Abstract Rabaul in Papua-New-Guinea is an extremely active andesitic caldera complex that displays a large spectrum of eruption styles. Since 1878, four sub-plinian (VEI-4) and ten VEI 1–3 (effusive, strombolian, vulcanian) eruptions occurred from Tavurvur and Vulcan, the two main active vents. We study pumiceous tephra, ballistic bombs and a lava flow from five of these eruptions to investigate (1) magma ascent rates and (2) volatile escapement processes during the ascent. We measured total and connected porosities, permeability, and connectivity and related these results with measurements of crystallinity, Microlite Number Density (MND) and Microlite Size Distribution (MSD) of plagioclases and pyroxenes. From the application of existing percolation models, we find that explosive products yield a percolation threshold comprised between 50 and 60 vol% total porosity, while petrophysical parameters of the lava flow and some of the ballistic bombs are explained by bubble collapse driven by surface tension. Sub-plinian products show low phenocryst contents (5–15 vol%), microlites generated by nucleation-driven crystallization or glassy textures due to kinetic crystallization lags. Vulcanian, strombolian and effusive products on the other hand, show medium to high phenocryst contents (15–40 vol%) and microlites that crystallised by growth-dominated processes. MSDs in sub-plinian pumiceous tephra constitute a partial record of drastic magma acceleration. In comparison, MSDs in vulcanian and strombolian ballistic bombs and lava flow show a classic pattern of crystallization at a steady-state due to gradual magma ascent. We find that magma feeding sub-plinian eruptions ascends 2–3 orders of magnitude faster than magma feeding vulcanian/effusive eruptions (≥ 1–100 m/s vs 0.1 m/s). In the case of sub-plinian products, due to the important kinetic crystallization lag, these speeds cannot be estimated using microlite crystallization triggered by decompression as a proxy. Combining petrophysical and textural measurements we suggest that slight changes in initial conditions in the reservoir such as crystallinity or the presence of exsolved volatiles, can have a profound impact on the ascent rate and generate positive or negative feedback effects leading to powerful sub-plinian activity or intermittent vulcanian/ passive effusive activity respectively.

中文翻译：

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巴布亚新几内亚拉包尔火山喷发方式的控制——来自微晶岩、孔隙度和渗透率测量的见解

摘要 巴布亚-新几内亚的拉包尔火山是一个极其活跃的安山岩火山口群，具有多种喷发方式。自 1878 年以来，Tavurvur 和 Vulcan 这两个主要的活动喷口发生了四次亚平面 (VEI-4) 喷发和十次 VEI 1-3（溢流型、斯特龙波型、火山喷发）喷发。我们研究了其中五次喷发的火山灰、弹道炸弹和熔岩流，以调查 (1) 岩浆上升速率和 (2) 上升过程中的挥发性逃逸过程。我们测量了总的和连通的孔隙度、渗透率和连通性，并将这些结果与斜长石和辉石的结晶度、微晶石数密度 (MND) 和微晶石尺寸分布 (MSD) 的测量结果相关联。从现有渗透模型的应用来看，我们发现爆炸产品产生的渗透阈值占总孔隙度的 50% 到 60%，而熔岩流和一些弹道炸弹的岩石物理参数可以用表面张力驱动的气泡坍塌来解释。Sub-plinian 产品显示出低斑晶含量（5-15 vol%），由成核驱动结晶或由于动力学结晶滞后产生的玻璃状纹理产生的微晶。另一方面，Vulcanian、strombolian 和渗出性产品显示出中等至高的斑晶含量（15-40 vol%）和通过生长主导过程结晶的微晶。亚平层火山灰中的 MSD 构成了剧烈岩浆加速的部分记录。相比下，由于岩浆逐渐上升，火山和斯特龙博利安弹道炸弹和熔岩流中的 MSD 显示出稳定状态下的经典结晶模式。我们发现，以岩浆为食的次斜坡喷发比以火山岩/喷发的岩浆为食的火山喷发上升 2-3 个数量级（≥ 1-100 m/s vs 0.1 m/s）。在亚平面产品的情况下，由于重要的动力学结晶滞后，这些速度不能使用减压触发的微晶结晶作为代理来估计。结合岩石物理和质地测量，我们建议储层中初始条件的轻微变化，例如结晶度或外溶挥发物的存在，