Crystalline domains in rhyolitic domes and coulees are commonly characterized by spherulites and lithophysae. Spherulites typically range from microscopic to a few centimeters in diameter. Larger spherulites, termed “megaspherulites,” are rare but have been reported in the USA and Mexico. The uncommon nature of such structures supports the need for a study to explore the factors and special conditions that allow them to reach such a large size. In the Southern Puna of Argentina, El Viejo Coulee includes megaspherulites up to 4 m in diameter. We present observations from field work, petrography, scanning electron microscopy, bulk geochemistry, and Fourier transform infrared spectroscopy. The megaspherulites occur in obsidian lenses that differ in phenocryst content and composition from the foliated coherent facies that forms the rest of the coulee. The obsidian lacks vesicles and microlites and is unaltered. The megaspherulites comprise growth cones consisting of micropoikilitic texture where quartz encloses potassium feldspar. The growth cones are separated by interconal areas composed of lithophysae. We propose that the megaspherulites were formed above the glass transition temperature ( T g ) and are the product of primary crystallization of rhyolitic melt. The exceptionally large size of the megaspherulites implies high diffusion rates which are favored by temperatures above T g during crystallization. The large size also suggests scarcity of nucleation sites, which is consistent with the megaspherulites being hosted by unaltered microlite- and vesicle-free glass. The position of the obsidian lens at the base of the coulee may have played a critical role in maintaining the temperature above the T g long enough to allow the crystallization of the megaspherulites. These conditions also favored crystallization in the most advanced stage where micropoikilitic texture replaced the fans of crystal fibers typical of spherulites. Crystallization of anhydrous quartz and feldspar in the growth cones led to the concentration of volatiles in the melt in the interconal areas, resulting in volatile exsolution and formation of vesicles that became nucleation sites for lithophysae. The study advances our understanding of some of the special processes that are involved in the cooling and solidification of rhyolitic magmas. Fundamentally, we find that the position of the obsidian at the base of the coulee was critically important because this position favored maintenance of the temperature above the T g which, in turn, favored high diffusion rates. Also, the scarcity of nucleation sites in the obsidian melt allowed only a small number of spherulites to nucleate; those that nucleated therefore grew very large. The meter-scale megaspherulites may have taken ~ 55 years to create.

中文翻译：

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阿根廷南部普纳 El Viejo 流纹岩古力（更新世）巨球晶的成因

流纹质圆顶和古井中的结晶域通常以球晶和石泡为特征。球晶的直径通常从微观到几厘米不等。较大的球晶，称为“巨球晶”，虽然很少见，但在美国和墨西哥已有报道。这种结构的不寻常性质支持需要进行研究，以探索使它们达到如此大尺寸的因素和特殊条件。在阿根廷南部的普纳，El Viejo Coulee 包括直径达 4 m 的巨型球晶。我们展示了野外工作、岩石学、扫描电子显微镜、体地球化学和傅里叶变换红外光谱的观察结果。大球晶出现在黑曜石透镜中，其斑晶含量和成分与形成其余古力的叶状连贯相不同。黑曜石没有囊泡和微晶，并且没有改变。巨型球晶包括生长锥，由石英包裹钾长石的微变晶结构组成。生长锥被由石泡组成的锥间区域隔开。我们认为巨球晶形成于玻璃化转变温度 (Tg) 以上，并且是流纹岩熔体初级结晶的产物。超大尺寸的巨球晶意味着高扩散速率，这有利于结晶过程中高于 Tg 的温度。大尺寸还表明成核位点的稀缺，这与由未改变的微晶和无囊泡玻璃承载的巨球晶一致。黑曜石透镜在古力底部的位置可能在将温度保持在 T g 以上足够长的时间以允许巨球晶结晶方面发挥了关键作用。这些条件也有利于在最先进的阶段结晶，在这个阶段，微球晶纹理取代了典型球晶的晶体纤维扇形。生长锥中无水石英和长石的结晶导致锥间区域熔体中挥发物的浓度，导致挥发物出溶和囊泡的形成，成为石泡的成核位点。这项研究增进了我们对流纹质岩浆冷却和凝固所涉及的一些特殊过程的理解。从根本上说，我们发现黑曜石在古力底部的位置非常重要，因为这个位置有利于将温度维持在 Tg 以上，进而有利于高扩散速率。此外，黑曜石熔体中成核位点的稀缺性仅允许少量球晶成核；因此，那些成核的变得非常大。米级巨球晶可能需要大约 55 年的时间才能形成。