SGER: Detailed Lava Surface Age Map and Pre-eruptive Magma Aging for the 2005-6 Volcanic Eruption at 9 46'-9 56'N EPR

在美国国家科学基金会资助的东太平洋隆起9N综合研究基地的快速反应考察期间所做的观测证实，在2006年5月和6月的考察前不久发生了一次或多次火山爆发。这是一个理想的地点，可以尝试从深海喷发中绘制出第一张详细的熔岩表面年龄图。收集的快速响应工作包括NSF资金，用于启动使用210Po-210Pb放射性不平衡的熔岩测年研究。快速衰变时间尺度（半衰期=138天）允许非常高的熔岩年龄的时间分辨率，如果工作是在疑似喷发后不久开始的。等待开始分析的时间越长，年龄分辨率就会降低，因为自然的、火山喷发引起的放射性特征会在大约2年内衰减。目前的项目将对现在被认为是多阶段喷发的火山进行大规模的测年研究，在此期间，复合熔岩流被放置在约18公里的山脊轴和周围的近轴地形上。这项工作提供的高分辨率熔岩流年龄几乎是之前五项研究中的任何一项的四倍（几乎是所有这些研究总和的两倍）。该项目验证了这样一种假设，即火山喷发是分阶段发生的，伴随着热量和物质流过地壳的影响，这将反映在熔岩流场中地理子域的可识别的年龄差异上。此外，该地区2005-2006年的喷发与1991- 1992年喷发的相似之处表明，其中涉及的岩浆大部分相同，我们可以在这里进行测试。a) 10个alvin潜水器采集的、空间分布均匀的熔岩样品的喷发日期为210Po-210Pb。与先前SGER资助的6个样本一起，这些数据将揭示喷发序列的细节，直接为多个研究团队正在进行的熔岩就位和当地热液作用和生态反应的研究提供信息。高密度熔岩流表面年龄图的变革潜力促使PI在收集后（即2006年夏秋期间）尽快开始对所有样本进行年代分析，以获得尽可能高的年龄分辨率，并希望获得额外的研究资金。b)对10-11个样品的喷发前岩浆年龄进行210Pb-226Ra放射性不平衡定年，以确定喷发前亚脊轴向岩浆储层中新岩浆和1991-92年岩浆的相对比例。该方法还需要在熔岩中进行230 - 238u - 232th分析。这项研究的这一部分风险更高，因为它建议以一种新的方式使用在洋中脊玄武岩中新发现的放射性特征，这可能是由于该地点唯一记录的重复火山活动。结合其他合作研究人员正在进行的熔岩体积估计和熔岩成分研究，我们希望在这次喷发之前估计岩浆和热通量进入熔体库。本研究解决了RIDGE2000计划的关键时间研究和EPR综合研究站点（ISS）组件的主要目标，并为研究ISS的地质学、地球物理学和地震学、生物学和水文学的其他研究人员提供了重要的综合数据；它还包括博士后培训。最后，PI参与了一个教育和推广项目，与R2K办公室的工作人员合作，为他们的网站提供海底喷发取证和相关故事，作为该项目的一部分。

Abstract Observations made during NSF-funded rapid response expeditions to the East Pacific Rise 9N Integrated Studies Site of the Ridge2000 Program confirmed that one or more volcanic eruptions had taken place shortly before the expeditions of May and June, 2006. This is an ideal locale to attempt the first ever detailed lava surface age map from a deep submarine eruption. The collected rapid response effort included NSF funds to initiate a lava dating study using 210Po-210Pb radioactive disequilibrium. The rapid decay timescale (half life=138 days) allows for very high temporal resolution of lava ages if work is begun soon after a suspected eruption. The age resolution is diminished the longer one waits to initiate analyses since the natural, eruption-induced radioactive signature decays away in ~2 years. The present project will enable a large-scale dating study of what is now thought to be a multi-phase eruption during which a compound lava flow was emplaced over ~18 km of ridge axis and surrounding near-axis terrain. The work will provide nearly four times as many high resolution lava flow ages as any of five prior studies (and nearly twice as many as all of those studies combined). The project tests the hypothesis that this eruption occurred in stages, with attendant affects for heat and material flow through the crust, and that this will be reflected in discernable age difference in geographic sub-domains within the lava flow field. Furthermore, the similarity of 2005-2006 eruptives to those erupted in 1991-92 in this area suggests that much of the same magma was involved, which we can test for here. The dating has 2 components:a) 210Po-210Pb eruption dates on 10 Alvin-submersible-collected, spatially well-distributed lava samples. Along with the 6 samples funded with the prior SGER, this data will reveal details of the eruption sequence, feeding directly into on-going studies of lava emplacement and responses in local hydrothermalism and ecology by multiple research teams. The transformative potential of a high density lava flow surface age map caused the PI to begin dating analyses on all of the samples as soon as possible after collection (i.e., during the summer-fall of 2006), to obtain the highest possible age resolution with the hope that additional research funds would be forthcoming.b) 210Pb-226Ra radioactive disequilibrium dating of pre-eruptive magma age on 10-11 samples to determine the relative proportion of new and 1991-92 magma in the sub-ridge axial magma reservoir just prior to this eruption. This method also requires 230Th-238U-232Th analyses in the lavas. This component of the research is higher-risk as it proposes to use a newly discovered radioactive signature in mid-ocean ridge basalts in a new fashion, made possible by the uniquely documented repeat volcanism at this site. Coupled with underway lava volume estimate and lava composition studies by other collaborating investigators, we hope to estimate magma and heat fluxes into the melt reservoir before this eruption.Broader impacts This research addresses main goals of the of the Time Critical Studies and EPR Integrated Studies Site (ISS) components of the RIDGE2000 program and provides important, integrative data to other researchers studying the geology, geophysics and seismology, biology, and hydrology of the ISS; it also involves Postdoc training. Finally, the PI is involved in an Education and Outreach project in collaboration with the R2K office staff on submarine eruption forensics and related stories for their website as part of this project.