Evaluation of Source Components and Processes Forming the Koloa Volcanics of Kauai, Hawaii, Based on Pb, Sr and Nd Isotopic Characteristics

The Koloa Volcanics are the most voluminous (~58 km3) and long-lived (2.4 m.y.) example of Hawaiian rejuvenated volcanism. Rock types range from alkali basalts to melilite foidites with ages from 2.6 to 0.2 Ma; however, there is no temporal correlation in rock types. New, unaltered to weakly altered samples were analyzed for Pb, Sr and Nd isotopes to compare with compositional and age data to evaluate the source components and processes responsible for Kauai's rejuvenated volcanism. The three current models for rejuvenated volcanism are: secondary plume melting, flexure-induced decompression melting and lithospheric melting by conductive heating. The new Pb isotopic ratios define a tight linear trend extending to more radiogenic values than previously reported for rejuvenated lavas, or any other lava in Hawaii. The ^{208}Pb/204Pb vs. 206Pb/204Pb linear trend does not intersect the depleted MORB field; alternatively, it lies within the field defined by Kea trend volcanoes, despite being on the Loa trend geographically. On the same diagram, the rejuvenated Koloa Volcanics also almost entirely overlap the field of the main-stage Kauai tholeiites but lie on a slightly different trend. In a Sr vs. Pb diagram, the rejuvenated lavas plot off the hyperbola defined by main-stage volcanism on Kauai and other Hawaiian volcanoes, and away from the MORB field, indicating the involvement of a unique component. No correlation exists between Pb or Sr isotopes and rock types of the Koloa Volcanics. There is also no discernible trend of Pb or Sr isotopic ratios with trace elements (La/Ce, Zr/Nb, Sr/Y or Zr/Y) or incompatible elements (P2O5). Temporal variations, however, are observed for Pb isotopic ratios, particularly for the alkali basalts and the basanites. The younger lavas are less radiogenic suggesting a temporal trend towards a less radiogenic source component. The overlap of the Pb isotopic trends between the rejuvenated and the main-shield volcanics, and the lack of correlation between trace elements and isotopes indicate the involvement of a heterogeneous mantle plume source. Therefore, the secondary plume melting and flexure-induced decompression melting models, which both suggest a plume-derived source, are preferred over the lithospheric melting by conductive heating model.