Understanding the petrogenesis of silicic magmas is critical for understanding the volcanic hazards they pose, their geothermal energy potential, and the creation of continental crust. In this study we explore the origin of rhyolitic magmas in basaltic crust at the Krafla Central Volcano in Iceland. We present laser fluorination oxygen isotope analyses of plagioclase, pyroxene, and groundmass from eight rhyolites and six selected basalts, as well as in situ oxygen isotope analyses and U-Th geochronology of zircons from three rhyolitic domes erupted around the caldera margins. Zircon U-Th geochronology for the rhyolite domes yields ages of 88.7 ± 9.9 ka for Jörundur, 83.3 ± 9.2 ka for Hlíðarfjall, and 85.5 ± 9.4 ka for Gæsafjallarani, some 20–30 ka after the eruption of the zoned rhyolite to basalt Halarauður ignimbrite during a major collapse of the Krafla caldera. We suggest that the domes represent a renewed episode of silicic magma production in the pre-heated crust. Oxygen isotope analyses of single and bulk plagioclase and pyroxene identify some instances of isotopic disequilibrium with groundmass (~3.5‰) reflecting assimilation of diverse low δ¹⁸O crustal material. However, zircon is largely in equilibrium with groundmass analyses, suggesting it crystallized directly from low δ¹⁸O magma. Zircon trace elements (Hf, Yb, Th, U) for all three domes show trends indicative of fractional crystallization. Pairing these observations with two-dimensional thermal modeling using the Heat2D model, and chemical modeling using the Magma Chamber Simulator, we suggest that petrogenesis of rhyolitic magma at Krafla requires at least two-steps: the δ¹⁸O of basaltic parental magmas are first lowered through assimilation of hydrothermally altered material (generated in the high temperature region in the crust surrounding the magma chamber) to produce low δ¹⁸O mafic to intermediate magmas, which then ascend from magma generation zones into colder crust where they undergo further fractional crystallization at shallower depths. Our models suggest that prior hydrothermal alteration of the mafic crust greatly increases the volume of partial melt that can be produced and assimilated, and we thus suggest that long-lived hydrothermal systems may play an important role in further encouraging the production of larger volumes of rhyolitic magmas in basalt-dominated environments.

了解硅质岩浆的成岩作用对于理解它们构成的火山灾害，地热能潜力以及形成大陆壳至关重要。在这项研究中，我们探索了冰岛克拉夫拉中央火山的玄武质地壳中流纹岩浆的起源。我们提供了来自八种流纹岩和六种选定玄武岩的斜长石，辉石和地层的激光氟化氧同位素分析，以及来自在火山口边缘喷出的三个流纹穹顶的原位氧同位素分析和锆石的U-Th地质年代学。流纹岩穹顶的锆石U-Th年代学得出Jörundur的年龄为88.7±9.9 ka，Hlíðarfjall的年龄为83.3±9.2 ka，Gæsafjallarani的年龄为85.5±9.4 ka，在克拉夫拉火山口的一次大崩塌期间，将带状流纹岩喷发到玄武岩哈拉劳德乌尔火成岩后约20-30 ka。我们建议穹顶代表预热地壳中硅质岩浆生产的新事件。对单斜长石和斜长石和辉石的氧同位素分析确定了一些同位素不平衡的情况，其地基质量（〜3.5‰）反映了不同低δ的同化作用¹⁸ O地壳材料。然而，锆石是大致符合groundmass分析平衡，这表明它从低δ直接结晶¹⁸ ö岩浆。所有三个圆顶的锆石微量元素（Hf，Yb，Th，U）均显示出指示部分结晶的趋势。配对这些观察与使用Heat2D模型使用岩浆模拟器化学建模二维热建模，并且，我们建议在卡拉夫拉流纹岩浆的那成因需要至少两个步骤：δ ¹⁸玄武母岩浆的O的第一降低通过水热改变材料（在周围的岩浆室地壳中的高温区域中产生的）的同化来产生低δ ¹⁸镁铁质至中型岩浆，然后从岩浆生成区上升到较冷的地壳，在那里它们在较浅的深度进行进一步的分步结晶。我们的模型表明，铁镁壳的先期水热蚀变极大地增加了可产生和吸收的部分熔体的体积，因此，我们认为长寿命的热液体系可能在进一步鼓励生产更大数量的流纹岩中起重要作用。玄武岩为主的环境中的岩浆。