Insights into shallow magma storage and crystallization at Volcán Llaima (Andean Southern Volcanic Zone, Chile)

Abstract

Scoriae produced by four mafic historic eruptions of Volcán Llaima are used to elucidate magma differentiation, recharge, mixing, and eruption triggering. Whole-rock, mineral, and olivine-hosted melt inclusion chemistry suggest that basaltic andesites (~ 52–53.5 wt.% SiO 2) stall at immediately sub-edifice depths (≤ 4 km beneath the base of the volcano), where they crystallize to large extents and form mush bodies. Melt inclusion trends overlap with the whole-rock data trends defined by the entire volcano up to 53–55% SiO 2, but more evolved compositions define divergent trends for some elements, with up to 2.2 wt.% TiO 2 (increasing Zr) and Al 2O 3 as low as 12.5 wt.% (decreasing Sr) at 57–58 wt.% SiO 2. These more evolved melt compositions are inferred to be the result of shallow evolution of interstitial melt during the formation of crystal mush bodies, as a consequence of degassing and crystallization with a reduced participation of Fe–Ti-oxides. The inferred suppression of Fe–Ti-oxide stability and the modal dominance of plagioclase crystallization are consistent with low fluid-saturation pressures of ~ 40–70 MPa inferred from average H 2O contents for mafic arc magmas (dominantly 1–4 wt.%) but low CO 2 contents (dominantly 0–300 ppm) in melt inclusions. The broad range in olivine core compositions (Fo 69–83) and the absence of correlated degassing and magma evolution trends in historic Llaima magmas suggest that they are stored as multiple dike-like bodies created by a high frequency of magma replenishment relative to the frequency of large eruptions. This temporal–spatial disconnection leads to isolated evolution and degassing of discrete magma batches, followed by remobilization and assembly just prior to eruption. Eruptions are probably triggered by recharge of relatively hot, mafic, and much less degassed magma, in accord with dominantly reversely-zoned olivine crystals, and higher olivine-melt temperatures recorded by the relatively primitive matrix glasses and coexisting olivine rims.