Magnesium isotopic variation of oceanic island basalts generated by partial melting and crustal recycling

Abstract

Ocean island basalts (OIBs) are geochemically diverse in radiogenic isotopes, a feature that is commonly ascribed to record the chemical heterogeneity of their deep-mantle source, where significant compositional variation relates to variable amounts of ancient recycled crustal material. Although Mg is a major constituent of the mantle, it is still unclear whether Mg isotopes of OIBs predominantly correspond to deep-mantle source heterogeneity or processes such as partial melting. Here, we present Mg isotopic and trace-element compositional data for OIBs from the Hawaii islands, the Louisville seamounts, and for altered oceanic crust samples from the South Pacific. The δ26Mg value range of these OIBs is −0.29±0.07‰ (2SD, n=17), which is a variation approximately twice as large as the known compositional variation of the peridotitic mantle (−0.23±0.04‰, 2SD). Moreover, alkaline basalt (−0.31±0.04‰, 2SD, n=12) is relatively enriched in light Mg isotopes compared to tholeiitic basalt (−0.24±0.02‰, 2SD, n=5). In contrast, altered oceanic crust analyzed in this study has heavier Mg isotopic composition (−0.18±0.08‰, 2SD, n=13) relative to the basalts and to the peridotitic mantle. An evaluation of our and published data shows that the δ26Mg values of most OIBs negatively correlate with melting-sensitive trace-element ratios, but that they are uncorrelated with source-sensitive elemental ratios. This implies that Mg isotopic variation in most OIBs is largely controlled by variable degrees of partial melting and not by source heterogeneity. Negative correlation between Nb/Zr (or La/Sm) versus δ26Mg suggests that altered oceanic crust with heavier Mg isotopic composition is a more suitable source candidate for common OIBs. However, for a given melting degree, Louisville basalts have lower δ26Mg values than other OIBs, suggesting a different source, e.g. a peridotitic mantle. Modeling calculations suggest that melting of both garnet pyroxenite (recycled altered oceanic crust) and garnet peridotite can generate melts with low-δ26Mg signature for low-degree partial melting. Therefore, if the degree of partial melting can be independently constrained for the generation of parental OIB magma, the Mg isotopic compositions of their source can be estimated to investigate the chemical heterogeneity of the deep mantle.