Ultramafic Volcanism Associated With the El Golfo Giant Lateral Collapse, El Hierro, Canary Islands

It is firmly established that giant landslides play a key role in the evolution of ocean island volcanoes. On- and off-shore studies of e.g. Réunion Island, the Hawaiian, Canarian, and Cape Verdean archipelagoes confirm enormous landslide volumes up to thousands of km3 and runout distances frequently exceeding a hundred km [1]. Accelerated post-collapse volcanism has been witnessed at strato-volcanoes (e.g. Bezymianny, Mt. St. Helens), but may also be common for shield-volcanoes (Waianae, Oahu [2]; Teno, Tenerife [3]). The extent of this phenomenon and its consequences for the magmatic plumbing system of the volcanic edifice are poorly constrained. Between 15-134 ka, the El Golfo giant lateral collapse removed a 150-180 km3 sector of the volcanic edifice of El Hierro, the youngest and westernmost of the Canary Islands. Although no major volcanic edifice has been established since the collapse, post-El Golfo landslide volcanic vents concentrate at the base of the collapse scarp while additonal eruptions occurred along the triaxial rift system of the island [4]. Whilst pre- collapse volcanics include relatively differentiated rocks (up to trachytes, SiO2 ~55 wt%, MgO ~1.5 wt% [4]), a drastic change in the volcanic regime is reflected in the post-collapse eruptions of crystal-rich (up to 60 volume %, with megacrysts up to 3 cm in size) lava flows and pyroclastics (basanites, ankaramites, and picrites, SiO2 <45 wt%, MgO = 6-15 wt%). Chemical thermobarometry [5] using augite and olivine (Fo 71-81) rims, fused groundmass, and whole rock compositions of post-collapse samples indicates that crystallisation mostly occurred in the uppermost mantle at pressures and temperatures in the range of 7-13 kbar and 1150-1250°C, respectively. Most volcanics include several crystal populations, including complex normal and reverse zonation patterns. Narrow outer rims (last ~20 μm) of the augite and olivine crystals commonly show a sharp decrease in Mg# (from ~80 to ~70). We propose that the decompression of the magma plumbing system following the El Golfo giant lateral collapse may have triggered rapid ascent of melts stored at depth, causing mixing of multiple magma batches and the aggregation of their crystal populations. The last stage, involving steep normal zonation at the rims of many augite and olivine crystals may be attributed to a `sudden' change in the P-T conditions and/or the melt chemical composition. [1] McGuire 1996, Volcano instability on the Earth and other planets. [2] Presley et al. 1997, Bull Volcanol. [3] Walter & Schmincke 2002, Int J Earth Sci. [4] Carracedo et al. 2001, Est Geol. [5] Putirka et al. 2003, Am Min.