SGER: 210Po-210Pb Constraints on Time-Scales of Degassing and Crystallization for Young Lavas and Tephras

正在收集具有广泛成分的“零年龄”火山样品，以进行全面的U系列分析（即，~（238）U-~（230）Th-~（226）Ra-~（210）Pb-~（210）Po和（230）Th/（232）Th）。进行这些分析的主要目标是约束的时间尺度的结晶和脱气的岩浆具有广泛多样的组合物。寿命较长的核素分析将用于调查喷发前千年或更长时间尺度上发生的过程。该项目的最大努力将是加强火山弧熔岩中210 Pb，特别是210 Po的微薄数据库。因此，本研究主要集中在火山爆发前的最后世纪期间发生的岩浆过程。通过关注喷发前的时期，我们希望了解岩浆从深室系统向地表迁移的触发因素，以及上升岩浆中质量分馏过程的性质和时间。本研究分析的第一批样品来自圣海伦火山、特里斯坦-达库尼亚和伊豆岛弧。我们对2004年10月4日从圣海伦斯火山喷发的火山灰进行的210 Po初步分析表明，大部分喷出物含有在岩浆温度下挥发的210 Po，然后在导致喷发的一年中在浅火山系统中升华。在早期火山灰中识别幼年岩浆成分的可能性有待第二组分析，这也将提供所有这些样品的初始210 Pb值。将在加州大学圣克鲁斯分校和布里斯托大学对所有地点的样品进行238 U和232 Th衰变系列中的226 Ra和寿命更长的核素分析。对SGER的资助也将支持爱荷华州大学的本科生和研究生的培训。

Zero-aged" volcanic samples with a broad range of compositions are being gathered for comprehensive U-series analyses (i.e., 238U-230Th-226Ra-210Pb-210Po and (230Th)/(232Th)). The principal goals for conducting these analyses are to constrain the time scales of crystallization and degassing in magmas with widely diverse compositions. The longer-lived nuclide analyses will be used to investigate processes occurring on the millenium or longer time-scale before eruption. This greatest effort for this project will be directed toward enhancing the meager database for 210Pb and especially 210Po in volcanic arc lavas. Therefore, this study has a primary focus on the magmatic processes that occur during the last century before a volcanic eruption. By focusing on the period just before eruption, we hope to gain an understanding of the triggers of magma migration from deep chamber systems toward the surface, as well as the nature and timing of mass fractionation processes in the rising magma. The first samples to be analyzed for this study are from Mount St. Helens, Tristan da Cunha, and the Izu arc. Our preliminary analyses of the October 4, 2004 tephras from Mount St. Helens for 210Po indicate that most of the ejecta contains 210Po that was volatilized at magmatic temperatures, then sublimated in the shallow fumarolic system in the year leading up to the eruption. The potential for recognizing a juvenile magmatic component in this early tephra awaits a second set of analyses, which also will provide the initial 210Pb values for all of these samples. The samples from all locations will be analyzed for 226Ra and longer-lived nuclides in the 238U and 232Th decay series at UC Santa Cruz and the University of Bristol. Funding for this SGER will also support the training of undergraduates and a graduate student ath the University of Iowa.