SUBMARINE LAVAS FROM MAUNA-KEA VOLCANO, HAWAII - IMPLICATIONS FOR HAWAIIAN SHIELD STAGE PROCESSES

The island of Hawaii is composed of five voluminous shields but only the youngest, active and well-exposed shields of Mauna Loa and Kilauea have been studied in detail. The shield lavas forming Kohala, Hualalai and Mauna Kea are largely covered by postshield lavas with geochemical characteristics that differ from the shield lavas. In order to determine the geochemical characteristics of the Mauna Kea shield which is adjacent to the Kilauea and Mauna Loa shields, 12 Mauna Kea shield basalts dredged from the submarine east rift were analyzed for major and trace element contents and isotopic (Sr, Nd, and Pb) ratios. The lavas are MgO-rich (11 to 20%), submarine erupted, tholeiitic basalts, but they are not representative of crystallized MgO-rich melts. Their whole rock and mineral compositions are consistent with mixing of an evolved magma, <7% MgO, with a magma containing abundant olivine xenocrysts, probably disaggregated from a dunitic cumulate. At a given MgO content, some of the Mauna Kea whole rocks have lower abundances of CaO and higher abundances of incompatible elements. The evolved melt component in these lavas reflects significant fractionation of plagioclase and clinopyroxene and in some cases even the late crystallizing phases orthopyroxene and Fe-Ti oxide. Although these Mauna Kea lavas are not isotopically homogeneous, in general their Sr, Nd and Pb isotopic ratios overlap with the fields for lavas from Loihi and Kilauea volcanoes. Lavas from these three shields define the low Sr-87/Sr-86, high Pb-206/Pb-204, and low Zr/Nb ranges for Hawaiian shield lavas, and they may contain a larger proportion of a component with high U-238/Pb-204. Most of these Mauna Kea shield lavas, however, are distinguished from Kilauea and Loihi tholeiitic basalts by having lower La/Ce and La/Yb. These data provide additional evidence that each Hawaiian shield has distinctive geochemical characteristics.