Experimental constraints on the textures and origin of obsidian pyroclasts
Experimental constraints on the textures and origin of obsidian pyroclasts
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黑曜岩火山碎屑的结构和起源的实验限制
DOI：
10.1007/s00445-019-1283-z
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发表时间：
2019
影响因子：
3.5
通讯作者：
Coumans, Jason P.
中科院分区：
文献类型：
--
作者：
Gardner, James E.;Wadsworth, Fabian B.;Llewellin, Edward W.;Watkins, James M.;Coumans, Jason P.
黑铁矿火山碎屑通常保存在硅质喷发的秋季沉积物中。最近的研究表明，它们是由火山灰颗粒在破碎深度以上的管道壁上烧结形成的，随后被撕裂并在气粒弥散中运输。尽管烧结假说与黑麻岩中一般的气泡结构和溶解挥发分是一致的，但以前的烧结实验并没有捕捉到天然火山碎屑中观察到的所有结构的复杂性。在这里，我们设计了实验，在与浅层火山管道相关的温度和H2O压力下，在不同的冷却速度下，单峰和双峰分布的流纹岩火山灰被烧结。这些实验产生了致密的焊接黑铁矿，具有一系列类似于在天然火山碎屑中观察到的结构。我们发现，使用颗粒的单峰分布产生具有均匀分布的囊泡的黑晶石，而双峰初始颗粒分布产生具有较差的泡状玻璃区域的黑麻岩，其中具有较多的泡孔丰富的玻璃的区域。我们还发现，缓慢的冷却会导致捕获的囊泡被吸收，产生完全致密的黑铁矿。这些广泛的特征与在北莫诺(美国加州)流纹岩喷发中发现的黑麻岩火山碎屑中发现的特征相匹配，为黑岩可以通过在爆炸爆发时在碎屑深度以上的火山灰烧结而产生的假说提供了强有力的支持。
Obsidian pyroclasts are commonly preserved in the fall deposits of explosive silicic eruptions. Recent work has suggested that they form by sintering of ash particles on the conduit walls above the fragmentation depth and are subsequently torn out and transported in the gas-particle dispersion. Although the sintering hypothesis is consistent with the general vesicle textures and dissolved volatiles in obsidian pyroclasts, previous sintering experiments do not capture all of the textural complexities observed in the natural pyroclasts. Here, we design experiments in which unimodal and bimodal distributions of rhyolitic ash are sintered at temperatures and H2O pressures relevant to shallow volcanic conduits and under variable cooling rates. The experiments produce dense, welded obsidian that have a range of textures similar to those observed in natural pyroclasts. We find that using a unimodal distribution of particles produces obsidian with evenly distributed trapped vesicles, while a bimodal initial particle distribution produces obsidian with domains of poorly vesicular glass among domains of more vesicle-rich glass. We also find that slow cooling leads to resorption of trapped vesicles, producing fully dense obsidian. These broad features match those found in obsidian pyroclasts from the North Mono (California, USA) rhyolite eruption, providing strong support to the hypothesis that obsidian can be produced by ash sintering above the fragmentation depth during explosive eruptions.