The use of trace element abundances from Loihi Seamount to investigate the chemical heterogeneity of the Hawaiian mantle plume
May 8th, 2013, 10:20am
The hypothesis that oceanic crust enters the mantle at subduction zones and is subsequently recycled as it returns to the surface through plumes plays an important role in the understanding of the chemical composition of Earth’s mantle. This recycled material resurfaces at hotspots such as Iceland, Samoa, and the Hawaiian Islands. Observational data has revealed compositional variations in the lavas of several Hawaiian volcanoes that may be related to this recycling hypothesis. These differences form two geographic trends, the Kea trend and the Loa trend, which each have a distinct isotopic signature. Kea-type lavas (including Kilauea) exhibit characteristics of a long-term depleted source, while Loa-type lavas (including Mauna Loa) have features typically seen in long-term enriched sources. Various models have been proposed to explain these trends, including a large-scale bilateral asymmetry or radial zonation in the composition of the Hawaiian plume. Loihi Seamount is an active volcano located 35 km south of the island of Hawaii. It displays a wide range of rock types from tholeiites to basanitoids and offers much insight into the chemical heterogeneity of the mantle. Geographically, Loihi is situated on the Loa trend and therefore is expected to erupt Loa-type lavas, according to the bilateral asymmetry or radial zonation models. However, previous research shows that, although Loihi lavas exhibit Loa-like Pb isotope values, incompatible trace element ratios from this volcano plot near Kilauea (Kea-type) values. There is very little high-quality trace element abundance data available for Loihi lavas. In order to further investigate these observations, we obtained LA-ICP-MS trace element abundances from 16 fresh glass samples from Loihi. Here we compare these incompatible trace element abundances with literature data from three active Hawaiian volcanoes (Kilauea, Mauna Loa, and Loihi) and one extinct volcano (Koolau). These data provide further evidence that these compositional trends cannot be explained by either bilateral asymmetry or radial zonation. The findings of this study instead offer support for a current model suggesting that the mantle source beneath Hawaiian volcanoes is derived from recycled oceanic crust that has been altered by seawater and variably dehydrated during subduction. The model also proposes that the plume consists of small-scale heterogeneities, oriented NW-SE, that differ in the amount, type, and extent of dehydration of the recycled crust.