Ubride, and three co-authors, reported mid-2021 that 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 remains poorly assessed. The authors investigated volcanic products from El Hierro island (Canary Islands) from textural and chemical perspectives. The majority of geochemical data cluster at relatively fractionated basaltic compositions of 5 wt% MgO. Compositions ≥10 wt% MgO are porphyritic whole rocks that accumulate mafic minerals. Near-primary melts do not erupt. Instead, the authors showed that carrier melts (crystal-free whole rocks, glasses, and melt inclusions) are consistently buffered to low-MgO compositions during passage through the plumbing system.
The authors tested our model of melt fractionation and crystal accumulation on a global compilation of 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, they also showed that with 50% crystallization, OIB melts reach 5 wt% MgO with reduced density, increased volatile content, and overall low viscosity, becoming positively buoyant relative to wall rocks and highly eruptible when reaching volatile saturation at depths around the crust-mantle boundary. Under these conditions, 5 wt% MgO OIB “sweet spot” melts are propelled to the surface and erupt carrying an assortment of recycled crystals. This mechanism is consistent with the petrography and chemistry of erupted products and suggests OIB volcanoes are dominated by low-MgO basaltic melts.
The authors concluded that liquid compositions in OIB magma chambers are relatively evolved basaltic products of picritic parental melts. This occurs during transport through vertically extended feeder systems that control the growth and evolution of OIBs (Klügel et al., 2015). Basaltic liquids with low-MgO contents (5 wt% MgO, less than half the >10 wt% MgO traditionally considered parental to OIBs) may reach volatile saturation at the base of the
crust, increasing buoyancy and accelerating to the surface with a cargo of recycled crystals that obfuscate bulk-rock chemistries.
Petrographic analysis is required to discriminate between petrogenetic hypotheses. Antecryst accumulation obscures whole-rock chemical data, with aphyric samples and glasses representing the best proxies for liquid compositions with which to model melting, fractionation, assimilation, and mixing. Together, they found that magma filtering effects of the volcano plumbing systems were prevalent, which may have implications for understanding crustal growth and mantle evolution in terrestrial planets.
Managed & Built by I2M Consulting, LLC | © 2010-
