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Volcanic plumbing filters on ocean-island basalt geochemistry

Abstract:

Ocean-island basalts (OIBs) are considered to be messengers from the deep mantle, yet the filtering effect of the plumbing systems that bring OIB melts to the surface remainspoorly assessed. We investigated volcanic products from El Hierro island (Canary Islands)from textural and chemical perspectives. The majority of geochemical data cluster at relativelyfractionated basaltic compositions of 5 wt% MgO. Compositions ≥10 wt% MgO areporphyritic whole rocks that accumulate mafic minerals. Near-primary melts do not erupt.Instead, we show that carrier melts (crystal-free whole rocks, glasses, and melt inclusions) areconsistently buffered to low-MgO compositions during passage through the plumbing system.We tested our model of melt fractionation and crystal accumulation on a global compilationof OIBs. Similar to El Hierro, the majority of data cluster at evolved compositions of 5 wt%MgO (alkaline) to 7 wt% MgO (tholeiitic). Modeling the fractionation of OIB parental melts,we show that with 50% crystallization, OIB melts reach 5 wt% MgO with reduced density,increased volatile content, and overall low viscosity, becoming positively buoyant relative towall rocks and highly eruptible when reaching volatile saturation at depths around the crustmantleboundary. Under these conditions, 5 wt% MgO OIB “sweet spot” melts are propelledto the surface and erupt carrying an assortment of recycled crystals. This mechanism is consistentwith the petrography and chemistry of erupted products and suggests OIB volcanoesare dominated by low-MgO basaltic melts