Hawaii is the archetypal example of intra-plate 'hotspot' volcanism, yet the mechanisms of plume formation, hotspot volcano formation, and the nature of chemical heterogeneity in hotspot lavas such as those at Hawaii remain in question. Of particular interest is understanding what physical and chemical processes lead to the distinctive ‘double-track’ geochemical trend, whereby the volcanoes on the south-west side of the Hawaiian volcanic chain (‘Loa’ track) vary systematically compared with those on the north-east side (‘Kea’ track) with respect to major elements, trace elements, and isotopic ratios.

In order to make meaningful inferences about how the parallel distinction may arise, it is necessary to understand how the chemistry of magma evolves at Hawaii starting with the composition and melting of the mantle source, followed by emplacement of the magma into the crust with associated residence times and mixing, the progression of the crystallization process as the magma cools, and finally the eventual eruption through the volcanic edifice. Detangling of these distinct magmatic processes allows us to determine how the chemistry of the lava sampled at the surface has been altered from the original ‘primary’ magma in the mantle, and the extent to which lavas can be used to answer questions about the broad-scale geochemistry of Hawaii.

I will present results of major element, trace element, and Sr and Nd isotopic analyses of tholeiites from submarine Mauna Loa volcano, and will use these and compiled literature data of Mauna Loa and Kilauea Volcanoes to investigate both shallow magmatic processes in the crust and deep mantle melting in the upwelling plume. I will show how computational geochemical modelling may be used as an effective tool to model shallow magma chamber processes, and discuss its limitations when modelling deep mantle melting. I will show why whole-rock lava from Hawaii can be used to infer parental magma compositions, and that contrary to some visions of a spatially differentiated magmatic plumbing system, both submarine and subaerially erupted lavas are remarkably similar in composition. I will present a compilation of experimental melt studies and trace element models that suggest the source of Hawaii’s lava is most probably a garnet-bearing peridotite with a major element composition similar to that of primitive mantle or Hawaiian pyrolite, contradicting some current hypotheses, which argue for a pyroxenitic source.