Crystal scale anatomy of a dying supervolcano: an isotope and geochronology study of individual phenocrysts from voluminous rhyolites of the Yellowstone caldera
Crystal scale anatomy of a dying supervolcano: an isotope and geochronology study of individual phenocrysts from voluminous rhyolites of the Yellowstone caldera
复制标题
垂死超级火山的晶体尺度解剖：黄石火山口大量流纹岩中单个斑晶的同位素和地质年代学研究
DOI：
10.1007/s00410-012-0724-x
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发表时间：
2012
影响因子：
3.5
通讯作者：
A. Schmitt
中科院分区：
文献类型：
--
作者：
K. Watts;I. Bindeman;A. Schmitt
在黄石火山640年前的火山口崩塌之后，出现了规模巨大（约600立方千米）且持续时间较长的流纹岩喷发期（约520-75 ka）。这些喷发是否代表了一个垂死的循环，还是一个新的岩浆库的生长，仍然是一个重要的问题。利用离子探针测定的新锆石U-Th年龄和δ18O值，以及激光烧蚀测定的铅同位素比值，研究了大量破火山口后流纹岩的成因。最古老的后破火山口流纹岩，喷发于~520 ~ 470 ka之间，表现出极端的年龄和氧同位素不均匀性，需要从个别低δ 18o熔体中推导。从~260 ~ 75 ka，流纹岩喷发的锆石均一性逐渐增强，低δ 18o锆石均一值为2.7 ~ 2.8‰，在大多数最年轻喷发中与低δ 18o主体熔体处于平衡。新的铅同位素数据定义了破火山口后流纹岩和之前形成破火山口的凝灰岩的单独阵列，表明它们不是来自破火山口崩塌后留下的糊状熔岩溪凝灰岩基。相反，我们的新时代和同位素数据表明，破火山口后流纹岩是由各种破火山口内源岩重熔形成的，包括前熔岩河凝灰岩火山岩和深成岩以及早期喷发的熔岩河凝灰岩后流纹岩。从~260 ka开始的低δ 18o熔体的成批组合导致了渐次均质化，然后是分异和冷却，直到最后一次流纹岩喷发~75 ka，我们认为这一趋势是黄石火山口下垂死岩浆储层的特征。
A voluminous (>600 km3) and long-lived (~520–75 ka) phase of rhyolitic eruptions followed collapse of the Yellowstone caldera 640 ka. Whether these eruptions represent a dying cycle, or the growth of a new magma chamber, remains an important question. We use new U–Th zircon ages and δ18O values determined by ion microprobe, and sanidine Pb isotope ratios determined by laser ablation, to investigate the genesis of voluminous post-caldera rhyolites. The oldest post-caldera rhyolites, erupted between ~520 and 470 ka, exhibit extreme age and oxygen isotopic heterogeneity, requiring derivation from individual parcels of low-δ18O melts. We find a progressive increase in zircon homogeneity for rhyolite eruptions from ~260 to 75 ka, with homogeneous low-δ18O zircon values of 2.7–2.8‰ that are in equilibrium with low-δ18O host melts for the majority of the youngest eruptions. New sanidine Pb isotope data define separate arrays for post-caldera rhyolites and the caldera-forming tuffs that preceded them, indicating that they were not sourced from a mushy Lava Creek Tuff batholith that remained after caldera collapse. Rather, our new age and isotopic data indicate that the post-caldera rhyolites were generated by remelting of a variety of intracaldera source rocks, consisting of pre-Lava Creek Tuff volcanic and plutonic rocks and earlier erupted post-Lava Creek Tuff rhyolites. Batch assembly of low-δ18O melts starting at ~260 ka resulted in progressive homogenization, followed by differentiation and cooling up until the last rhyolite eruption ~75 ka, a trend that we interpret to be characteristic of a dying magma reservoir beneath the Yellowstone caldera.