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We present new evidence that shows magma mingling can be a key process during highly explosive eruptions. Using fractal analysis of the size distribution of trachybasaltic fragments found on the inner walls of bubbles in trachytic pumices, we show that the more mafic component underwent fracturing during quenching against the trachyte. We propose a new mechanism for how this magmatic interaction at depth triggered rapid heterogeneous bubble nucleation and growth and could have enhanced eruption explosivity. We argue that the data support a further, and hitherto unreported contribution of magma mingling to highly explosive eruptions. This has implications for hazard assessment for those volcanoes in which evidence of magma mingling exists.

The El Abrigo member of the Diego Hernández Formation (Tenerife, Canary Islands) represents the final (170 ka) and most voluminous eruption (>20 km3 DRE) of the last cycle of explosive activity of the Las Cañadas volcano. It is a dominantly phonolitic ignimbrite containing both mafic and banded pumices, suggesting that magma mixing played an important role in triggering the eruption and modulating eruptive dynamics. Here we use petrology, together with glass and mineral geochemistry of enclave-rich mafic scoriae, banded and phonolitic pumices from El Abrigo ignimbrite, to shed light on the pre-eruptive storage conditions and processes governing magma ascent and interaction dynamics and to provide a first-order assessment of the contribution of magma mixing and crystal mush melting to the dynamics of this eruptive event. The distribution of major elements in glasses is consistent with diffusive exchange between the interacting melts whereby Na transfers from the phonolite to the tephriphonolitic melt. However, V, Zr, Ba and Eu suggest a complex scenario in which an intruding tephritic to phonotephritic magma interacted with two distinct zones of a phonolitic magma chamber, one occupied by a crystal rich, low-Zr and high Ba phonolite, and the other by an evolved, crystal poor, high-Zr phonolite. These results, coupled with mineral-melt thermobarometry, allow us to reconstruct the Las Cañadas plumbing system at the end of the Diego Hernández cycle, and to evaluate the contribution of cumulate mush melting and magma mixing in as follows: (1) the parental tephritic magma was stored at or near the Moho (410-450 MPa) at 1050°C where it was periodically replenished by more primitive basanitic magma; (2) upon ascent, the tephrite intruded into a shallow and zoned phonolitic storage system, triggering the disruption of a crystal mush in its base, and (3) subsequently interacted with a crystal-poor zone within the reservoir. Energy balance evaluations suggest that relative mafic volume ratios ranged between 20 and 43 vol %, and the conservation of small-scale magma mingling structures and their geochemical distribution suggest that the mixing event took place very shortly before the eruption, on a timescale of hours.

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