Cryptic lower crustal signature in the source of the Ontong Java Plateau revealed by Os and Hf isotopes

Abstract

Previous studies of Ontong Java Plateau (OJP) basalts reveal that they are products of 20–30% degree of melting and possess ocean-island-like Pb, Nd, and Sr isotopic signatures that fall within a limited range yet define two distinct isotopic groups represented by the Kroenke- and Kwaimbaita-type basalts and by the Singgalo-type basalts. The origin of the two groups is not clear, although the Kroenke–Kwaimbaita-type signature has been suggested to represent the plateauʼs main mantle source, probably originating from the lower mantle. In this study, samples from central Malaita, Solomon Islands and Ocean Drilling Program (ODP) Sites 807, 1185B, and 1187 were analyzed for Os and Hf isotopic composition to further investigate the origin of the bimodal isotopic composition of OJP basalts. The Kroenke-type basalts, which are parental to Kwaimbaita-type basalts, show the least effect of post-emplacement alteration on their Os isotopic composition and have εHf(t)=+10.8 to +13.3, Os = 118–174 ppt, Re = 161–1111 ppt, and near-chondritic (Os187/Os188)t=0.1322±0.0029 [γOs(t)=2.7±2.2 and 4.3±2.3 (n=6; 1σ) relative to primitive upper mantle (PUM) and average chondrite values, respectively], suggesting a near-primitive mantle source for the OJP. Up to 25% assimilation of altered Jurassic Pacific MORB crust into fractionating Kroenke-type magma could explain the lower, 11–51 ppt, Os contents and more radiogenic, (Os187/Os188)t=0.1395±0.0020 [γOs(t)=8.4±1.5 and 10.0±1.5 (n=3; 1σ)], composition of some Kwaimbaita-type basalts. These compositions are markedly different from those of the least altered Singgalo-type basalts with εHf(t)=+10.2 to +11.5 and (Os187/Os188)t=0.3301±0.0175 [γOs(t)=156±14 and 160±14 (n=8, 1σ)] and low, 21–37 ppt, Os contents that are lower oceanic crust-like. Despite the scatter in the Re–Os isotope data, meaningful isochron ages and initial values were obtained. These ages are 121.4±4.6 Ma for Malaitan Kwaimbaita Fm. basalts and 123±24 Ma for Singgalo-type basalts, with initial 187Os/188Os of 0.129±0.025 and 0.325±0.098, respectively. The Re–Os ages agree very well with the average ∼122 Ma 40Ar–39Ar age for the OJP. The Re–Os and Lu–Hf results reinforce the interpretation of a two-component mantle source for the OJP consisting of a dominant near-primitive mantle similar to that involved in the sources of several ocean islands, represented by the Kroenke- and Kwaimbaita-type basalts, and old, recycled lower continental crustal material expressed subtly in the isotopic signature of the Singgalo-type basalts. A lower crustal influence may be explained by incorporation of ancient, delaminated mafic continental crust into a thermochemical plume; incorporation could provisionally involve dense ex-crustal material that had accumulated in the lower mantle or less dense material that had stagnated at mid-mantle depths.