Mafic explosive volcanism at Llaima Volcano: 3D x-ray microtomography reconstruction of pyroclasts to constrain shallow conduit processes
Mafic explosive volcanism at Llaima Volcano: 3D x-ray microtomography reconstruction of pyroclasts to constrain shallow conduit processes
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DOI：
10.1007/s00445-021-01514-8
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发表时间：
2021-12
影响因子：
3.5
通讯作者：
Pedro Valdivia;A. Marshall;B. Brand;M. Manga;C. Huber
中科院分区：
文献类型：
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作者：
Pedro Valdivia;A. Marshall;B. Brand;M. Manga;C. Huber
镁铁质火山活动主要是喷发到轻度爆发。普林尼和熔结凝灰岩形成的镁铁质喷发虽然罕见，但也有可能;然而，促进这种爆炸性的条件仍在探索中。喷发方式取决于岩浆上升时气体逃逸的能力，这在低粘度的镁铁质岩浆中往往更容易。如果岩浆的渗透性足够高，以减少上升过程中的气泡超压，挥发物可能会从岩浆中逸出，抑制剧烈的爆炸活动。相反，如果渗透率低到足以在上升过程中保持岩浆中的气相，气泡超压可能会驱动岩浆破碎。快速上升可能会导致不平衡结晶，增加粘度，影响气泡网络，从而影响渗透性，从而影响爆炸性。为了探索条件，促进强烈的爆炸性镁铁质火山作用，我们结合联合收割机微晶结构分析与同步加速器X射线计算机显微断层扫描的10个火山碎屑从12.6万年的镁铁质Curacautín火山灰岩（Llaima火山，智利）。我们量化的微晶晶体尺寸分布（CSD），微晶数密度，孔隙度，气泡的互连性，气泡数密度，和多孔介质的几何特性，岩浆脱气过程中的作用进行调查在镁铁质爆炸喷发。我们使用的分析技术来估计渗透率和曲折性结合Kozeny-Carman关系，曲折因子，火山碎屑泡纹理。我们样品的基质由高达44%的斜长石微晶组成，其中> 85%的长度< 10 µm。此外，我们在我们的样品中鉴定了两种囊泡群体：（1）由较小囊泡（> 99%的孔体积）的广泛聚结产生的回旋互连囊泡网络，以及（2）非常小且完全孤立的囊泡群体（< 1%的孔隙率）。计算出的渗透率范围为3.0 × 10− 13至6.3 × 10− 12平方米，低于新西兰的塔拉韦拉和埃特纳（公元前112年，意大利）的类似爆发性镁铁质喷发。我们的CSD，微晶数密度和3D囊泡纹理的组合证据快速上升，诱导高不平衡条件，促进快速同步喷发结晶的微晶内浅管道。我们解释说，微晶结晶增加粘度，同时迫使气泡变形，因为它们一起生长，导致在可渗透的高度曲折的网络囊泡。使用分离的囊泡的气泡数密度（0.1-3 - 3× 104个气泡/mm 3），我们得到最小平均减压速率为1.4 MPa/s。尽管有纹理证据表明，库拉索岩浆达到渗流阈值，我们建议，快速上升抑制放气和增加气泡超压，导致爆炸性破碎。此外，利用我们样品的孔隙度和渗透率，我们估计气泡超压> 5 MPa可能足以使库拉索岛岩浆破碎。其他镁铁质爆发报告类似的不平衡条件引起的快速上升速率，这意味着同喷发不平衡条件可能控制爆炸性的镁铁质喷发更普遍。
Mafic volcanic activity is dominated by effusive to mildly explosive eruptions. Plinian and ignimbrite-forming mafic eruptions, while rare, are also possible; however, the conditions that promote such explosivity are still being explored. Eruption style is determined by the ability of gas to escape as magma ascends, which tends to be easier in low-viscosity, mafic magmas. If magma permeability is sufficiently high to reduce bubble overpressure during ascent, volatiles may escape from the magma, inhibiting violent explosive activity. In contrast, if the permeability is sufficiently low to retain the gas phase within the magma during ascent, bubble overpressure may drive magma fragmentation. Rapid ascent may induce disequilibrium crystallization, increasing viscosity and affecting the bubble network with consequences for permeability, and hence, explosivity. To explore the conditions that promote strongly explosive mafic volcanism, we combine microlite textural analyses with synchrotron x-ray computed microtomography of 10 pyroclasts from the 12.6 ka mafic Curacautín Ignimbrite (Llaima Volcano, Chile). We quantify microlite crystal size distributions (CSD), microlite number densities, porosity, bubble interconnectivity, bubble number density, and geometrical properties of the porous media to investigate the role of magma degassing processes at mafic explosive eruptions. We use an analytical technique to estimate permeability and tortuosity by combing the Kozeny-Carman relationship, tortuosity factor, and pyroclast vesicle textures. The groundmass of our samples is composed of up to 44% plagioclase microlites, > 85% of which are < 10 µm in length. In addition, we identify two populations of vesicles in our samples: (1) a convoluted interconnected vesicle network produced by extensive coalescence of smaller vesicles (> 99% of pore volume), and (2) a population of very small and completely isolated vesicles (< 1% of porosity). Computed permeability ranges from 3.0 × 10−13to 6.3 × 10−12m2, which are lower than the similarly explosive mafic eruptions of Tarawera (1886; New Zealand) and Etna (112 BC; Italy). The combination of our CSDs, microlite number densities, and 3D vesicle textures evidence rapid ascent that induced high disequilibrium conditions, promoting rapid syn-eruptive crystallization of microlites within the shallow conduit. We interpret that microlite crystallization increased viscosity while simultaneously forcing bubbles to deform as they grew together, resulting in the permeable by highly tortuous network of vesicles. Using the bubble number densities for the isolated vesicles (0.1-3−3× 104bubbles per mm3), we obtain a minimum average decompression rate of 1.4 MPa/s. Despite the textural evidence that the Curacautín magma reached the percolation threshold, we propose that rapid ascent suppressed outgassing and increased bubble overpressures, leading to explosive fragmentation. Further, using the porosity and permeability of our samples, we estimated that a bubble overpressure > 5 MPa could have been sufficient to fragment the Curacautín magma. Other mafic explosive eruptions report similar disequilibrium conditions induced by rapid ascent rate, implying that syn-eruptive disequilibrium conditions may control the explosivity of mafic eruptions more generally.