Oceanic Volcanism from the Low-velocity Zone - without Mantle Plumes

• Published in: Journal of Petrology
• Main Authors: Presnall, Dean C., Gudfinnsson, Gudmundur H.
• Format: Text
• Language: English
• Published: Oxford University Press 2011
• Subjects: Original Papers; Pacific; Iceland
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/52/7-8/1533; https://doi.org/10.1093/petrology/egq093

We develop a model for oceanic volcanism that involves fracturing of the seismic lithosphere to access melts from the partly melted seismic low-velocity zone. Data on global seismic shear-wave velocities are combined with major-element compositions of global mid-ocean ridge basalt glasses, Hawaiian basalt glasses, and phase relations in the CaO-MgO-Al 2 O 3 -SiO 2 -CO 2 and CaO-MgO-Al 2 O 3 -SiO 2 -Na 2 O-FeO systems at pressures from 1 atm to 6 GPa. We use these data to constrain the pressure–temperature conditions for melt extraction at Hawaii and mid-ocean ridges (including Iceland), and to evaluate the existence of hot mantle plumes. In the low-velocity zone, the maximum reduction and maximum anisotropy of seismic shear-wave velocity (maximum melt fraction) occurs at a depth of ∼140–150 km for crustal ages >∼50 Ma, and a depth of ∼65 km at the East Pacific Rise axis. Seismic data indicate a smooth depth transition between these extremes. Experimental phase-equilibrium data, when combined with natural glass compositions, show that pressure–temperature conditions of tholeiitic melt extraction at Hawaii (∼4–5 GPa, 1450–1500°C) and the global oceanic ridge system (∼1·2–1·5 GPa, 1250–1280°C) are an excellent match for the two depth ranges of maximum melting indicated by seismic shear-wave data. At Hawaii, magmas escape to the surface along a fracture system that extends through the lithosphere into the low-velocity zone. This allows eruption of progressively deeper melts from the low-velocity zone, which exist at equilibrium along a normal geotherm. No significant decompression melting occurs. This produces the characteristic sequence at each volcano of initial low-volume alkalic magmas, then voluminous tholeiitic magmas that show low-pressure olivine-controlled crystallization, and final low-volume alkalic magmas from extreme depths. At the East Pacific Rise, the more shallow depth of magma extraction is caused by a perturbed ridge geotherm that grazes the lherzolite solidus within the thermal boundary layer. ...