Collaborative Research: A 210Pb Paradox?

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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.