Hawaiian Volcanoes Deep Underwater Perspectives

Abstract

Hawaii is currently the most magmatically productive hotspot on the earth. Its track across the Pacific plate has been recovered for more than 70 Ma. Although Hawaiian volcanoes and their magma genesis have been studied extensively compared with other hotspots, their deep-ocean features have been little known until recently. The final section of this volume contains papers on geophysical observations and magma genesis in the Hawaiian mantle plume. Because of its central location in the Pacific Basin where the distribution of seismic stations is poor, seismic imaging of the mantle plume underneath Hawaiian hot spot is difficult. New seismic global tomography to look at the roots of major hot spots (e.g., Hawaii, Iceland, South Pacific, East Africa) have been carried out by Zhao et al. [this section] using body-wave first arrivals. Using advanced techniques (e.g., 3D-grid parameterization and incorporating the topography of Moho, 410 and 660 km discontinuities), they define structures under the hotspot with improved resolution. A low-velocity anomaly beneath Hawaii was found to extend westward down to at least the mid-lower mantle (ca. 1500 km depth). Slow velocity anomalies for other hotspots are found to extend to the core-mantle boundary but not as vertical pillars. Instead, Zhao et al. [this section] discovered that the plumes are deflected and they hypothesize that they are blown by mantle winds. Heat flow anomalies around hot spots have been regarded as direct evidence of hot upwelling (mantle plume). Contrary to previous interpretations, McNutt [this section] shows that variations in ocean-bottom heat flow around the Hawaiian archipelago are best correlated with thermal conductivity of near-surface sediments. Observed heat flow patterns are largely determined by lateral flux in permeable aquifers in the oceanic crust, and thermal structure of the underlying mantle plume is therefore difficult to determine from heat-flow data. Distinct anomalies in rare-gases (e.g., 3He/4He in lavas from Loihi Seamount is up to 35 times that in the atmosphere) are a key signature of mantle-plume magmas that originated from a deep undegassed reservoir in the mantle. Kaneoka et al. (this section] report new analyses of rare gas isotopes (He, Ne, Ar, Xe, and Kr) for deep-ocean samples from Loihi, Kilauea, and Koolau volcanoes. The new data and their previous studies document large variations in He isotopes among volcanoes in the Hawaiian hot spot. The highly variable He values are interpreted to represent different degrees of interaction between the rising plume magma and the uppermost asthenosphere. The variable values for the heavy rare gases document incorporation of seawater into the magma. Kaneoka et al. [this section] propose that the magnitude of these interactions with the upper mantle and crust is related to the evolutionary stage of the volcano. Koolau volcano on Oahu island was known to be an unusual Hawaiian shield because its basaltic lavas are high in SiO2, and have the highest 87Sr/86Sr and lowest 144Nd/143Nd isotopic ratios in Hawaii. Previous workers interpreted these features to recycling of old oceanic crust in the Hawaiian plume. Garcia [this section] examined olivine-rich lavas (picrite) recovered for the first time from the deep submarine flank of Koolau volcano. Although subaerial slopes of Koolau are covered by high-silica tholeiite, the sampled submarine parts contain abundant picrite. From detailed petrologic observations, Garcia concludes that the primary magma for Koolau volcano contains high MgO (14-15 wt%), as in other Hawaiian volcanoes. Takahashi and Nakajima [this section] explore an alternative model for the Koolau magma genesis. In order to simulate melting processes in the Hawaiian plume consisting of peridotite and entrained oceanic crust (eclogite), they conducted series of high-pressure melting experiments. They showed that high-silica tholeiite characteristic of the subaerial Koolau volcano could be produced by direct partial melting of recycled eclogite at temperatures slightly below the peridotite dry solidus. At temperatures slightly above the peridotite dry solidus, the melt compositions produced from a basalt-peridotite hybrid source change drastically toward picritic. Based on the experiments, they propose a model of magma genesis to explain the abrupt change in magma composition during the growth history of Koolau, from Mauna Loa-type main shield stage to the silica-rich Makapuu stage [Shinozaki et al., Section 3; Tanaka et al., Section 31. According to their model, temperatures within the Hawaiian plume are 100-150°C lower than previous estimates.