Explosive Ocean Island Volcanism Explained by High Magmatic Water Content Determined Through Nominally Anhydrous Minerals

Abstract Ocean island basalt (OIB) magmas are generally water poor and usually contain less than 1 wt.% of H 2 O. Explosive eruption styles are therefore rare. When explosive eruptions occur, they are thought to be driven by either volatile‐enriched mantle sources or by gas segregation processes during magma differentiation. Here we report on crystal‐ and water‐rich porphyritic basanites and ankaramites from El Hierro in the Canary Islands, Spain, that erupted inside the El Golfo giant landslide collapse embayment that formed at ≥39 ka. Using rock and mineral chemistry in combination with H 2 O contents of nominally anhydrous minerals (olivine and clinopyroxene), we show that despite their relatively primitive composition, the post‐collapse ankaramites are not primary mantle melts. Instead, they record high crystal contents as well as unusually high water contents of up to 3.20 ± 0.64 wt.% H 2 O, and likely represent a normally inaccessible snapshot of dense crystal‐rich magma compositions that reside in the sub‐island underplating zone. We hypothesize that their eruption was facilitated by sudden decompression from crustal unloading, implying that the El Golfo landslide may have affected the deeper portions of the plumbing system and triggered the ascent of volatile‐rich, crystal‐laden magmas from the underplating zone. We propose that some “wet” and explosive ocean island eruptions might result from the ascent of deep‐seated water‐rich magmas in the aftermath of vertical unloading and associated decompression.