Rejuvenated stage lavas on and around Hawaii
Hawaiian volcanoes progress through a series of eruptive stages. The final stage, which occurs on only some volcanoes, consists of small-volume eruptions of strongly alkalic lavas that follow a pronounced erosional period. This stage is restricted to volcanoes older than Haleakala Volcano on Maui. It consists of only a few vents and associated flows on West Maui (Lahaina Basalt) and Molokai (Kalaupapa Basalt), about 37 vents and groups of vents and associated flows on Koolau Volcano on Oahu (Honolulu Volcanics), but numerous vents and flows on Kauai (Koloa Volcanics) and Niihau (Kiekie Basalt). The lavas range from alkalic basalt to nepheline melilitite and include lavas with as little as 35.5% SiO2. The striking characteristic of these lavas is that their isotopic compositions are highly depleted, but they are strongly enriched in incompatible trace elements. Most models for their formation suggest that they are small degree partial melts of a recently enriched (metasomatized) source.
Geochemically similar lavas occur on the submarine flanks of several volcanoes as well as on the deep sea floor far from the islands. We have found and sampled (using the Pisces V submersible) a single vent and flow on the submarine north flank of Molokai that is geochemically similar to the Kalaupapa basalt. Offshore Oahu, complete mapping of the flanks of the island reveal only a few submarine vents, one off the northeast coast, and several more extending along the Koko Rift to the southwest of Hanauma Bay. These vents were sampled using the Remotely Operated Vehicle (ROV) Tiburon and found to be alkalic basalt and nephelinite. Incomplete mapping around Kauia has revealed a number of submarine vents and we have sampled several using the ROV Tiburon to recover a range of lavas from nephelinite to alkalic basalt. Volcaniclastic deposits from one cone contained abundant lherzolite xenoliths. All these cones were steep pointed, in contrast to most of the mapped and samples vents offshore Niihau, which are flat-topped volcanoes that formed as overflowing lava ponded. The submarine rejuvenated stage lavas are extremely abundant and voluminous offshore Niihau, where single cones are as large as 9 km3. These lavas are in all ways similar to their equivalents on land, except that they have less depleted isotopic ratios of Sr and Nd.
On the deep sea floor around Hawaii, mapping has shown that a large number of cones and flows exist between Oahu and Kauai, in the Southwest Oahu, North Arch, and South Arch volcanic fields. These lavas are mainly alkalic basalt and basanite, but nephelinite has also been recovered, as have a few samples of more fractionated teprophonolite, which occur as pumice fragments between Oahu and Kauai and in the Southwest Oahu volcanic field. These lavas commonly form extensive sheet flows that travel long distances (as much as 110 km) on only slightly sloping sea floor (a few m vertical change per km). The North Arch lavas are geochemically and isotopically similar to the rejuvenated stage lavas on the islands, whereas those from the Southwest Oahu volcanic field are more similar to those recovered from the island flanks, except that they also have intermediate He isotopic ratios. The South Arch lavas are unique in having high He isotopic ratios more similar to Loihi Seamount and distinct from the MORB-like ratios of all the other samples. The spatial distribution of more and less depleted sources suggests that the melting that produced the islands has added an enriched component to the source away from the center of the shield volcanoes, but that this added component is restricted to only the island flanks and deep sea floor relatively near the islands (Southwest Oahu volcanic field).
These strongly alkalic lavas contained large volatile inventories and commonly are characterized by pyroclastic (strombolian) eruptions that formed volcaniclastic deposits adjacent to the vents, even in water as deep as 4,200 m. At shallower depths, such deposits are common and even include spatter and bombs. At low eruption rates, steep cones form whereas at moderate eruption rates, flat-topped cones form. At the highest eruption rates, extensive sheet flows form and cover large areas of the sea floor. In the North Arch, sills as large as 10-12 km across, are commonly emplaced below the roughly 50 m of sediment that covers the seafloor. Many of the sheet flows may be fed from the edges of sills that breached the sediment.