Shield-stage alkalic volcanism on Mauna Loa Volcano, Hawaii

Alkalic lavas have been discovered at two young (∼15–20 ka) submarine radial vents on the western flank of Mauna Loa, a volcano that was thought to have produced only tholeiitic lavas for at least the last ∼240,000 years. Major and trace element data show that the lavas from only one of these eruptions have undergone lower degrees of partial melting, the assumed dominant process for creating alkalic lavas on Hawaiian volcanoes. However, major and trace element analyses also suggest that lavas from both of the alkalic eruptions are evolved and have experienced moderate-pressure (∼1.0 GPa) fractionation of pyroxene, an alternative mechanism for producing an alkalic magma from a tholeiitic parent. Pyroxene fractionation is also indicated by MELTS modeling and phase diagram projections. Although lavas with similar compositions have been erupted from neighboring Mauna Kea and Hualalai volcanoes, Pb isotope and trace element data indicate that the submarine alkalic lavas were produced from the same source generating Mauna Loa tholeiites. Thus, these are the first alkalic lavas to be identified from this volcano. Holocene eruptions of Mauna Loa have all produced tholeiitic lavas and eruption rates have remained relatively high. Thus, the volcano has probably not yet entered the postshield stage of Hawaiian volcanism, which is characterized by alkaline lavas and less frequent eruptions. The occurrence of the alkalic lavas only at radial vents may indicate that these magmas bypassed the primary conduit, which supplies the summit reservoir with predominantly tholeiitic magma.