The isotopic origin of Lord Howe Island reveals secondary mantle plume twinning in the Tasman Sea

Thermochemical convective instabilities in the mantle, often referred to as mantle plumes, cause mantle melting that give rise to ocean island basalts (OIB) in intraplate settings. The array of radiogenic isotope signatures in global intraplate OIB indicate that although plumes are individual entities, they share enriched components that resemble various parts of subducted crust and a hypothetical mantle matrix termed ‘focal zone’ (FOZO). Each plume is expected to rise with an individual buoyancy flux, thereby producing variable volumes of melting with some OIB forming subaerial islands, whereas others produce submarine volcanoes. Here, we report the first radiogenic isotope data (Sr, Nd, Pb and Hf) for lavas of Lord Howe Island (LHI) in the Tasman Sea, the most prominent subaerial expression of the hypothesised Lord Howe mantle plume. Major element data are consistent with a moderate degree of partial melting, and heavy rare earth element depletion indicates melting occurred in the presence of garnet, consistent with other global plume lavas. Radiogenic SrNd isotopic data are similar to those defining the relatively primitive FOZO component with no clear enriched mantle affinity. The nearby Tasmantid Seamounts are also sourced from a mantle plume, and have similar SrNd character. However, combined Pb ratios of LHI lavas are inconsistent with a FOZO-type source. Instead, Pb isotopes overlap with typical enriched mantle 1 (EM1) lavas, trending slightly higher than the Northern Hemisphere Reference Line. Hafnium isotopes follow trends observed in lavas of the archetypal EM1 Pitcairn-Gambier islands. The combined element-isotope data indicate that LHI forms the most recent expression of a mantle plume track in the Tasman Sea.

Chemical Geology