The origin and evolution of lavas from Haleakala Crater, Hawaii
West, Howard Bruce
Leeman, William P.
Doctor of Philosophy
Sr, Nd, and Pb isotope systematics of lavas from the Maui Volcanic Complex (MVC) are consistent with a three-component petrogenetic mixing model. MVC shield-building (SB) lavas define linear trends on isotope-isotope plots, consistent with two-component mixing between primitive (PM) and enriched (EM) mantle components. The two-component (PM-EM) Hawaiian plume source is variable in composition during production of tholeiite magmas even within a single shield. Sr and Pb isotopic ratios of Haleakala post shield-building (PSB) lavas define a strong positively correlated array that deviates from the SB array towards an unradiogenic end-member. The PSB array may therefore result from time- and volume-dependent binary mixing between Hawaiian plume melts and a depleted (DM) mantle (i.e. MORB source) component. Several trace element ratios in Haleakala PSB lavas are correlated with isotopic compositions but not with major and trace element contents, and therefore appear to reflect changes in source composition. Trace element mixing systematics for these lavas indicate that the DM component must be a melt. The inferred PM component has chondritic ratios for several trace elements, consistent with it representing primitive mantle. The EM component may represent a part of the Hawaiian plume source that was either metasomatized or metasomatically scavenged. Alkalic cap lavas exposed in the northwest wall of Haleakala Crater display systematic, upsection geochemical variations indicative of the repetitive intrusion of discrete magma batches. Magma batches are separated by geochemical discontinuities characterized by abrupt upsection increases in incompatible element contents and commensurate drops in compatible element contents. In contrast, lava compositions within magma batches vary upsection progressively, and geochemical variations are opposite to those observed for interbatch discontinuities. Together, these geochemical variations are interpreted as resulting from the cyclic operation of a dynamic, evolving, open system magma chamber. Interbatch transitions appear to reflect periods of eruptive quiessence characterized by low magma recharge rates and relatively high degrees of crystal fractionation. Intrabatch variations appear to represent eruptive periods characterized by relatively high recharge rates, low degrees of crystal frationation, and progressive mixing of evolved rest magma with more primitive recharge magma.