NOBLE GAS AND GEOCHRONOLOGY STUDY OF THE HANA RIDGE, HALEAKALA VOLCANO, HAWAII; IMPLICATIONS TO THE TEMPORAL CHANGE OF MAGMA SOURCE AND THE STRUCTURAL EVOLUTION OF THE SUBMARINE RIDGE

Abstract

In order to constrain the magma sources and the structural evolution of Hana Ridge, noble gas isotope ratios and 40Ar/39Ar ages were determined for this submarine extension of the east rift zone of Haleakala volcano. 40Ar/39Ar ages of ten lavas from Hana Ridge are bimodally distributed. Three samples have ages of 1.8-1.9 Ma and seven samples have ages of 1.4-1.5 Ma. The observation of older lavas overlying younger ones in two locations suggests that growth of Hawaiian rift zones occurs endogenously by continuous intrusion of magmas and patchy resurfacing at outbreak points. The 0.5 Ma age range for submarine Hana Ridge lavas, coupled with previously published K/Ar ages of 0.97-1.1 Ma for subaerial Honomanu lavas on the Haleakala Volcano, indicates that Haleakala shield volcanism persisted for nearly 1 My. Furthermore, the new 40Ar/39Ar ages, when considered with isotopic compositions of the lavas, suggests that the source for Hana Ridge magmas gradually shifted over a period of 0.5-1.0 My. The majority of samples from the ridge have relatively uniform 3He/4He ratios between 18 and 22 Ra, which is higher than 3He/4He of subaerial Honomanu tholeiites. 20Ne/22Ne and 21Ne/22Ne ratios define linearly correlated trend that overlaps with the Loihi-Kilauea trend. He-Ne systematics of the Hana Ridge indicate that the magmas comprising most parts of the ridge were derived from a source with a primordial less-degassed mantle component. Since Hana Ridge lavas predate the subaerial Honomanu lavas, temporal decrease of 3He/4He ratios suggests that contribution of this primordial component had decreased in the magma source during establishment of Hana Ridge and Haleakala volcano. Pb-Sr isotopes demonstrate that Hana Ridge magmas are representative of the Kea component. However, such isotopic signatures associated with elevated high 3He/4He ratios precludes that the Kea component is a distinct endmember, such as recycled oceanic crust or lithospheric mantle. Alternatively, we propose that it is a common sub-component that is a mixture of Loihi endmember and recycled oceanic crust. Ne isotope ratios of the Hawaiian samples, including the Hana Ridge, show primordial signature irrespective of 3He/4He ratios. Such apparent decoupling of He and Ne isotopes may be also attributed to mixing of the Loihi component and recycled component in the mantle plume, either of which components needs to be elementally fractionated prior to mixing. © 2006 Elsevier B.V. All rights reserved.