The U-Series Evolution of Stromboli Volcano in the Last Century

The typical volcanic activity of Stromboli consists of small explosions erupting a highly porphyritic (hp) magma. In addition, every few months/years, a magma with low phenocryst (lp) content can also occur during more energetic eruptions. This magma is thought to bypass the shallow hp magma chamber and the associated crystal mush, and represents the most primitive liquid at Stromboli volcano. We integrate the U-series dataset of Bragagni et al. [1] with 228Ra/232Th measurements and additional U-Th disequilibrium data. The U-series dataset cover two periods of intense volcanic activity (1930-1955 AD and 1996-2007 AD) and one of more moderate activity (1965-1986 AD). 234U/238U is in secular equilibrium (with one exception), suggesting no U mobility during alteration or hydrothermal activity. Moreover, the (228Ra/232Th)≈1 attests the lack of significant Ra/Th fractionation in the last decades due to magmatic or surface processes. On the equiline diagram, the lp and hp magmas follow two distinct and sub parallel trends, with younger products having lower 238U/232Th and 230Th/232Th. At a given time, hp magmas have 238U/232Th higher than lp magmas due to the assimilation of ancient material (crystals and especially melts) within the hp magna chamber [1]. The persistence of such compositional gap (with one exception) reflects the steady state of the shallow plumbing system during the last century. In the most recent period (1996-2007 AD), the constant 238U/232Th within lp and hp magmas, made it possible to investigate the variation in 230Th/232Th in terms of melting rate (lp magmas) [2] and shallow residence times (hp magmas) [1]. In contrast, the previous two volcanic periods show variable 238U/232Th. This suggests changes in the primary magma, which likely reflect different mantle sources as also confirmed by 87Sr/88Sr data [3].