The Petrology and Geochemistry of St. Helena Alkali Basalts: Evaluation of the Oceanic Crust-recycling Model for HIMU OIB

Abstract

We present major element, trace element, and petrographic data on alkali basalts from St. Helena, and examine the geochemical characteristics of a recycled component involved in the source of HIMU (Pb/ Pb420·5) ocean island basalts. Petrographic and compositional variations in the St. Helena basalts are best explained by the combined effect of fractional crystallization and accumulation of phenocrysts. Primary melt compositions are estimated by correcting for the effects of crystal^liquid differentiation by reconstructing the order of crystallization and the relative amount of fractionated phases.This calculation indicates that the St. Helena alkali basalts are derived from a common primary magma with 14^20 wt % MgO. Simple partial melting of fertile mantle peridotite, depleted mid-ocean ridge basalt (MORB)-source mantle, or garnet pyroxenite fails to produce the St. Helena primary melt. Instead, this primary melt can be reproduced if there are contributions from ancient recycled oceanic crust and depleted peridotite [(Rb/ Nb)PM1⁄4 0·38^0·80]. Subducted sediment can be excluded to explain the low (Rb, Ba, U)/Nb and Ce/Pb of St. Helena basalts. Geochemical modeling using major and trace element abundances, together with Sr, Nd, Pb, and Hf isotope ratios, indicates that the St. Helena primary melt can be formed by 1^2% melting of a peridotitic source that was refertilized by a small amount (8^18%) of melt derived from recycled oceanic crust. This source has a similar trace element pattern to modern normal (N)-MORB, but element abundances are 0·1^0·2 times N-MORB values. The calculated recycled crust has a wide range of present-day Pb isotopic ratios (Pb/ Pb of 21·7^79·3 and Pb/ Pb of 40·8^89·3), Sr/ Sr of 0·7018^0·7028, Nd/Nd of 0·51274^0·51285, and Hf/Hf of 0·28262^0·28293 after a residence time of 1·2^2·8 Gyr. Rb, Ba, Pb, Sr, and light rare earth element abundances in the recycled crust are depleted compared with modern N-MORB, whereasTh, U, Sm, and Nd abundances fall within the range of compositional variations in modern N-MORB. The trace element compositions of the recycled oceanic crust can be explained by element behavior during seafloor alteration and subduction zone dehydration of oceanic crust. Therefore, recycling of ancient subducted oceanic crust is a potential process for producing the St. Helena HIMU basalts.