Vanderbilt University, Earth & Environmental Sciences, Nashville, Tennessee, United States of America.
PLoS One. 2012;7(5):e37492. doi: 10.1371/journal.pone.0037492. Epub 2012 May 30.
Supereruptions violently transfer huge amounts (100 s-1000 s km(3)) of magma to the surface in a matter of days and testify to the existence of giant pools of magma at depth. The longevity of these giant magma bodies is of significant scientific and societal interest. Radiometric data on whole rocks, glasses, feldspar and zircon crystals have been used to suggest that the Bishop Tuff giant magma body, which erupted ~760,000 years ago and created the Long Valley caldera (California), was long-lived (>100,000 years) and evolved rather slowly. In this work, we present four lines of evidence to constrain the timescales of crystallization of the Bishop magma body: (1) quartz residence times based on diffusional relaxation of Ti profiles, (2) quartz residence times based on the kinetics of faceting of melt inclusions, (3) quartz and feldspar crystallization times derived using quartz+feldspar crystal size distributions, and (4) timescales of cooling and crystallization based on thermodynamic and heat flow modeling. All of our estimates suggest quartz crystallization on timescales of <10,000 years, more typically within 500-3,000 years before eruption. We conclude that large-volume, crystal-poor magma bodies are ephemeral features that, once established, evolve on millennial timescales. We also suggest that zircon crystals, rather than recording the timescales of crystallization of a large pool of crystal-poor magma, record the extended periods of time necessary for maturation of the crust and establishment of these giant magma bodies.
超级喷发在数天内将大量（100 到 1000 立方公里）的岩浆剧烈地输送到地表，并证明了深部存在巨大的岩浆池。这些巨型岩浆体的长寿具有重要的科学和社会意义。对全岩、玻璃、长石和锆石晶体的放射性测年数据表明，约 76 万年前喷发并形成长谷火山口（加利福尼亚州）的比肖夫凝灰岩巨型岩浆体寿命长（超过 10 万年），且演化相当缓慢。在这项工作中，我们提出了四条证据来限制比肖夫岩浆体的结晶时间尺度：（1）基于 Ti 分布扩散弛豫的石英居留时间，（2）基于熔体包裹体成核动力学的石英居留时间，（3）使用石英+长石晶体尺寸分布得出的石英和长石结晶时间，以及（4）基于热力学和热流模型的冷却和结晶时间尺度。我们所有的估计都表明，石英结晶的时间尺度小于 10000 年，更典型的是在喷发前 500-3000 年。我们得出的结论是，体积大、晶体少的岩浆体是短暂存在的特征，一旦形成，就会在千年时间尺度上演化。我们还认为，锆石晶体记录的不是贫晶体大岩浆池的结晶时间尺度，而是记录了地壳成熟和这些巨型岩浆体形成所需的长时间。
