Magmatic Processes During the Prolonged Pu’u ’O’o Eruption of Kilauea Volcano, Hawaii

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Abstract
The Pu’u ’O’o eruption is exceptional among historical eruptions of Kilauea Volcano for its long duration (∼17 years and continuing), large volume (∼2 km3), wide compositional range (5·6–10·1 wt % MgO) and the detailed monitoring of its activity. The prolonged period of vigorous effusion (∼300 000 m3/day) and the simple phenocryst mineralogy of the lavas (essentially only olivine) has allowed us to examine the volcano’s crustal and mantle magmatic processes. Here we present new petrologic data for lavas erupted from 1992 to 1998 and a geochemical synthesis for the overall eruption. The dominant crustal magmatic processes are fractionation and accumulation of olivine, which caused short-term (days to weeks) compositional variations. Magma mixing was important only during the early part of the eruption and during episode 54. The overall systematic decrease in MgO-normalized CaO content and abundances of highly incompatible elements, without significant Pb, Sr and Nd isotope compositional variation, is interpreted to be caused by mantle melting processes. Experimental results and modeling of trace element variations indicate that neither batch melting nor simple progressive melting can explain these compositional variations. Instead, a more complex progressive melting model is needed. This model involves two source components with the same isotopic composition, but one was melted ∼3% in the Hawaiian plume. The model results indicate that the amount of this depleted source component progressively increased during the eruption from 0 to ∼25%. Given the isotopic similarity of Pu’u ’O’o lavas to many lavas from Loihi Volcano and the small extent of prior melting to form the depleted source component, the melting region for Pu’u ’O’o magmas may partially overlap with that of the adjacent, younger volcano, Loihi.