Hilo Ridge

A student-run and organized cruise from the UW on the R/V Thompson had as one of its objectives obtaining bathymetry and samples of the Hilo Ridge. Robin Holcomb, Brian West and Courtenay Wilkerson (UW Oceanography) complied this map. Consideration of the bathymetry, petrology and isotope geochemistry of the dredged samples leads to the (controversial) possibility that much of Hilo Ridge is a rift zone of Kohala and not Mauna Kea. The arguments were presented in a poster at the 1997 San Francisco AGU meeting.

Newly dredged samples of submarine lava flows from the Hilo Ridge rift zone, directly east of Mauna Kea, include both tholeiitic and alkalic basalts and display a much larger range of Sr-, Nd- and Pb-isotope compositions than previously reported for Mauna Kea. Picrites and tholeiitic basalt from 5000 m below sea level near the base of the ridge, and tholeiitic basalts from 1100-m depth on the ridge have higher 87Sr/86Sr (0.703534-0.703899), lower eNd (6.65-5.91), and a much larger range of Pb-isotope comp ositions (e.g., 206Pb/204Pb = 18.276-18.520) than subaerial Mauna Kea lavas. Only tholeiites recovered from a 400-m depth submarine terrace at the shallowest part of the rift zone have Sr-, Nd-, Pb-isotope compositions within the fields of subaerial Mauna Kea basalts. In addition to tholeiities, alkalic basalts were also recovered from 1100 m depth on the ridge, at the location of a newly-discovered submarine terrace. This terrace may be correlative with one between 1145- and 950-m depth on the north flan k of Kohala volcano to the northwest, which has been interpreted as representing the end of Kohalažs tholeiitic shield-building stage. If all of these lavas are from the Mauna Kea shield, then this volcano has a much larger range of isotopic compositions than previously recognized, and lavas of the rift zone record a wider range of mantle sources (and possibly a longer period of time) than subaerial lavas. However, both tholeiitic and alkalic basalts from the 1100-m terrace, as well as tholeiites between 1600-3300 m studied by previous workers, have Sr- and Nd-isotopic compositions very similar to those of tholeiites from the Kohala shield. An alternative explanation for these data is that the Hilo Ridge rift zone comprises, at least in part, lavas of the Kohala shield and the 1100-m terrace on the Hilo Ridge represents the end of Kohala shield-building.