this area, we identify distinct gradients of surface coloration and temperature that indicate a local variability in the degassing and alteration intensity and define several structural units within the fumarole field. At least seven such larger units of increased activity could be constrained. Through mineralogical and geochemical analysis of samples from the different alteration units, we define a relationship between surface appearance in drone imagery and the mineralogical and chemical composition. Gradients in surface color from reddish to gray correlate with a reduction in Fe2O3 from up to 3.2 % in the unaltered regime to 0.3 % in the altered regime, and the latter coincides with the area of increased diffuse acid gas flux. As the pixel brightness increases towards higher alteration gradients, we note a loss of the initial (igneous) mineral fraction and a change in the bulk chemical composition with a concomitant increase in sulfur content from close to 0 % in the unaltered samples to up to 60 % in samples from the altered domains. Using this approach of combined remote-sensing and in situ analyses, we define and spatially constrain several alteration units and compare them to the present-day thermally active surface and degassing pattern over the main crater area. The combined results permit us to present a detailed anatomy of the La Fossa fumarole field, including high-temperature fumaroles and seven larger units of increased alteration intensity, surface temperature, and variably intense surface degassing. Importantly, we also identify apparently sealed surface domains that prevent degassing, likely as a consequence of mineral precipitation from degassing and alteration processes. By assessing the thermal energy release of the identified spatial units quantitatively, we show that thermal radiation of high-temperature fumaroles accounts for < 50 % of the total thermal energy release only and that the larger part is emitted by diffuse degassing units. The integrated use of methods presented here has proven to be a useful combination for a detailed characterization of alteration and activity patterns of volcanic degassing sites and has the potential for application in alteration research and for the monitoring of volcanic degassing systems.
火口场剖析:无人机遥感和岩石学方法揭示意大利武尔卡诺火山 La Fossa 锥体的脱气和蚀变结构
摘要。热液蚀变和矿化过程会影响火山岩的物理和化学特性。侵蚀性酸性脱气和流体流动通常也会导致岩石外观发生变化,如表面颜色变化或强烈漂白。虽然热液蚀变会对岩石的稳定性和渗透性产生深远影响,但人们对热液喷发系统近地表的详细结构、范围和动态变化了解有限。通过将无人机摄影测量与岩石样本的矿物学和化学分析以及地表气体通量相结合,我们研究了意大利武尔卡诺岛 La Fossa 锥体不断演变的火山脱气和蚀变系统的结构。我们的图像分析结合了主成分分析(PCA)、图像分类和热分析,通过这些分析,我们确定了一个面积约为 70 000 平方米的区域,该区域勾勒出地表热液蚀变效应的最大范围,表现为岩石颜色从红色向灰色的转变。在这一区域内,我们确定了地表颜色和温度的明显梯度,这表明了当地脱气和蚀变强度的变化,并确定了热液场内的几个结构单元。至少可以确定七个这样的较大的活动增加单元。通过对来自不同蚀变单元的样本进行矿物学和地球化学分析,我们确定了无人机图像中的表面外观与矿物学和化学成分之间的关系。地表颜色从红色到灰色的渐变与 Fe2O3 的减少有关,Fe2O3 从未改变区的高达 3.2% 减少到改变区的 0.3%,而后者与扩散酸性气体通量增加的区域相吻合。随着像素亮度向更高的蚀变梯度增加,我们注意到初始(火成岩)矿物成分的损失和主体化学成分的变化,同时硫含量从未变质样本的接近 0% 增加到蚀变区域样本的高达 60%。利用这种遥感和现场分析相结合的方法,我们确定并在空间上限制了几个蚀变单元,并将它们与主要陨石坑区域的现今热活跃表面和脱气模式进行了比较。综合分析结果使我们能够对拉佛萨火山口热液场进行详细解剖,包括高温热液场和七个较大的蚀变强度、表面温度和表面脱气强度不同的单元。重要的是,我们还发现了表面明显密封的区域,它们阻止了脱气,这可能是脱气和蚀变过程中矿物沉淀的结果。通过定量评估已识别空间单元的热能释放情况,我们发现高温富马瘤的热辐射仅占总热能释放量的小于 50%,而更大的部分是由弥散脱气单元释放的。事实证明,本文介绍的各种方法的综合使用,是详细描述火山脱气地点的蚀变和活动模式的有效组合,具有应用于蚀变研究和火山脱气系统监控的潜力。
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来源期刊
Solid Earth
Solid Earth GEOCHEMISTRY & GEOPHYSICS-
CiteScore
6.90
自引率
8.80%
发文量
78
审稿时长
4.5 months
期刊介绍: Solid Earth (SE) is a not-for-profit journal that publishes multidisciplinary research on the composition, structure, dynamics of the Earth from the surface to the deep interior at all spatial and temporal scales. The journal invites contributions encompassing observational, experimental, and theoretical investigations in the form of short communications, research articles, method articles, review articles, and discussion and commentaries on all aspects of the solid Earth (for details see manuscript types). Being interdisciplinary in scope, SE covers the following disciplines: geochemistry, mineralogy, petrology, volcanology; geodesy and gravity; geodynamics: numerical and analogue modeling of geoprocesses; geoelectrics and electromagnetics; geomagnetism; geomorphology, morphotectonics, and paleoseismology; rock physics; seismics and seismology; critical zone science (Earth''s permeable near-surface layer); stratigraphy, sedimentology, and palaeontology; rock deformation, structural geology, and tectonics.
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