Pre-Eruptive Conditions and Dynamics Recorded in Banded Pumices from the El Abrigo Caldera-Forming Eruption (Tenerife, Canary Islands)

Abstract

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.

Ba, Sr, and Rb feldspar/melt partitioning in recent eruptions from Teide-Pico Viejo volcanic complex, Tenerife: New insights into pre-eruptive processes

“…This is the first time that this type of magmatic process has been proposed in the T-PV active system. Also noteworthy is the presence of banded pumice in the Montaña Reventada eruption deposits, very similar to those described in El Abrigo and several other units of the Diego Hernández Formation (Edgar et al, 2002;Pittari, 2004;Edgar et al, 2007;Edgar et al, 2017;González-García et al, 2022). All of these characteristics are further evidence that the T-PV volcanic system is beginning to reveal its capacity for magma-magma interactions leading to highly explosive eruptions, similar to those of previous volcanic cycles of the Las Cañadas Edifice.…”

“…This eruption extruded less SiO 2 -undersaturated magma (trachyte rather phonolite), with low Zr, high Ba, and Sr, and a positive Eu anomaly. These are clear indicators for the partial melting of an alkali feldsparrich mush, as reported extensively in the literature (e.g., Sliwinski et al, 2015;Wolff, 2017;Astbury et al, 2018;Wolff et al, 2020;González-García et al, 2022). Figure 3 shows how the whole rock composition of Montaña Reventada deviates from phonolite towards alkali feldspar.…”

“…Mafic recharge leading to partial melting and remobilisation of a feldspar-rich crystal mush has been previously documented in Tenerife but only for Las Cañadas Edifice eruptions (Sliwinski et al, 2015;González-García et al, 2022). This is the first time that this type of magmatic process has been proposed in the T-PV active system.…”

Bubble-enhanced basanite–tephrite mixing in the early stages of the Cumbre Vieja 2021 eruption, La Palma, Canary Islands

“…Filament lengths are in the order of 150–300 µm and widths do not exceed 20 µm. Such features are reminiscent of magma mingling structures (i.e., advection or physical mixing), being dominated by stretching and folding of a primitive magma inside a more evolved one 48 , 49 , and strongly resemble those observed in banded pumices with notably larger scale and compositional contrast 9 . These structures are very often associated to bubbles, with filaments commonly radiating from them, but some of them are detached, likely due to sectioning effects (Fig.…”

“…A common feature among them is a multi-level plumbing system spanning from the deep mantle to shallow reservoirs in the island edifice [1][2][3][4][5][6] . In this architecture, interactions between magmas stored at different levels of the system are common and are most conspicuously recorded in bimodal mafic-felsic systems by e.g., the occurrence of banded pumices [7][8][9] . In basaltic systems, the evidence of magma mixing is commonly cryptic due to the compositional similarity of end-members, high temperature, and low viscosity contrast, making them more susceptible to quick homogenization 10 .…”