Dense, glassy pyroclasts found in products of explosive eruptions are commonly employed to investigate volcanic conduit processes through measurement of their volatile inventories. This approach rests upon the tacit assumption that the obsidian clasts are juvenile, that is, genetically related to the erupting magma. Pyroclastic deposits within the Yellowstone-Snake River Plain province almost without exception contain dense, glassy clasts, previously interpreted as hyaloclastite, while other lithologies, including crystallised rhyolite, are extremely rare. We investigate the origin of these dense, glassy clasts from a coupled geochemical and textural perspective combining literature data and case studies from Cougar Point Tuff XIII, Wolverine Creek Tuff, and Mesa Falls Tuff spanning 10 My of silicic volcanism. These results indicate that the trace elemental compositions of the dense glasses mostly overlap with the vesiculated component of each deposit, while being distinct from nearby units, thus indicating that dense glasses are juvenile. Textural complexity of the dense clasts varies across our examples. Cougar Point Tuff XIII contains a remarkable diversity of clast appearances with the same glass composition including obsidian-within-obsidian clasts. Mesa Falls Tuff contains clasts with the same glass compositions but with stark variations in phenocryst content (0 to 45%). Cumulatively, our results support a model where most dense, glassy clasts reflect conduit material that passed through multiple cycles of fracturing and sintering with concurrent mixing of glass and various crystal components. This is in contrast to previous interpretations of these clasts as entrained hyaloclastite and relaxes the requirement for water-magma interaction within the eruptive centres of the Yellowstone-Snake River Plain province.

在爆炸性爆发产物中发现的致密的玻璃状破火山岩通常用于通过测量其挥发性物质清单来研究火山管道的过程。这种方法基于隐性假设，即黑曜石碎屑是幼小的，即与喷发的岩浆有遗传关系。黄石-蛇河平原省内的火山碎屑沉积物几乎毫无例外地含有致密的玻璃状碎屑，以前被称为透明质碎屑岩，而其他岩性，包括结晶的流纹岩，则极为罕见。我们从地球化学和构造学的角度出发，结合文献数据和来自Cougar Point Tuff XIII，Wolverine Creek Tuff和Mesa Falls Tuff跨越10 My硅质火山作用的案例研究，从地球化学和纹理学的角度研究了这些致密的玻璃状碎屑的起源。这些结果表明，致密玻璃的痕量元素组成大部分与每个沉积物的囊状组分重叠，而与附近的单元不同，因此表明致密玻璃是幼稚的。在我们的示例中，密集的碎屑的纹理复杂性各不相同。美洲狮点凝灰岩XIII具有相同的玻璃成分（包括黑曜石-黑曜石内部碎屑）的大量碎屑外观。梅萨瀑布凝灰岩包含的玻璃碎片具有相同的玻璃成分，但表晶的含量却有明显的变化（0至45％）。累积地，我们的结果支持了一个模型，其中最致密的玻璃状碎屑反映了导管材料，该导管材料经过多个破裂和烧结循环，同时混合了玻璃和各种晶体成分。