Systematic geochemical and eruptive relations in the late stage evolution of volcanoes from the Hawaiian plume: With case studies of Waianae and East Molokai volcanoes.

摘要

A significant geochemical event in the evolution of Hawaiian volcanoes is the decline in magmatic flux from shield to post-shield volcanism as each edifice is carried away from the mantle-plume source. This eruptive decline is accompanied by a shift from tholeiitic to alkalic basalt volcanism, although a full spectrum exists in the extent to which any particular volcano undergoes the transition. Evidence shows that systematic geochemical relations exist among volcanoes for this late-shield stage. Compositional correlations in subaerial tholeiitic basalt among Pb, Sr, and Nd isotope ratios and SiO 2, TiO2, FeO, CaO, and Na2O content indicate that, for any particular volcano, the proportion of the two identified endmember isotopic components of the Hawaiian shield-building stage, KOO and KEA, are related to what extent eruptions subsequently shift from tholeiitic to alkalic basalt. The spectrum is described by a model in which KOO-dominated lavas result from melting of a mantle plume with a steep marginal temperature gradient, which accounts for large degree melt production followed by abrupt termination as the volcano moves off the plume. KEA-dominated lavas result from melting of a plume that has developed a long lateral temperature gradient, where large degree melts are followed by continued melt production of increasingly smaller degree. Detailed geochemical data for lavas of Waianae and East Molokai Volcanoes show evolutionary characteristics consistent with the overall model and provide further insight into mantle component characteristics and mixing. The Waianae tholeiite source is an intermediate mix of KEA and KOO. Otherwise apparently well-mixed in the source, the components are systematically sampled by late-shield lavas, where compositions vary stratigraphically from increased KEA to increased KOO. The appearance at Waianae of endmember KOO, previously identified only at Koolau, Lanai, and Kahoolawe, expands the identified isotopic range of Hawaiian lavas and defines KOO isotopic heterogeneity. The East Molokai tholeiite source is KEA-dominated. Late-shield compositions shift toward the isotopically-deleted Post-Erosional (PE) component, which is shown to be characteristic of KEA-dominated volcanoes. Mixing models, accounting for both Sr and Pb isotopic variations, describe PE involvement as metasomatism of the plume margin by small degree melts of a MORB source.