Timescales of degassing and crystallization implied by (super 210) Po- (super 210) Pb- (super 226) Ra disequilibria for andesitic lavas erupted from Arenal Volcano; Arenal Volcano, Costa Rica; magma genesis and volcanological processes
Journal of Volcanology and Geothermal Research
DOI of Published Version
Disequilibrium between (super 210) Po, (super 210) Pb, and (super 226) Ra was measured on whole rocks and plagioclase mineral separates erupted between 1968 and 2003 from Arenal volcano with a goal of monitoring the volatile fluxing and crystallization in the decades and years leading up to eruption. Degassing during the eruption was found to remove nearly all (super 210) Po from Arenal lavas, which appears to be true of lava eruptions in general. Most of Arenal's lavas have ( (super 210) Pb)/( (super 226) Ra) ratios within 20% of equilibrium, indicating that most of the magmas involved in this eruption did not have strong, persistent fluxes of (super 222) Rn in or out of the system during the decades leading to eruption. This is consistent with a time-frame of differentiation from basalt to basaltic andesite exceeding a century. Lava erupted in 1971 had ( (super 210) Pb) in excess of ( (super 226) Ra) by as much as a factor of 2. These lavas were the first to mark the change in geochemical trends that were likely caused by the arrival of a new magma at the surface at Arenal [Ryder, C.H., Gill, J.B., Tepley III, F., Ramos, F., Reagan, M., this issue. Closed to open system differentiation at Arenal Volcano (1968-2003). Journal of Volcanology and Geothermal Research.], suggesting that the (super 210) Pb excess was related to the first appearance of this magma. The high ( (super 210) Pb)/( (super 226) Ra) ratio in this lava apparently reflects Rn-degassing from large volumes of underlying magma and/or extraction of Rn from conduit-area rocks or fluids due to deformation and heating. Plagioclase mineral separates had (super 210) Po- (super 210) Pb- (super 226) Ra disequilibrium patterns suggesting a growth period stretching over a period of more than 50 years up to the time of eruption.
Published Article/Book Citation
Journal of Volcanology and Geothermal Research, 157:1-3 (2006) pp.135-146.