Compaction and gas loss in welded pyroclastic deposits as revealed by porosity, permeability, and electrical conductivity measurements of the Shevlin Park Tuff
Compaction and gas loss in welded pyroclastic deposits as revealed by porosity, permeability, and electrical conductivity measurements of the Shevlin Park Tuff
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DOI：
10.1130/b30668.1
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发表时间：
2014
影响因子：
4.9
通讯作者：
H. Wright;K. Cashman
中科院分区：
文献类型：
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作者：
H. Wright;K. Cashman
大型火山喷发产生的火山碎屑流通常在侵位后变得致密。火山碎屑流沉积物中的气体逸出、压实和焊接过程受组成材料的物理和热性质控制。通过测量基质孔隙度、渗透率和电导率，我们提供了一个框架来了解这些过程中孔隙结构的演变。利用美国俄勒冈州中部 Shevlin Park 凝灰岩的数据和文献中的数据，我们发现在 0%–70% 的孔隙度范围内，基质渗透率变化近 10 个数量级（从 10 –20 到 10 –11 m 2 ），在任何给定的孔隙度下，变化超过三个数量级。给定孔隙度的部分变化是由于渗透率各向异性造成的，其中定向岩心样本表明平行于叶理（水平）的渗透率高于垂直于叶理（垂直）的渗透率。这表明孔隙空间在压实过程中被压平，形成各向异性的裂纹状网络，这是一种由电导率测量支持的几何形状。我们发现幂律方程：k 1 = 1.3 × 10 –21 × phi 5.2 提供了主要水平气体损失的最佳近似值，其中 k 1 = 渗透率，phi = 孔隙度。 Kozeny-Carman 流体流动近似的应用表明，Shevlin Park 凝灰岩的渗透率是由最小宽度为 0.3 和 7.5 μm 的裂缝或盘状孔隙孔径控制的。我们发现基质渗透性限制了短时间内的压实，但变形是通过冷却、压实、水吸收和可渗透气体逸出之间的竞争来控制的。这些竞争过程控制着超压（和二次爆炸）发展的可能性以及沉积物中的焊接程度，这些过程通常适用于火山沉积物的粘性致密化。此外，孔隙度、渗透率和孔隙几何形状之间的一般关系与任何流体通过冰晶基质的流动相关。
Pyroclastic flows produced by large volcanic eruptions commonly densify after emplacement. Processes of gas escape, compaction, and welding in pyroclastic-flow deposits are controlled by the physical and thermal properties of constituent material. Through measurements of matrix porosity, permeability, and electrical conductivity, we provide a framework for understanding the evolution of pore structure during these processes. Using data from the Shevlin Park Tuff in central Oregon, United States, and from the literature, we find that over a porosity range of 0%–70%, matrix permeability varies by almost 10 orders of magnitude (from 10 –20 to 10 –11 m 2 ), with over three orders of magnitude variation at any given porosity. Part of the variation at a given porosity is due to permeability anisotropy, where oriented core samples indicate higher permeabilities parallel to foliation (horizontally) than perpendicular to foliation (vertically). This suggests that pore space is flattened during compaction, creating anisotropic crack-like networks, a geometry that is supported by electrical conductivity measurements. We find that the power law equation: k 1 = 1.3 × 10 –21 × ϕ 5.2 provides the best approximation of dominant horizontal gas loss, where k 1 = permeability, and ϕ = porosity. Application of Kozeny-Carman fluid-flow approximations suggests that permeability in the Shevlin Park Tuff is controlled by crack- or disk-like pore apertures with minimum widths of 0.3 and 7.5 μm. We find that matrix permeability limits compaction over short times, but deformation is then controlled by competition among cooling, compaction, water resorption, and permeable gas escape. These competing processes control the potential for development of overpressure (and secondary explosions) and the degree of welding in the deposit, processes that are applicable to viscous densification of volcanic deposits in general. Further, the general relationships among porosity, permeability, and pore geometry are relevant for flow of any fluid through an ignimbritic host.