Lanai

Lanai volcano exposes no known alkalic or post-erosional volcanism. Our analyses of tholeiites from Lanai confirm their "primitive" (low Nd and high Sr) isotopic signatures relative to other Hawaiian volcanoes, but also show that the isotopic range is muc h larger than first thought. A remarkable linear correlation of Nd and Sr isotope compositions (linear to within analytical error) also characterizes the sample suite we have analyzed to date.

Detailed isotopic characterization of ocean island basalts that represent extreme mantle compositions are of considerable use in defining the chemical identity and history of mantle reservoirs that are rarely, if ever, sampled in their undiluted form. Of the Hawaiian volcanoes, Koolau, Kahoolawe and Lanai represent the purest samples of the enriched plume component. Lanai and Kahoolawe are two of the volcanoes that comprise the Maui Volcanic Complex (MVC). The few isotopic data available for Lanai samples (4 with Nd-Sr-Pb (West et al., 1987) and 7 with Nd-Sr (Basu and Faggart, 1996)) show compositional overlap with Koolau lavas and are similar to or somewhat depleted with respect to primitive mantle. In order to better constrain the mantle sources of Lana i lavas, we analyzed an additional 18 samples that extend both the geographic and stratigraphic range of the available data base.

We collected tholeiitic basalt samples from Manele Bay, Kaumalapau and Maunalei Gulches, the Kaholo Pali in the southwest, the northwestern area and several other localities in order to encompass the stratigraphic and geographic range of exposures on Lana i. Sr and Nd isotope ratios are strongly correlated in our sample suite, ranging from bulk earth values toward depleted mantle values (87Sr/86Sr = 0.70395 to 0.70466; eNd = ‚0.24 to +3.78). Pb isotope compositions are near the geochron (206Pb/204Pb = 17. 87 to 18.02). The Nd, Sr and Pb isotope compositions are extremes for the Hawaiian islands. We also identify a general negative correlation of Pb with Sr isotope ratios, the slope of which is on an extension of the trend observed overall for tholeiites fr om the MVC. No systematic spatial distribution of the Lanai data are apparent. Samples collected from a continuous 7 meter section on the Kaholo Pali reveal no systematic temporal variations in isotopic values. The isotopic variability within this single short section spans nearly the complete range observed for the entire island.

Our data offer further insight into the compositionally primitive component of the Hawaiian plume. Nd-Sr and Sr-Pb correlations manifested in the overall growth of the MVC (West and Leeman, 1987) indicate mixing between Koolau-Lanai and Kilauea endmembers . We also now observe this process during the temporally and spatially more restricted volcanism on Lanai. The isotopic range of Lanai lavas is very similar to that of the Koolau lavas, suggesting that similar small extents of mixing, dominated by an isot opically primitive endmember, occurred at different times and places in the Hawaiian chain.