PETROGENESIS OF HIGH-CAO LAVAS FROM MAUNA KEA, HAWAII: CONSTRAINTS FROM TRACE ELEMENT ABUNDANCES

Mauna Kea volcano at Hawaii is one of the best studied intra-plate oceanic volcanoes, and it has been drilled by the Hawaii Scientific Drilling Project (HSDP), a 1-km pilot hole (HSDP-1) drilled in 1996 followed by a 3-km hole (HSDP-2) drilled in 1999 and deepened by another 400 m in 2005. The drill cores provide a series of samples from the interior of this Hawaiian volcano that are not exposed on the surface of the volcano, and their relative ages are well constrained. Consequently, HSDP provides a unique opportunity to study the magmatic history of a Hawaiian volcano. Mauna Kea HSDP tholeiitic lavas form several distinctive compositional and isotopic groups, which play an important role in studying the geochemical structure of the Hawaiian plume and the relative roles of peridotite and pyroxenite in generating Hawaiian lavas. In detail, Mauna Kea HSDP tholeiites can be divided into three groups based on their major element compositions: High-SiO₂, Low-SiO₂, and High-CaO groups. Detailed geochemical and isotopic studies have been focused on the High- and Low-SiO₂ group lavas, and High-CaO lavas were not well studied because they were not included in the original reference suite samples. Here we report trace element compositions determined on a suite of High-CaO glasses, and use these data to constrain the petrogenesis of High-CaO lavas. When normalized to Low-SiO₂ lavas, High-CaO lavas form a U-shaped trace element pattern. That is, High-CaO lavas are enriched in both the most (Nb, Th) and the least (Sc, V) incompatible elements. This trace element difference is best explained if High-CaO parental magma represents a mixture of low degree partial melt of the Low-SiO₂ mantle source and a mafic cumulate component. This mafic cumulate must be clinopyroxene-rich, and it could be delaminated mafic cumulate formed under arcs during continent formation, lower continental crust, or lower oceanic crust.