FOUR VOLCANOES IN THE PACIFIC

I compare the transition from tholeiitic to alkalic basalt on four volcanic on the Pacific plate. These are 1) Haleakala, Maui, Hawaii; 2) Tutuila, American Samoa; 3) Fagaloa shield volcano of Upolu, Samoa; and 4) Tahiti-nui, the largest volcano of Tahiti, French Polynesia. All four exhibit shifts through time from hypersthene-normative tholeiitic compositions toward mildly alkalic, nepheline-normative basalt and then to more strongly undersaturated basanite. The latter two suites erupted sequentially during waning stages of shield volcanism, when felsic alkalic magmatic lineages were well developed, to a final rejuvenated or post-erosional stage. A precursor alkalic stage has not been sampled at any of these volcanoes, but does exist at Hawaii and Samoa.

Despite directional consistency in bulk and normative compositions, strong differences in alkalicity and in trends of trace element and isotopic enrichment or depletion occur that are related to source compositions. Quantitative models for partial melting for one place based on isotopes and trace elements cannot be used at the others and no common pattern of zoned distribution of source heterogeneity exists. At Samoa, temporal trace element and isotopic directionality differs between adjacent islands. At Tutuila and Tahiti-nui, significant variation can be sampled in stratigraphic successions and from flow to flow, to indicate an extremely small scale of significant heterogeneity at the source. I infer similar ranges of temperature, extent of partial melting and depth control major-oxide compositions, and interpret the complex geochemical patterns to indicate juxtaposition of mantle lithofacies with different isotopic characteristics at each of these places. Lesser partial melting were sampled through time. Melt regimes deepened in regions of shear within the low-velocity zone that lies within the boundary layer at the base of the Pacific plate moving overhead. The source of tholeiites is near 75 km at all four volcanoes; the base, where olivine nephelinites derive, is near 150 km. The boundary layer is the geochemically diverse source for all parental basaltic magmas of these volcanoes, regardless of how that geochemistry was imparted to it.