HAWAIIAN-EMPEROR RIDGE AND NINETYEAST RIDGE: A COMPARISON OF TWO HOTSPOT TRACKS

The Hawaiian-Emperor Ridge (HER) and the Ninetyeast Ridge (NER) are the longest (>5 km) submarine volcanic chains on Earth, and range from from 0 to > 81 Ma and 43 to 77 Ma respectively. These seamount chains are interpreted as hotspot tracks formed as the Pacific and Indian Oceanic plates passed over the Hawaiian and Kerguelen mantle plumes. Both hotspot tracks have complexities: paleomagnetic and age data show that from 81 – 49 Ma the Hawaiian hotspot moved southward; magnetic anomaly and age data show that the NER is 11° longer than adjacent contemporaneous oceanic crust. This discrepancy has been explained by southward spreading ridge jumps towards the Kerguelen hotspot.

Hawaiian volcanoes grow through well-defined compositional stages (pre-shield alkalic to shield tholeiitic to post-shield alkalic) consistent with varying extents of melting as the volcano approaches, overrides and recedes from the hotspot. Although low in volume, post-shield lavas almost completely cover the shield lavas at Mauna Kea, Hualalai and Haleakala. Alkalic basalt has also been found overlying tholeiitic basalt in drill cores from Emperor Seamounts. Alkalic lavas related to the Kerguelen hotspot have erupted in the <25 Ma eastern Kerguelen Archipelago and on Heard and McDonald Islands. However, alkalic lavas are rare on the NER; only 1 of 21 dredges along the ridge recovered alkalic basalt. All NER drill sites recovered tholeiitic lavas that are compositionally distinct from the olivine-bearing tholeiitic basalt that forms Hawaiian shields.

From Kilauea to Koolau (O to 3 Ma) Hawaiian volcanoes define the spatial Loa and Kea trends. Correlations between ratios of incompatible elements and isotopic ratios of Nd and Pb in these lavas require that the geochemical extreme ciomponenti in Loa shield lavas was derived from an ancient plagioclase-rich cumulate. Submarine volcanoes of the HER and NER have not been studied in similar detail. Correlations between incompatible element and isotopic ratios of NER samples ranging in age from 43 to 77 Ma are complex, reflecting a more enriched and heterogeneous component than the Loa component and a depleted component that is different from MORB erupted at the Southeast Indian Ridge.