Collaborative Research: A 210Pb Paradox?

岩浆脱气作用是火山爆发的主要驱动力。因此，为了评估火山爆发造成的危害程度，重要的是要了解不同成分岩浆的脱气速率、总体程度和机制。作为该项目的一部分，该团队将通过测量在这些环境中喷发的熔岩中的（210 Pb/226 Ra）活性比来研究俯冲带和板块内部岩浆脱气的速率和程度。这个比例对于这样的研究是理想的，因为熔岩中（210 Pb）相对于（226 Ra）的不足可能是由于喷发前岩浆脱气导致的222 Rn的持续损失，而（210 Pb）过量可能是由于气体通过岩浆流动导致的222 Rn的持续过量。210 Pb的半衰期短（T1/2 = 22.6.最近的观测表明，与俯冲作用有关的火山喷发的熔岩具有趋于平衡（1.0）的（210 Pb/226 Ra）值，而洋岛玄武岩（OIB），如夏威夷火山喷发的玄武岩，往往具有210 Pb亏损。这似乎是自相矛盾的，因为它是低挥发分含量的OIB熔岩似乎持续多年失去222 Rn，而弧熔岩，失去更多的气体，似乎很少这样做超过一年或两年前爆发。在拟议的研究中，该团队将确定这种差异的根本原因是否确实与脱气有关，或者与硫化物分馏等其他过程有关。计划通过将（210 Pb）/（226 Ra）值的变化与铂族元素Os、Ir、Pt和Pd以及Pb、Cu、Zn、Ni、Re和其他微量元素的浓度变化进行比较来完成这一工作。例如，如果（210 Pb/226 Ra）值的变化与OIB和裂谷相关熔岩中硫化物分馏的测量一致（例如Ir/Pt和Pb/Ba正相关），则有可能将这些变化归因于硫化物分馏，最有可能是在熔融期间，而不是脱气。这意味着大多数OIB和俯冲相关的岩浆从其来源到地表的过渡时间非常快（几十年）。与此相反，（210 Pb/226 Ra）和岩浆中的脱气措施（如Re/Os和Os/Ir值）之间的协变，将支持（210 Pb/226 Ra）值是由持续的222 Rn脱气控制在几年和几十年导致喷发的假设。这一结果意味着与俯冲相关的火山相比，洋岛玄武岩的最终脱气时间更长。

Magma degassing is the principal driving force for volcanic explosions. Thus, to evaluate the extent of the hazards posed by explosive volcanic eruptions, it is important to understand the rates, overall extents, and mechanisms of degassing for magmas with differing compositions. As part of this project, the team will investigate the rates and extents of magma degassing for subduction zones and plate interiors by measuring (210Pb/226Ra) activity ratios in lavas erupted in these settings. This ratio is ideal for such a study because deficits in (210Pb) with respect to (226Ra) in lavas can result from persistent losses of 222Rn as magmas degas before eruption, whereas (210Pb) excesses can result from persistent excesses of 222Rn due to streaming of gasses through magmas. The short half-life of 210Pb (T1/2 = 22.6. y) will facilitate measurement of the crucial period of the final century before an eruption.Recent observations suggest that lavas erupted from volcanoes associated with subduction have (210Pb/226Ra) values that tend towards equilibrium (1.0), whereas ocean island basalts (OIB), like those erupting from Hawaiian volcanoes tend to have 210Pb deficits. This appears paradoxical, as it is the low-volatile-content OIB lavas that appear to persistently lose 222Rn for years, whereas arc lavas, which lose much more gas, appear to rarely do so for more than a year or two before eruption. In the proposed research, the team will determine whether the root cause of this difference is indeed related to degassing, or to other processes such as fractionation of sulfides. It is planned to do this by comparing variations in (210Pb)/(226Ra) values to variations in concentrations of Platinum-group elements Os, Ir, Pt, and Pd, as well as Pb, Cu, Zn, Ni, Re, and other trace elements. For example, if (210Pb/226Ra) values vary consistently with measures of sulfide fractionation in OIB and rift-related lavas (e.g. Ir/Pt and Pb/Ba positively correlate), then it is possible to attribute these variations to sulfide fractionation, most likely during melting, rather than to degassing. This would imply very fast transit times ( several decades) for most OIB and subduction related magmas from their sources to the surface. In contrast, covariations between (210Pb/226Ra) and measures of degassing in magmas (e.g. Re/Os and Os/Ir values), would support the hypothesis that (210Pb/226Ra) values are controlled by persistent 222Rn degassing during the years and decades leading to eruption. This result would imply longer time periods of final degassing for ocean island basalts compared to subduction-related volcanoes.