Collaborative Research: A 210Pb Paradox?

岩浆脱气是火山爆发的主要驱动力。因此，要评估火山喷发造成的危害程度，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俯冲带和板块内部喷发的熔岩的(210Pb/226Ra)活度比来调查俯冲带和板块内部的岩浆脱气速率和程度。这一比率对于这样的研究是理想的，因为熔岩中(210Pb)相对于(226Ra)的亏损可能是由于喷发前岩浆退化而导致的222Rn的持续损失，而(210Pb)的过剩可能是由于气体流经岩浆而导致的222Rn的持续过剩。~(210)Pb的短半衰期(t1/2=22.6。Y)将有助于测量火山喷发前最后一个世纪的关键时期。最近的观察表明，与俯冲有关的火山喷发的熔岩(210Pb/226Ra)的值趋于平衡(1.0)，而洋岛玄武岩(OIB)，如夏威夷火山喷发的玄武岩，往往具有210Pb值。这似乎是自相矛盾的，因为低挥发性的OIB熔岩似乎多年来一直在流失222Rn，而失去更多气体的弧形熔岩似乎很少在喷发前超过一到两年的时间。在这项拟议的研究中，该团队将确定这种差异的根本原因是否确实与脱气有关，还是与硫化物分馏等其他过程有关。计划通过比较(210Pb)/(226Ra)值的变化与铂族元素Os、Ir、铂和Pd以及铅、铜、锌、镍、Re和其他微量元素的浓度变化来实现这一点。例如，如果(210Pb/226Ra)值与OIB和裂谷相关熔岩中硫化物分馏的量度一致(例如Ir/铂和Pb/BA正相关)，那么有可能将这些变化归因于硫化物分馏，最有可能是在熔融过程中，而不是脱气。这将意味着大多数大洋钻探和与俯冲有关的岩浆从其源头到地表的过渡时间非常快(几十年)。相反，(210Pb/226Ra)与岩浆脱气措施(例如Re/Os和Os/Ir值)之间的协变将支持(210Pb/226Ra)值受导致喷发的数年和数十年持续的222Rn脱气控制的假设。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.