Formation of obsidian pyroclasts by sintering of ash particles in the volcanic conduit
Formation of obsidian pyroclasts by sintering of ash particles in the volcanic conduit
复制标题
DOI：
10.1016/j.epsl.2016.11.037
复制
发表时间：
2017-02-01
影响因子：
5.3
通讯作者：
Befus, Kenneth S.
中科院分区：
文献类型：
--
作者：
Gardner, James E.;Llewellin, Edward W.;Befus, Kenneth S.
火山喷发的强度和类型的范围，从高度爆炸性的普林尼式喷发到静止的熔岩喷出，取决于上升岩浆中气体损失的类型和效率。黑曜岩火山碎屑--在一些火山熔岩床中发现的小的玻璃状的淬火岩浆碎片--可能在它们的泡状结构和挥发性成分中保存了关于岩浆脱气的有价值的信息。然而，对它们的结构和挥发物的准确解释需要了解火山碎屑的形成机制。来自加州的黑曜石火山碎屑。公元1325-1350年对美国加利福尼亚州莫诺火山口的北莫诺喷发进行了分析，发现其含有H2O和CO2，表明它们是在大约3-40 MPa的压力范围内形成的。许多还包含具有不同囊泡纹理的结构域，由包含捕晶物质的边界分隔，表明它们由缝合在一起的较小片段组成。超过一半的火山碎屑分析包含小（类似于10 μ m），高度扭曲的囊泡，与多尖形形态，解释为残留的间隙气体之间的熔体/玻璃烧结碎片被困。圆形囊泡也很常见，并被解释为由扭曲囊泡的表面张力驱动的松弛引起的。计算的烧结和松弛的时间尺度是一致的火山碎屑形成的H2O内的异质火山碎屑再平衡的时间尺度。这种烧结模型的黑曜石火山碎屑的起源进一步支持的观察，发现球形囊泡主要是在富H2O火山碎屑，和扭曲囊泡主要是在H2O贫火山碎屑。我们的结论是，黑曜石火山碎屑在北莫诺火山喷发形成的周期的碎片化，烧结/烧结，和放松，在一个非常广泛的深度内的管道，我们没有发现证据支持浮石（泡沫）崩溃的形成机制。类似的纹理，并在其他喷发的黑曜岩火山碎屑的捕虏体材料的发生表明，烧结可能是一般的黑曜岩火山碎屑的起源负责。我们的概念模型表明，黑曜岩火山碎屑中的挥发分含量反映了气泡岩浆的脱气和烧结颗粒之间的气体成分。(C)2016作者由爱思唯尔公司出版
The ranges in intensity and style of volcanic eruptions, from highly explosive Plinian eruptions to quiescent lava extrusions, depend on the style and efficiency of gas loss from ascending magma. Obsidian pyroclasts - small, glassy pieces of quenched magma found in some volcanic tephra beds - may preserve valuable information about magma degassing in their vesicle textures and volatile contents. Accurate interpretation of their textures and volatiles, however, requires understanding the mechanism of formation of the pyroclasts. Obsidian pyroclasts from the ca. 1325-1350 C.E. North Mono eruption of Mono Craters (CA, USA) were analyzed and found to have H2O and CO2 contents indicating that they were formed at pressures in the approximate range of 3-40 MPa. Many also contain domains with differing vesicle textures, separated by boundaries containing xenocrystic material, indicating that they are composed of smaller fragments that have sutured together. More than half of the pyroclasts analyzed contained small (similar to 10 mu m), highly distorted vesicles, with multi-cuspate morphology, interpreted as the remnants of interstitial gas trapped amongst sintered fragments of melt/glass. Rounded vesicles are also common and are interpreted to result from surface tension-driven relaxation of the distorted vesicles. Calculated timescales of sintering and relaxation are consistent with timescales for pyroclast formation indicated by H2O re-equilibration within the heterogeneous pyroclasts. This sintering model for the origin of obsidian pyroclasts is further supported by the observation that spherical vesicles are found mainly in H2O-rich pyroclasts, and distorted vesicles mainly in H2O-poor pyroclasts. We conclude that obsidian pyroclasts generated during the North Mono eruption were formed by cycles of fragmentation, sintering/suturing, and relaxation, over a very wide range of depths within the conduit; we find no evidence to support pumice (foam) collapse as the formation mechanism. Similar textures, and the occurrence of xenolithic material, in obsidian pyroclasts in other eruptions suggest that sintering may be generally responsible for the origin of obsidian pyroclasts. Our conceptual model indicates that volatile contents in obsidian pyroclasts reflect both degassing of bubbly magma and the composition of gas trapped between sintering particles. (C) 2016 The Authors. Published by Elsevier B.V.