Timescales of magma degassing — Insights from U-series disequilibria, Mount Cameroon, West Africa

Abstract Short-lived uranium-series isotope data from rocks erupted under explosive and effusive regimes are presented and used to provide constraints on the timescales of magmatic degassing and volatile fluxing during the 1999 and 2000 AD eruption of Mt. Cameroon. In contrast to the relatively homogenous major and trace element data of the analysed rocks, volcanic rocks from Mt. Cameroon reveal a spread of 230 Th– 226 Ra isotope data. Volcanic rocks erupted along the southwest rift in 1999 have ( 226 Ra/ 230 Th) ratios of ~ 1.25, whereas rocks erupted more axially in 2000 have relatively low ( 226 Ra/ 230 Th) ratios of 1.09–1.2 and concomitant low Ba/Th ratios. These differences imply separate magma chamber systems and probably reflect differences in the concentrations of water within the primary magmas, which led to different amounts of amphibole fractionation at depth. Variations in the ( 210 Pb/ 226 Ra) 0 ratios are used to track degassing or volatile accumulation within the magma system. The near equilibrium ( 210 Pb/ 226 Ra) values for effusively erupted rocks from Mt. Cameroon suggest that this magma resided for more than several decades and less than a few thousand years before it erupted. The small excesses of 210 Pb over 226 Ra in some samples indicate that some of this magma was fluxed by a Rn-bearing gaseous phase for weeks to years before the eruption. In contrast, most explosively erupted rocks from Mt. Cameroon have deficits of 210 Pb relative to 226 Ra that require years to decades of degassing before eruption. We suggest that the magmas erupted as scoria at Mt. Cameroon degassed as they rose from lithospheric depths. Deep degassing was CO 2 -dominated, whereas shallower magma degassing involved more water and crystallisation of an anhydrous mineral assemblage.