Major element chemistry of ocean island basalts — Conditions of mantle melting and heterogeneity of mantle source

Abstract

We estimate average compositions of near-primary, ‘reference’ ocean island basalts (OIBs) for 120 volcanic centers from 31 major island groups and constrain the depth of lithosphere–asthenosphere boundary (LAB) at the time of volcanism and the possible depth of melt–mantle equilibration based on recently calibrated melt silica activity barometer. The LAB depth versus fractionation corrected OIB compositions (lava compositions, X, corrected to Mg# 73, X OIB #73, i.e., magmas in equilibrium with Fo 90, if olivine is present in the mantle source) show an increased major element compositional variability with increasing LAB depths. OIBs erupted on lithospheres < 40 km thick approach the compositions (e.g. SiO 2 #73, TiO 2 #73, [CaO/Al 2O 3] #73) of primitive ridge basalts and are influenced strongly by depth and extent of shallow melting. However, X OIB #73 on thicker lithospheres cannot be explained by melt–mantle equilibration as shallow as LAB. Melt generation from a somewhat deeper (up to 50 km deeper than the LAB) peridotite source can explain the OIB major element chemistry on lithospheres ≤ 70 km. However, deeper melting of volatile-free, fertile peridotite is not sufficient to explain the end member primary OIBs on ≥ 70 km thick lithospheres. Comparison between X OIB #73 and experimental partial melts of fertile peridotite indicates that at least two additional melt components need to be derived from OIB source regions. The first component, similar to that identified in HIMU lavas, is characterized by low SiO 2 #73, Al 2O 3 #73, [Na 2O/TiO 2] #73, and high FeO⁎ #73, CaO #73, [CaO/Al 2O 3] #73. The second component, similar to that found in Hawaiian Koolau lavas, is characterized by high SiO 2 #73, moderately high FeO⁎ #73, and low CaO #73 and Al 2O 3 #73. These two components are not evenly sampled by all the islands, suggesting a heterogeneous distribution of mantle components that generate them. We suggest that carbonated eclogite and volatile-free, silica-excess eclogite are the two most likely candidates, which in conjunction with fertile mantle peridotite, give rise to the two primitive OIB end members.